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  • 1. Andersson, Ove
    et al.
    Carvalho, Paulo H. B. Brant
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hsu, Ying-Jui
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transitions in pressure-amorphized clathrate hydrates akin to those of amorphous ices2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 1, article id 014502Article in journal (Refereed)
    Abstract [en]

    Type II clathrate hydrates (CHs) were studied by thermal and dielectric measurements. All CHs amorphize, or collapse, on pressurization to 1.3 GPa below 135 K. After heating to 160 K at 1 GPa, the stability of the amorphous states increases in a process similar to the gradual high density to very high density amorphous ice (HDA to VHDA) transition. On a subsequent pressure decrease, the amorphized CHs expand partly irreversibly similar to the gradual VHDA to expanded HDA ice transformation. After further heating at 1 GPa, weak transition features appear near the HDA to low density amorphous ice transition. The results suggest that CH nucleation sites vanish on heating to 160 K at 1 GPa and that a sluggish partial phase-separation process commences on further heating. The collapsed CHs show two glass transitions (GTs), GT1 and GT2. GT1 is weakly pressure-dependent, 12 K GPa(-1), with a relaxation time of 0.3 s at 140 K and 1 GPa; it is associated with a weak heat capacity increase of 3.7 J H2O-mol(-1) K-1 in a 18 K range and an activation energy of only 38 kJ mol(-1) at 1 GPa. The corresponding temperature of GT2 is 159 K at 0.4 GPa with a pressure dependence of 36 K GPa(-1); it shows 5.5 times larger heat capacity increase and 4 times higher activation energy than GT1. GT1 is observed also in HDA and VHDA, whereas GT2 occurs just above the crystallization temperature of expanded HDA and only within its similar to 0.2-0.7 GPa stable pressure range.

  • 2. Andersson, Ove
    et al.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    A Second Glass Transition in Pressure Collapsed Type II Clathrate Hydrates2018In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 122, no 15, p. 4376-4384Article in journal (Refereed)
    Abstract [en]

    Type II clathrate hydrates (CHs) M-17 H2O, with M = tetrahydrofuran (THF) or 1,3-dioxolane, are known to collapse, or amorphize, on pressurization to similar to 1.3 GPa in the temperature range 77-140 K. On heating at 1 GPa, these pressure-amorphized CH states show a weak, stretched sigmoid-shaped, heat-capacity increase because of a glass transition. Here we use thermal conductivity and heat capacity measurements to show that also type II CH with M = cyclobutanone (CB) collapses on isothermal pressurization and undergoes a similar, weak, glass transition upon heating at 1 GPa. Furthermore, we reveal for both THF CH and CB CH a second, much more pronounced, glass transition at temperatures above the thermally weak glass transition on heating in the 0.2-0.7 GPa range. This result suggests the general occurrence of two glass transitions in water-rich (94 mol %) pressure-collapsed CHs. Because of a large increase in dielectric permittivity concurrently as the weak heat capacity increase, the first glass transition must be due to kinetic unfreezing of water molecules. The thermal features of the second glass transition, measured on isobaric temperature cycling, are typical of a glass liquid glass transition, which suggests that pressure-amorphized CHs transform reversibly to liquids.

  • 3. Auer, Henry
    et al.
    Nedumkandathil, Reji
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kohlmann, Holger
    The Hydrogenation of the Zintl Phase NdGa Studied by in situ Neutron Diffraction2019In: Zeitschrift für Anorganische und Allgemeines Chemie, ISSN 0044-2313, E-ISSN 1521-3749, Vol. 645, no 3, p. 175-181Article in journal (Refereed)
    Abstract [en]

    The hydrogenation of the Zintl phase NdGa was studied by in situ neutron powder diffraction. We find a compositional range of 0.1 < x < 0.8 in NdGaH1+x. Hydrogen atoms are located in two different positions, in HNd4 tetrahedra, and close to the polyanionic chains. For the latter, the Ga-H distance in NdGaH1.66 is quite long (ca. 200 pm) with a trigonal bipyramidal Nd3Ga2 surrounding of hydrogen atoms. Hydrogen poor NdGaH<1 phases as known for similar systems were not observed. The changing hydrogen content shows no measureable effect on the unit cell volume, but on lattice parameter ratios. Superstructures occur for 0.53 < x < 0.66 and 0.73 < x < 0.8, leading to a doubling or tripling of the lattice parameter a. They are probably caused by partial hydrogen ordering. The threefold superstructure contains a (1)[(Ga-H-Ga-H-Ga)(6-)] moiety with hydrogen bridging two gallium chains.

  • 4. Benson, Daryn
    et al.
    Li, Yanling
    Luo, Wei
    Ahuja, Rajeev
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lithium and Calcium Carbides with Polymeric Carbon Structures2013In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 52, no 11, p. 6402-6406Article in journal (Refereed)
    Abstract [en]

    We studied the binary carbide systems Li2C2 and CaC2 at high pressure using an evolutionary and ab initio random structure search methodology for crystal structure prediction. At ambient pressure Li2C2 and CaC2 represent salt-like acetylides consisting of C-2(2-) dumbbell anions. The systems develop into semimetals (P (3) over bar m1-Li2C2) and metals (Cmcm-Li2C2, Cmcm-CaC2, and Immm-CaC2) with polymeric anions (chains, layers, strands) at moderate pressures (below 20 GPa). Cmcm-CaC2 is energetically closely competing with the ground, state structure. Polyanionic forms of carbon 4 stabilized by electrostatic interactions with surrounding cations add a new feature to carbon chemistry. SemimetallicP (3) over bar m1-Li2C2 displays an electronic structure close to that of graphene. The pi* band, however, is hybridized with Li-sp states and changed into a bonding valence band. Metallic forms are predicted to be superconductors. Calculated critical temperatures may exceed 10 K for equilibrium volume structures.

  • 5.
    Brant Carvalho, Paulo H. B.
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mace, Amber
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bull, Craig L.
    Funnell, Nicholas P.
    Tulk, Chris A.
    Andersson, Ove
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Elucidation of the pressure induced amorphization of tetrahydrofuran clathrate hydrate2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 20, article id 204506Article in journal (Refereed)
    Abstract [en]

    The type II clathrate hydrate (CH) THF center dot 17 H2O (THF = tetrahydrofuran) is known to amorphize on pressurization to similar to 1.3 GPa in the temperature range 77-140 K. This seems to be related to the pressure induced amorphization (PIA) of hexagonal ice to high density amorphous (HDA) ice. Here, we probe the PIA of THF-d(8)center dot 17 D2O (TDF-CD) at 130 K by in situ thermal conductivity and neutron diffraction experiments. Both methods reveal amorphization of TDF-CD between 1.1 and 1.2 GPa and densification of the amorphous state on subsequent heating from 130 to 170 K. The densification is similar to the transition of HDA to very-high-density-amorphous ice. The first diffraction peak (FDP) of the neutron structure factor function, S(Q), of amorphous TDF-CD at 130 K appeared split. This feature is considered a general phenomenon of the crystalline to amorphous transition of CHs and reflects different length scales for D-D and D-O correlations in the water network and the cavity structure around the guest. The maximum corresponding to water-water correlations relates to the position of the FDP of HDA ice at similar to 1 GPa. Upon annealing, the different length scales for water-water and water-guest correlations equalize and the FDP in the S(Q) of the annealed amorph represents a single peak. The similarity of local water structures in amorphous CHs and amorphous ices at in situ conditions is confirmed from molecular dynamics simulations. In addition, these simulations show that THF guest molecules are immobilized and retain long-range correlations as in the crystal.

  • 6. Efthimiopoulos, Ilias
    et al.
    Benson, Daryn E.
    Konar, Sumit
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nylén, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liebig, Stefan
    Ruschewitz, Uwe
    Vazhenin, Grigory V.
    Loa, Ingo
    Hanfland, Michael
    Syassen, Karl
    Structural transformations of Li2C2 at high pressures2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 6, article id 064111Article in journal (Refereed)
    Abstract [en]

    Structural changes of Li2C2 under pressure were studied by synchrotron x-ray diffraction in a diamond anvilcell under hydrostatic conditions and by using evolutionary search methodology for crystal structure prediction.We show that the high-pressure polymorph of Li2C2, which forms from the Immm ground-state structure (Z = 2)at around 15 GPa, adopts an orthorhombic Pnma structure with Z = 4. Acetylide C2 dumbbells characteristic ofImmm Li2C2 are retained in Pnma Li2C2. The structure of Pnma Li2C2 relates closely to the anticotunnite-typestructure. C2 dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structureof Immm Li2C2. First-principles calculations predict a transition of Pnma Li2C2 at 32 GPa to a topologicallyidentical phase with a higher Cmcm symmetry. The coordination of C2 dumbbell units by Li atoms is increasedto 11. The structure of Cmcm Li2C2 relates closely to the Ni2 In-type structure. It is calculated that Cmcm Li2C2becomes metallic at pressures above 40 GPa. In experiments, however, Pnma Li2C2 is susceptible to irreversibleamorphization.

  • 7. Ek, Gustav
    et al.
    Nedumkandathil, Reji
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johansson, Robert
    Montero, Jorge
    Zlotea, Claudia
    Andersson, Mikael S.
    Nordblad, Per
    Tang, Chiu
    Sahlberg, Martin
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydrogen induced structure and property changes in Eu3Si42019In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 277, p. 37-45Article in journal (Refereed)
    Abstract [en]

    Hydrides Eu3Si4H2-X were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 degrees C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25-200 degrees C afford a uniform hydride phase with an orthorhombic structure (Immm, a approximate to 4.40 angstrom, b approximate to 3.97 angstrom, c approximate to 19.8 angstrom), whereas at 300 degrees C mixtures of two orthorhombic phases with c approximate to 19.86 and approximate to 19.58 angstrom were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H-2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns pointed to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x < 2) and are disordered with respect to H in Si2Eu3 interstitials. Eu3Si4 is a ferromagnet with a Tc at about 120 K. Ferromagnetism is effectively quenched in Eu3Si4H2+x. The effective magnetic moment for both materials is 7.5 pg which is typical for compounds containing Eu2+ 4f(7) ions.

  • 8.
    Eklöf, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fischer, A.
    Wu, Y.
    Scheidt, E. -W
    Scherer, W.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transport properties of the ii v semiconductor znsb2013In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 1, no 4, p. 1407-1414Article in journal (Refereed)
    Abstract [en]

    The intermetallic compound ZnSb is an electron poor (II-V) semiconductor with interesting thermoelectric properties. Electrical resistivity, thermopower and thermal conductivity were measured on single crystalline and various polycrystalline specimens. The work establishes the presence of impurity band conduction as an intrinsic phenomenon of ZnSb. The impurity band governs electrical transport properties at temperatures up to 300-400 K after which ZnSb becomes an intrinsic conductor. Furthermore this work establishes an inherently low lattice thermal conductivity of ZnSb, which is comparable to the state-of-the- art thermoelectric material PbTe. It is argued that the impurity band relates to the presence of Zn defects and the low thermal conductivity to the electron-poor bonding properties of ZnSb.

  • 9. Eklöf-Österberg, Carin
    et al.
    Nedumkandathil, Reji
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pell, Andrew J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tyagi, Madhusudan
    Jalarvo, Niina H.
    Frick, Bernhard
    Faraone, Antonio
    Karlsson, Maths
    Dynamics of Hydride Ions in Metal Hydride-Reduced BaTiO3 Samples Investigated with Quasielastic Neutron Scattering2019In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 4, p. 2019-2030Article in journal (Refereed)
    Abstract [en]

    Perovskite-type oxyhydrides, BaTiO3-xHx, have been recently shown to exhibit hydride-ion (H-) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO3 samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400-700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, <= 250 K, all of the models feature a characteristic jump distance of about 2.8 angstrom, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 angstrom, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 X 10(-6) cm(-2) s(-1) at the lowest applied temperature of 225 K and between ca. 20 X 10(-6) and 100 X 10(-6) cm(-2) s(-1) at temperatures between 400 and 700 K. Activation energies E-a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.

  • 10. Evans, Michael J.
    et al.
    Wu, Yang
    Kranak, Verina F.
    Arizona State University, USA.
    Newman, N.
    Reller, Armin
    Garcia-Garcia, F. Javier
    Häussermann, Ulrich
    Arizona State University, USA.
    Structural properties and superconductivity in the ternary intermetallic compoundsMAB (M=Ca, Sr, Ba; A=Al, Ga, In; B=Si, Ge, Sn)2009In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 80, article id 064514Article in journal (Refereed)
    Abstract [en]

    The ternary intermetallic compounds MAB=CaAlSi, SrAlSi, BaAlSi, CaGaSi, SrGaSi, BaGaSi, SrAlGe, BaAlGe, CaGaGe, SrGaGe, BaGaGe, BaInGe, BaAlSn, CaGaSn, SrGaSn, and BaGaSn have been prepared by arc-melting stoichiometric elemental mixtures and structurally characterized by a combination of x-ray powder and electron diffraction. They crystallize as variants of the simple hexagonal AlB2 structure type where trivalent and tetravalent A- and B-type atoms, respectively, form commonly a planar hexagon layer, and structural variations arise from A/B ordering and/or puckering of hexagon layers. The silicides (B=Si) were previously investigated for their superconducting properties. By dc magnetization measurements it is demonstrated that also the germanides SrAlGe, BaAlGe, SrGaGe, and BaGaGe and the stannide BaAlSn are superconductors above 2 K.

  • 11.
    Filippov, Stanislav
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Linköping University, Sweden.
    Klarbring, Johan
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Simak, Sergei I.
    Temperature-induced phase transition and Li self-diffusion in Li2C2: A first-principles study2019In: Physical Review Materials, ISSN 2475-9953, Vol. 3, no 2, article id 023602Article in journal (Refereed)
    Abstract [en]

    Lithium carbide, Li2C2, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from the ground-state orthorhombic (Immm) to the high-temperature cubic (Fm (3) over barm) crystal structure is not well understood and here we elucidate it with help of first-principles calculations. We show that stabilization of the cubic phase is a result of a temperature-induced disorientation of the C-C dumbbells and their further thermal rotations. Due to these rotations rather large deviatoric stress, which is associated with the dumbbell alignment along one of the crystallographic axes, averages out making the cubic structure mechanically stable. At high temperature we observe a type-II superionic transition to a state of high Li self-diffusion involving collective ionic motion mediated by the formation of Frenkel pairs.

  • 12. Fischer, A.
    et al.
    Scheidt, E. -W.
    Scherer, W.
    Benson, D. E.
    Wu, Y.
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Haussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 22, article id 224309Article in journal (Refereed)
    Abstract [en]

    The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.

  • 13. Fischer, Andreas
    et al.
    Eklöf, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Benson, Daryn E.
    Wu, Yang
    Scheidt, Ernst-Wilhelm
    Scherer, Wolfgang
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Synthesis, Structure, and Properties of the Electron-Poor II-V Semiconductor ZnAs2014In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 53, no 16, p. 8691-8699Article in journal (Refereed)
    Abstract [en]

    ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil highpressure techniques and structurally characterized by single-crystal and powder X-ray 7 diffraction (space group Pbca (No. 61), a = 5.6768(2) angstrom, b = 7.2796(2) angstrom, c = 7.5593(2) angstrom, Z = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn2As2, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn3As2 and ZnAs2. When heating at a rate of 10 K/min decomposition takes place at similar to 700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2-400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are similar to 1 Omega cm and +27 mu V/K, respectively. These values are considerably higher and lower, respectively, compared to Zn Sb. Above 150 K the thermal conductivity attains low values, around 2 W/m.K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of theta(D) = 234 K, theta(E1) = 95 K, and theta(E2) = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties.

  • 14.
    Häussermann, Ulrich
    et al.
    Arizona State University, USA.
    Kranak, Verina F.
    Arizona State University, USA.
    Puhakainen, Kati
    Arizona State University, USA.
    Hydrogenous Zintl Phases: Interstitial Versus Polyanionic Hydrides2011In: Zintl Phases: Principles and Recent Developments / [ed] Thomas F. Fässler, Springer Berlin/Heidelberg, 2011, Vol. 139, p. 143-161Chapter in book (Refereed)
    Abstract [en]

    Hydrogen may be incorporated in Zintl phases in two different ways: either hydridic where H is exclusively coordinated by electropositive metals (interstitial hydrides), or as part of the polyanion where it acts as a covalently bonded ligand (polyanionic hydride). Both scenarios provide novel coordination environments and bonding scenarios for the atoms involved. This makes hydrogenous Zintl phases important model systems for fundamental studies of hydrogen–metal interactions. Simultaneously, hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science.

  • 15.
    Konar, Sumit
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häusserman, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Intercalation Compounds from LiH and Graphite: Relative Stability of Metastable Stages and Thermodynamic Stability of Dilute Stage I-d2015In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 7, p. 2566-2575Article in journal (Refereed)
    Abstract [en]

    The intercalation of lithium into graphite was studied at temperatures between 400 and 550 degrees C by heating mixtures of LiH and graphite powders with molar ratios 4:1, 1:1, and 1:6 under dynamic vacuum for periods between 1 and 72 h. These conditions probe the formation and thermal stability of metastable staged Ligraphite intercalation compounds (Li-GICs) close to the competing formation of the thermodynamically stable carbide Li(2)C2. Li-GICs of stages I (LiC6, A alpha), IIa (Li0.5C6, A alpha A), IIb (Li similar to C-0.33(6), A alpha AB beta B), III (Li similar to C-0.22(6), A alpha AB), IV (Li similar to C-0.167(6)), and dilute stage lithium Id have been identified and characterized by powder X-ray diffraction and Raman spectroscopy. The rate and extent of intercalation (i.e., the achieved stage of Li-GIC) depends on LiH activity and temperature. Stage I was only observed for temperatures above 500 degrees C. At 400 degrees C, the highest intercalation corresponded to stage IIb, which was obtained after 2 and 24 h for 4:1 and 1:1 reaction mixtures, respectively. Lower-staged Li-GICs attained at temperatures below 500 degrees C deintercalate upon prolonged dwelling with the exception of stage IIa, which can be maintained for very long periods (several days) in the presence of LiH. At temperatures above 500 degrees C, the kinetically controlled formation of Li-GICs is followed by Li2C2 carbide formation. It is shown that the Li-GIC I-d coexists with Li2C2 at temperatures up to 800 degrees C and that the Li content of I-d (solubility of Li in graphite) increases between 550 and 800 degrees C. Consequently, I-d with a temperature-dependent homogeneity range should be added as a stable phase in the Li-C phase diagram. A sketch of a revised Li-C phase diagram is provided.

  • 16.
    Konar, Sumit
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nylén, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bernin, Diana
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ruschewitz, Uwe
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    The many phases of CaC22016In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, p. 204-213Article in journal (Refereed)
    Abstract [en]

    Polymorphic CaC2 was prepared by reacting mixtures of CaH2 and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 degrees C under dynamic vacuum. These conditions provided a well controlled, homogeneous, chemical environment and afforded products with high purity. The products, which were characterized by powder X-ray diffraction, solid state NMR and Raman spectroscopy, represented mixtures of the three known polymorphs, tetragonal CaC2-I and monoclinic CaC2-II and -III. Their proportion is dependent on the nominal C/CaH2 ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC2-I, whereas high temperatures (above 1100 degrees C) and C-deficiency favored the formation of CaC2-I. From first principles calculations it is shown that CaC2-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC2-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC2-delta. It is proposed that monoclinic II is the ground state of CaC2 and polymorph III is stable at temperatures above 200 degrees C. Tetragonal I represents a metastable, heterogeneous, phase of CaC2. It is argued that a complete understanding of the occurrence of three room temperature modifications of CaC2 will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC2-IV, which is stable above 450 degrees C. The effect of high pressure on the stability of the monoclinic forms of CaC2 was studied in a diamond anvil cell using Raman spectroscopy. CaC2-II and -III transform into tetragonal CaC2-I at about 4 and 1GPa, respectively.

  • 17.
    Kranak, Verina F.
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Benson, Daryn E.
    Wollmann, Lukas
    Mesgar, Milad
    Shafeie, Samrand
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydrogenous Zintl Phase Ba3Si4Hx (x=1-2): Transforming Si-4 Butterfly Anions into Tetrahedral Moieties2015In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 3, p. 756-764Article in journal (Refereed)
    Abstract [en]

    The hydride Ba3Si4Hx (x = 1-2) was prepared by sintering the Zintl phase Ba3Si4, which contains Si-4(6-) butterfly-shaped polyanions, in a hydrogen atmosphere at pressures of 10-20 bar and temperatures of around 300 degrees C. Initial structural analysis using powder neutron and X-ray diffraction data suggested that Ba3Si4Hx adopts the Ba3Ge4C2 type [space group I4/mcm (No. 140), a approximate to 8.44 angstrom, c approximate to 11.95 angstrom, Z = 8] where Ba atoms form a three-dimensional array of corner-condensed octahedra, which are centered by H atoms. Tetrahedron-shaped Si-4 polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba3Si4H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba3Si4Hx, which is based on filling pyramidal Ba-5 interstices in Ba3Si4. The limiting composition is x = 2 [space group P4(2)/mmm (No. 136), a approximate to 8.4066 angstrom, c approximate to 12.9186 angstrom, Z = 8], and for x > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba3Si4Hx is heavily disordered in the c direction. Most plausible is to assume that Ba3Si4Hx has a variable H content (x = 1-2) and corresponds to a random intergrowth of P- and I-type structure blocks. In either form, Ba3Si4Hx is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive.

  • 18.
    Kranak, Verina F.
    et al.
    Arizona State University, USA.
    Evans, Michael J.
    Daemen, Luke L.
    Proffen, Thomas
    Lee, Myeong H.
    Sankey, Otto F.
    Häussermann, Ulrich
    Arizona State University, USA.
    Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH2009In: Solid State Sciences, ISSN 1293-2558, E-ISSN 1873-3085, Vol. 11, no 11, p. 1847-1853Article in journal (Refereed)
    Abstract [en]

    The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB2-type precursors SrAlGe and BaAlGe or reacting SrH2 with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.2435(2) and 4.3450(2) Å, c = 4.9710(3) and 5.2130(4) Å for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]2− which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]2− are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al–H stretching and bending mode frequencies at around 1250 and 870 cm−1, respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C. When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.

  • 19.
    Kranak, Verina F.
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lin, Yuan-Chih
    Karlsson, Maths
    Mink, Janos
    Norberg, Stefan T.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Structural and Vibrational Properties of Silyl (SiH3-) Anions in KSiH3 and RbSiH3: New Insight into Si-H Interactions2015In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 5, p. 2300-2309Article in journal (Refereed)
    Abstract [en]

    The alkali metal silyl hydrides ASiH(3) (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH(3) consist of metal cations and pyramidal SiH3 ions. At room temperature SiH3 moieties are randomly oriented (alpha modifications). At temperatures below 200 K ASiH(3) exist as ordered low-temperature (beta) modifications. Structural and vibrational properties of SiH3- in ASiH(3) were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered alpha-ASiH(3) SiH3 ions relate closely to freely rotating moieties with C(3)v symmetry (Si-H bond length = 1.52 angstrom; HSiH angle 92.2 degrees). Observed stretches and bends are at 1909/1903 cm(-1) (nu(1), A(1)), 1883/1872 cm(-1) (nu(3), E), 988/986 cm(-1) (nu(4), E), and 897/894 cm(-1) (nu(2), A(1)) for A = K/Rb. In ordered beta-ASiH(3) silyl anions are slightly distorted with respect to their ideal C-3v symmetry. Compared to a-ASiH(3) the molar volume is by about 15% smaller and the SiH stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3 anions in a-ASiH(3). SiH stretching force constants for SiH3 moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3- remains to be explored.

  • 20. Mink, Janos
    et al.
    Lin, Yuan-Chih
    Karlsson, Maths
    Österberg, Carin
    Udovic, Terrence J.
    Fahlquist, Henrik
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Vibrational properties of -KSiH3 and -RbSiH3: a combined Raman and inelastic neutron scattering study2017In: Journal of Raman Spectroscopy, ISSN 0377-0486, E-ISSN 1097-4555, Vol. 48, no 2, p. 284-291Article in journal (Refereed)
    Abstract [en]

    The hydrogen storage materials ASiH(3) (A=K and Rb) represent complex metal hydrides built from metal cations and pyramidal SiH3- ions. At room temperature, SiH3- moieties are randomly oriented because of dynamical disorder (-modifications). At temperatures below 200K, ASiH(3) exist as ordered low-temperature () modifications. The vibrational properties of -ASiH(3) were characterized by a combination of Raman spectroscopy and inelastic neutron scattering. Internal modes of SiH3- are observed in the spectral range 1800-1900cm(-1) (stretching modes) and 890-1000cm(-1) (bending modes). External modes are observed below 500cm(-1). Specifically, SiH3- librations are between 300-450cm(-1) and 270-400cm(-1) for A=K and Rb, respectively, SiH3- translations are between 95 and 160cm(-1), K+ translations are in the range 60-100cm(-1) and Rb+ translations in the range 50-70cm(-1). The red-shift of libration modes for A=Rb is associated with a 15-30% reduction of the libration force constants of SiH3- ions in -RbSiH3. This correlates with a lower temperature for the - order-disorder phase transition (278 vs 298K). Libration modes become significantly anharmonic with increasing temperature but are maintained up to at least 200K. The vibrational properties of ASiH(3) compare well to those of alkali metal borohydrides ABH(4) (A=Na-Cs).

  • 21.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Andersson, Mikael S.
    Nordblad, Per
    Johansson, Robert
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydrogen Induced Structure and Property Changes in Eu3Si4Manuscript (preprint) (Other academic)
    Abstract [en]

    Hydrides Eu3Si4H2+x were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 °C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25 – 200 ºC afford a uniform hydride phase with an orthorhombic structure (Immm, a ≈ 4.40 Å, b ≈ 3.97 Å, c ≈ 19.8 Å), whereas at 300 ºC mixtures of two orthorhombic phases with c ≈ 19.86 and ≈19.58 Å were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H to be occupied in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns point to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. However, the ordered monoclinic model structures do not provide a good match to the experimental, orthorhombic, PXRD patterns. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x < 2) and are disordered with respect to H in Si2Eu3 interstitials. Hydrides Eu3Si4H2+x decompose at temperatures above 300 °C in a dynamic vacuum into unidentified products. Thus the hydrogenation of Eu3Si4H2+x is not reversible. From magnetic measurements the Curie-Weiss constant and effective magnetic moment of Eu3Si4H2+x were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of ~8 mB which is typical for compounds containing Eu2+ 4f7 ions.  

  • 22.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Benson, Daryn E.
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Spektor, Kristina
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    The 3R polymorph of CaSi22015In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 222, p. 18-24Article in journal (Refereed)
    Abstract [en]

    The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA'BB'CC' fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 degrees C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 degrees C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects.

  • 23.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fischer, Andreas
    Österberg, Carin
    Lin, Yuan-Chih
    Karlsson, Maths
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pell, Andrew J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH32017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 9, p. 5241-5252Article in journal (Refereed)
    Abstract [en]

    The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3 moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3 rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH3, the SiH3 anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). 2H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from Cp measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH3 ions are only weakly coordinated in an environment of A+ cations. The orientational flexibility of SiH3 can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.

  • 24.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bernin, Diana
    Karlsson, Maths
    Eklof-Österberg, Carin
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pell, Andrew J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydride Reduction of BaTiO3 - Oxyhydride Versus O Vacancy Formation2018In: Acs Omega, ISSN 2470-1343, Vol. 3, no 9, p. 11426-11438Article in journal (Refereed)
    Abstract [en]

    We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4, and NaAlH4. The reactions employed molar BaTiO3/H ratios of up to 1.8 and temperatures near 600 degrees C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and H-1 magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products-indicative of the formation of BaTiO3-xHx. except for NaH. Lattice parameters were in a range between 4.005 angstrom (for NaBH4-reduced samples) and 4.033 A (for MgH2-reduced samples). With increasing H/BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger than 0.5. H-1 MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy square((x-y)) with x up to 0.6 and y in a range 0.04-0.25, rather than homogeneous solid solutions BaTiO3-xHx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3-xHy square((x-y)). Sintering of BaTiO3-xHy square((x-y)) in a gaseous H-2 atmosphere resulted in more ordered materials, as indicated by considerably sharper H-1 resonances.

  • 25.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bernin, Diana
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Karlsson, Maths
    Österberg, Carin
    Neagu, Alexandra
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pell, Andrew J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydride Reduction of BaTiO3 – Oxyhydride vs O-Vacancy FormationManuscript (preprint) (Other academic)
    Abstract [en]

    We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4 and NaAlH4. The reactions employed molar BaTiO3:H ratios of up to 1.8 and temperatures near 600 °C. The air stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA) and solid-state 1H NMR spectroscopy. PXRD showed the formation of cubic products - indicative of the formation of BaTiO3-xHx - except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4 reduces samples) and 4.033 Å (for MgH2 reduced samples). With increasing BaTiO3:H ratio, CaH2, NaAlH4 and MgH2 reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increase of up to 3.5 % for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3-xHx with x values larger 0.5. 1H NMR, however, revealed rather low concentrations of H, and, thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3-xHy(x-y) with x up to 0.6, y in a range 0.05 – 0.2 and (x-y) > y, rather than homogeneous solid solutions BaTiO3Hx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the -2 to -60 ppm spectral region. The large range of chemical shifts and breadth of the signals signifies the structural disorder in BaTiO3-xHy(x-y). Sintering of BaTiO3-xHy(x-y) in a gaseous H2 atmosphere resulted in more ordered materials as indicated by considerably sharper 1H resonances.

  • 26.
    Nedumkandathil, Reji
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kranak, Verina F.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johansson, Robert
    Ångström, Jonas
    Balmes, Oliver
    Andersson, Mikael S.
    Nordblad, Per
    Scheicher, Ralph H.
    Sahlberg, Martin
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hydrogenation induced structure and property changes in GdGa2016In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, p. 184-191Article in journal (Refereed)
    Abstract [en]

    Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.

  • 27. Puhakainen, Kati
    et al.
    Benson, Daryn
    Nylén, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Konar, Sumit
    Arizona State University, USA.
    Stoyanov, Emil
    Leinenweber, Kurt
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hypervalent octahedral SiH62- species from high pressure synthesis2012In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 51, no 13, p. 3156-3160Article in journal (Refereed)
  • 28.
    Samain, Louise
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jaworski, Aleksander
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ladd, Danielle M.
    Seo, Dong-Kyun
    Javier Garcia-Garcia, F.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Structural analysis of highly porous gamma-Al2O32014In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 217, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Two highly porous gamma-aluminas, a commercial catalyst obtained from the calcination of boehmite and a highly mesoporous product obtained from amorphous aluminum (oxy)hydroxide via a sol-gel-based process were investigated by Al-27 nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and atomic pair distribution function (PDF) analysis of synchrotron powder diffraction data. NMR data showed for both materials a distribution of tetrahedrally and octahedrally coordinated Al at a 0.30:0.70 ratio, which is typical for gamma-aluminas. TEM studies revealed that rod-shaped particles with about 5 nm in thickness are the building blocks of the porous structure in both materials. These particles often extend to a length of 50 nm in the commercial catalyst and are considerably shorter in the sol-gel-based material, which has a higher surface area. Refinement of PDFs revealed the presence of a similar to 1 nm scale local structure and the validity of a tetragonal average structure for both materials. This tetragonal average structure contains a substantial fraction of non-spinel octahedral Al atoms. It is argued that the presence of local structure is a general feature of gamma-alumina, independent of precursor and synthesis conditions. The concentration of non-spinel Al atoms seems to correlate with surface properties, and increases with increasing pore size/surface area. This should have implications to the catalytic properties of porous gamma-alumina.

  • 29. Spektor, Kristina
    et al.
    Crichton, Wilson A.
    Filippov, Stanislav
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Linköping University, Sweden.
    Simak, Sergei I.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Exploring the Mg-Cr-H System at High Pressure and Temperature via in Situ Synchrotron Diffraction2019In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 58, no 16, p. 11043-11050Article in journal (Refereed)
    Abstract [en]

    The complex transition metal hydride Mg3CrH8 has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H-7](5-). Here, we investigated the formation of Mg3CrH8 by in situ studies of reaction mixtures of 3MgH(2)-Cr-H-2 at 5 GPa. The formation of the known orthorhombic form (o-Mg3CrH8) was noticed at temperatures above 635 degrees C, albeit at a relatively slow rate. At temperatures around 750 degrees C a high temperature phase formed rapidly, which upon slow cooling converted into o-Mg3CrH8. The phase transition at high pressures occurred reversibly at similar to 735 degrees C upon heating and at similar to 675 degrees C upon slow cooling. Upon rapid cooling, a monoclinic polymorph (m-Mg3CrH8) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg3CrH8 was found to crystallize in P2(1)/n space group (a = 5.128 angstrom, b = 16.482 angstrom, c = 4.805 angstrom, beta = 90.27 degrees). Its structure elucidation from high resolution synchrotron powder diffraction data was aided by first-principles DFT calculations. Like the orthorhombic polymorph, m-Mg3CrH8 contains pentagonal bipyramidal complexes [CrH7](5-) and interstitial H-. The arrangement of metal atoms and interstitial H- resembles closely that of the high pressure orthorhombic form of Mg3MnH7. This suggests similar principles of formation and stabilization of hydrido complexes at high pressure and temperature conditions in the Mg-Cr-H and Mg-Mn-H systems. Calculated enthalpy versus pressure relations predict o-Mg3CrH8 being more stable than m-Mg3CrH8 by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg3CrH8 is very similar to that of o-Mg3CrH8. The stable 18-electron complex [CrH7](5-) is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.

  • 30. Spektor, Kristina
    et al.
    Crichton, Wilson A.
    Konar, Sumit
    Filippov, Stanislav
    Klarbring, Johan
    Simak, Sergei I.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Unraveling Hidden Mg-Mn-H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction2018In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 3, p. 1614-1622Article in journal (Refereed)
    Abstract [en]

    The MgMnH system was investigated by in situ high pressure studies of reaction mixtures MgH2MnH2. The formation conditions of two complex hydrides with composition Mg3MnH7 were established. Previously known hexagonal Mg3MnH7 (h-Mg3MnH7) formed at pressures 1.52 GPa and temperatures between 480 and 500 degrees C, whereas an orthorhombic form (o-Mg3MnH7) was obtained at pressures above 5 GPa and temperatures above 600 degrees C. The crystal structures of the polymorphs feature octahedral [Mn(I)H-6](5) complexes and interstitial H-. Interstitial H- is located in trigonal bipyramidal and square pyramidal interstices formed by Mg2+ ions in h- and o-Mg3MnH7, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg3MnH7 (m-Mg3MnH7). The structure elucidation of o- and m-Mg3MnH7 was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg3MnH7 to be more stable than h-Mg3MnH7 above 4.3 GPa. Phonon calculations revealed o-Mg3MnH7 to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg3MnH7 on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg3MnH7 is very similar. The stable 18-electron complex [MnH6](5-) is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of in situ investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides.

  • 31.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fischer, Andreas
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures-Dense Hydrous Aluminosilicates2016In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 16, p. 8048-8058Article in journal (Refereed)
    Abstract [en]

    High-pressure hydrothermal environments can drastically reduce the kinetic constraints of phase transitions and afford high-pressure modifications of oxides at comparatively low temperatures. Under certain circumstances such environments allow access to kinetically favored phases, including hydrous ones with water incorporated as hydroxyl. We studied the crystallization of glass in the presence of a large excess of water in the pressure range of 0.25 10 GPa and at temperatures from 200 to 600 degrees C. The p and T quenched samples were analyzed by powder X-ray diffraction, scanning electron microscopy, and IR spectroscopy. At pressures of 0.25-2 GPa metastable zeolite Li-ABW and stable alpha-eucryptite are obtained at low and high temperatures, respectively, with crystal structures based on tetrahedrally coordinated Al and Si atoms. At 5 GPa a new, hydrous phase of LiAlSiO4, LiAlSiO3(OH)(2) = LiAlSiO4 center dot H2O, is produced. Its crystal structure was characterized from single-crystal X-ray diffraction data (space group P2(1)/c, a = 9.547(3) angstrom, b = 14.461(5) angstrom, c = 5.062(2) angstrom, beta = 104.36(1)). The monoclinic structure resembles that of a-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edge-sharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. At 10 GPa another hydrous phase of LiAlSiO4 with presently unknown structure is produced. The formation of hydrous forms of LiAlSiO4 shows the potential of hydrothermal environments at gigapascal pressures for creating truly new materials. In this particular case it indicates the possibility of generally accessing pyroxene-type aluminosilicates with crystallographic amounts of hydroxyl incorporated. This could also have implications to geosciences by representing a mechanism of water storage and transport in the depths of the Earth.

  • 32.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Crystallization of LiAlSiO4 glass in a high pressure hydrothermal environment – new hydrous phases of eucryptiteManuscript (preprint) (Other academic)
  • 33.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). European Synchrotron Radiation Facility (ESRF), France.
    Nylen, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mathew, Renny
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Stoyanov, Emil
    Navrotsky, Alexandra
    Leinenweber, Kurt
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Formation of hydrous stishovite from coesite in high-pressure hydrothermal environments2016In: American Mineralogist, ISSN 0003-004X, E-ISSN 1945-3027, Vol. 101, no 11, p. 2514-2524Article in journal (Refereed)
    Abstract [en]

    In low-temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO2 source at temperatures of 350-550 degrees C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H2O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 degrees C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H(+) for Si4+ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm(-1), indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm(-1), respectively. Protons of the octahedral hydrogarnet defect produce H-1 MAS NMR signals in the 9-12 ppm region. The presence of multiple resonances suggests that the octahedral defect is associated with various proton arrangements. At elevated temperatures, the NMR signals narrow considerably because of proton dynamics.

  • 34.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry. Arizona State University USA.
    Nylen, Johanna
    Stoyanov, Emil
    Navrotsky, Alexandra
    Hervig, Richard L.
    Leinenweber, Kurt
    Holland, Gregory P.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO22011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 52, p. 20918-20922Article in journal (Refereed)
    Abstract [en]

    Stishovite (SiO2 with the rutile structure and octahedrally coordinated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350–550 °C near 10 GPa produces stishovite with significant amounts of H2O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 ± 0.2 wt % H2O and NMR suggests that the primary mechanism for the H2O uptake is a direct hydrogarnet-like substitution of 4H+ for Si4+, with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO2 + H2O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H2O at convergent plate boundaries, and in other relatively cool high-pressure environments.

  • 35.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nylen, Johanna
    Stoyanov, Emil
    Navrotsky, Alexandra
    Hervig, Richard L.
    Leinenweber, Kurt
    Holland, Gregory P.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO(2)2011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 52, p. 20918-20922Article in journal (Refereed)
    Abstract [en]

    Stishovite (SiO(2) with the rutile structure and octahedrally coordinated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350-550 degrees C near 10 GPa produces stishovite with significant amounts of H(2)O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 +/- 0.2 wt% H(2)O and NMR suggests that the primary mechanism for the H(2)O uptake is a direct hydrogarnet-like substitution of 4H(+) for Si(4+), with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO(2) + H(2)O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H(2)O at convergent plate boundaries, and in other relatively cool high-pressure environments.

  • 36.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Nylén, Johanna
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Mathew, Renny
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Stoyanov, Emil
    Navrotsky, Alexandra
    Leinenweber, Kurt
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Formation of hydrous stishovite from coesite in high pressure hydrothermal environmentsManuscript (preprint) (Other academic)
  • 37.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Tran, Dung Trung
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Leinenweber, Kurt
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Transformation of rutile to TiO2-II in a high pressure hydrothermal environment2013In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 206, p. 209-216Article in journal (Refereed)
    Abstract [en]

    The high pressure transformation of rutile to TiO2-II with the α-PbO2 structure is known to be kinetically hindered. In this study we show that a hydrothermal environment at 6 GPa and 650 °C provides appreciable rates for producing single phase bulk samples of TiO2-II. So obtained TiO2-II was characterized by scanning electron microscopy, powder X-ray diffraction, Raman and Far-IR spectroscopy. The structural properties are identical to TiO2-II from dry transitions. Transmission electron microscopy studies strongly indicate that Ostwald ripening processes play an important role in the hydrothermally assisted transformation and subsequent growth of TiO2-II crystals. TiO2-II is thermally stable to about 550 °C. At 600 °C the onset of the transformation to rutile is observed. The thermal expansion in the temperature range from room temperature to 500 °C is highly anisotropic, virtually affecting only the c unit cell parameter (αc=7.1(2)×10−6 °C−1). The pressure–temperature conditions for the hydrothermally assisted transformation of rutile are viable for industrial production settings, and in light of the large technological significance of TiO2, TiO2-II may present an interesting target for large-scale synthesis.

  • 38.
    Spektor, Kristina
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wan, Wei
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nedumkandathil, Reji
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Andersson, Ove
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Crystallization of mesoporous silica SBA-15 in a high pressure hydrothermal environment2017In: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 37, no 3, p. 345-359Article in journal (Refereed)
    Abstract [en]

    Mesoporous silica SBA-15 (with similar to 6 nm pore size and similar to 6 nm wall thickness) was exposed to a hydrothermal environment at 2 and 5GPa. The p,T quenched products were investigated by powder X-ray diffraction and transmission electron microscopy. Infrared spectroscopy and thermogravimetric analysis of a sample subjected to 5GPa at room temperature suggests functionalization of both inner and outer pore surface by silanol. Partial transformation to nano-sized (20-50 nm) coesite crystals with nonfaceted morphology was observed during short equilibration times of 2h at 125 degrees C, which is significantly below the melting point of water (similar to 250 degrees C). Untransformed SBA-15 maintained intact pore structure. At 175 degrees C and during 8h, SBA-15 transformed completely into faceted coesite crystals with dimensions 100-300 nm, suggesting Ostwald ripening and thus significant mass transport in the solid water environment. At 2GPa the melting point of water is near 70 degrees C. Partial transformation to nano-sized alpha-quartz was observed at 65 degrees C and during 2h. Untransformed SBA-15 partially pore collapsed. The reduced pore stability of SBA-15 at 2GPa is attributed to the presence of liquid water in the pores due to melting point depression of confined water.

  • 39.
    Tengå, Andreas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Garcia-Garcia, Javier
    Lehrstuhl für Festkörperchemie,Institutfür Physik,Universität Augsburg.
    Yang, Wu
    Department of Chemistry and Biochemistry, Arizona State University.
    Newman, Nathan
    Department of Chemistry and Biochemistry, Arizona State University.
    Häussermann, Ulrich
    Department of Chemistry and Biochemistry, Arizona State University.
    Metal–nonmetal transition in the sphalerite-type solid solution [ZnSnSb2]1-x[2(InSb)]x2009In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 182, no 6, p. 1438-1442Article in journal (Refereed)
    Abstract [en]

    Samples of the solid solution [ZnSnSb2]1-x[2(InSb)]x have been prepared over the whole range of composition by tin flux synthesis. The lattice parameter of the sphalerite-type average structure varies linearly between that of the end members ZnSnSb2 and InSb, a = 6.2849(2) and 6.4776(15), respectively. Electron diffraction shows different kinds of structured diffuse scattering for Zn and In rich samples, respectively. The former is attributed to compositional short range ordering, the latter to thermally excited phonon modes. A metal-nonmetal transition takes place between the compositions x = 0.8 and x = 0.9.

  • 40.
    Tengå, Andreas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    García-García, Javier
    Max-Planck Institute for Solid State Research.
    Mikhaylushkin, Arkady S
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Espinosa-Arronte, Beatriz
    Solid State Physics, IMIT, Royal Institute of Technology, KTH.
    Andersson, Magnus
    Solid State Physics, IMIT, Royal Institute of Technology, KTH.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Sphalerite-Chalcopyrite Polymorphism in Semimetallic ZnSnSb22005In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 17, no 24, p. 6080-6085Article in journal (Refereed)
    Abstract [en]

    We have investigated the system ZnSnSb2 in the course of our attempts to modify thermoelectricZn-Sb frameworks. ZnSnSb2 is only accessible when employing Sn as reactive flux in the synthesis.The material shows an order-disorder transition in the temperature interval between 225 and 240 °Cand decomposes peritectically at about 360 °C. The high-temperature form of ZnSnSb2 adopts the Zn/Sndisordered cubic sphalerite-type structure. Electron microscopy investigations reveal that samples quenchedfrom 350 °C already contain domains of the low-temperature form, which has the Zn/Sn ordered tetragonalchalcopyrite structure. The c/a ratio of the tetragonal structure is, within experimental errors, identical tothe ideal value 2. This gives rise to intricate microtwinning in the low-temperature chalcopyrite form ofZnSnSb2 as obtained in samples quenched from 250 °C. First principles electronic structure calculationsdemonstrate that the tetragonal low-temperature form of ZnSnSb2 has a narrow band gap of about 0.2eV. This is in agreement with the semimetallic behavior of the material found from resistivity measurement.The shape of the electronic density of states for ZnSnSb2 is similar to thermoelectric binary Zn-Sbframeworks. However, the thermopower of ZnSnSb2 is rather low with room-temperature values rangingfrom 10 to 30 íV/K.

  • 41.
    Tengå, Andreas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Lidin, Sven
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Belieres, Jean-Philippe
    Department of Chemistry and Biochemistry, Arizona State University.
    Newman, Nathan
    School of Materials, Arizona State University.
    Wu, Yang
    Department of Chemistry and Biochemistry, Arizona State University.
    Häussermann, Ulrich
    Department of Chemistry and Biochemistry, Arizona State University.
    Cd13-xInySb10 (x ≈ 2.7, y ≈ 1.5): An interstitial-free variant of thermoelectric β-Zn4Sb32009In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 15, no 27, p. 6704-6710Article in journal (Refereed)
    Abstract [en]

    Cd13-xInySb10 (x ≈ 2.7, y ≈ 1.5) was synthesized in the form of mm-sized crystals from reaction mixtures containing excess cadmium. The intermetallic compound crystallizes in the rhombohedral space group R-3m with a = 12.9704(4), c = 12.9443(5) Å, V = 1886.0(1), Z = 3 and is isostructural to thermoelectric β-Zn4Sb3 and β-Cd4Sb3. However, in contrast to the latter compounds Cd13-xInySb10 is free from interstitial atoms and does not display any temperature polymorphism. The electrical resistivity of Cd13-xInySb10 is considerably higher than that of Zn4Sb3 and Cd4Sb3 although the temperature behavior remains that of a metal. The thermal conductivity of Cd13-xInySb10 is low, with room temperature magnitudes around 0.8 Wm-1K-1, which is comparable to disordered or complex structured Cd4Sb3 and Zn4Sb3.

  • 42.
    Tengå, Andreas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Lidin, Sven
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Belieres, Jean-Philippe
    Department of Chemistry and Biochemistry, Arizona State University.
    Newman, Nathan
    School of Materials, Arizona State University.
    Wu, Yang
    Department of Chemistry and Biochemistry, Arizona State University.
    Häussermann, Ulrich
    Department of Chemistry and Biochemistry, Arizona State University,.
    Metastable Cd4Sb3: A Complex Structured Intermetallic Compound with Semiconductor Properties2009In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, no 46, p. 15564-15572Article in journal (Refereed)
    Abstract [en]

    The metastable binary intermetallic compound Cd4Sb3 was obtained as polycrystalline ingot by quenching stoichiometric Cd-Sb melts and as mm-sized crystals by employing Bi or Sn fluxes. The compound crystallizes in the monoclinic space group Pn with a = 11.4975(5) Å, b = 26.126(1) Å, c = 26.122(1) Å, b = 100.77(1)o and V = 7708.2(5) Å3. The actual formula unit of Cd4Sb3 is Cd13Sb10 and the unit cell contains 156 Cd and 120 Sb atoms (Z = 12). Cd4Sb3 displays a reversible order-disorder transition at 373 K and decomposes exothermically into a mixture of elemental Cd and CdSb at around 520 K. Disordered β-Cd4Sb3 is rhombohedral (space group R-3c, a ≈ 13.04 Å, c ≈ 13.03 Å) with a framework isostructural to β-Zn4Sb3. The structure of monoclinic α-Cd4Sb3 bears resemblance to the low-temperature modifications of Zn4Sb3, α- and α’-Zn4Sb3, in that randomly distributed vacancies and interstitial atoms of the high-temperature modification aggregate and order into distinct arrays. However, the nature of aggregation and distribution of aggregates is different in the two systems. Cd4Sb3 displays the properties of a narrow gap semiconductor. Between 10 and 350 K the resistivity of melt-quenched samples first increases with increasing temperature until a maximum value at 250 K and then decreases again. The resistivity maximum is accompanied with a discontinuity in the thermopower, which is positive and increasing from 10 to 350 K. The room temperature values of the resistivity and thermopower are about 25 mWcm and 160 mV/K, respectively. Flux synthesized samples show altered properties due to the incorporation of small amounts of Bi or Sn (less than 1 at. %). Thermopower and resistivity appear drastically increased for Sn doped samples. Characteristic for Cd4Sb3 samples is their low thermal conductivity, which drops below 1 W/mK above 130 K and attains values around 0.75 W/mK at room temperature, which is comparable to vitreous materials.

  • 43.
    Wu, Yang
    et al.
    Department of Chemistry and Biochemistry, Arizona State University.
    Lidin, Sven
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Tengå, Andreas
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Newman, Nathan
    School of Materials, Arizona State University.
    Häussermann, Ulrich
    Department of Chemistry and Biochemistry, Arizona State University.
    Phase relations and structural properties of the ternary narrow gap semiconductors Zn5Sb4In2-δ (δ = 0.15) and Zn9Sb6In22010In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 183, no 7, p. 1574-1581Article in journal (Refereed)
    Abstract [en]

    A systematic study of the Zn-rich corner of the ternary system Zn-Sb-In revealed the presence of two ternary compounds, stable Zn5Sb4In2-δ(δ = 0.15) and metastable Zn9Sb6In2, with closely related crystal structures. Their common motif is a tetragonal basic structure of 32434 nets formed by the Sb atoms that are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns which produces frameworks with a composition “ZnSb”. These frameworks represent orthorhombic superstructures, (2×1×1) for Zn5Sb4In2-δ (Z = 4) and (2×3×1) for Zn9Sb6In2 (Z = 8), with respect to the tetragonal arrangement of Sb atoms. The In and remaining Zn atoms are distributed in the channels formed by the square antiprisms. Phase relations in the Zn-Sb-In system are complex. Crystals of metastable Zn9Sb6In2 are regularly intergrown with various amounts of Zn5Sb4In2-δ. Additionally, a monoclinic variant to orthorhombic Zn9Sb6In2 could be identified. Zn9Sb6In2 decomposes exothermically into a mixture of Zn5Sb4In2-δ, Zn4Sb3 and elemental Zn at around 480 K. Both Zn5Sb4In2-δ and Zn9Sb6In2 are poor metals with resistivity values that are characteristic of heavily doped or degenerate semiconductors (0.2 to 3 mW cm at room temperature).

  • 44. Zeilinger, Michael
    et al.
    Baran, Volodymyr
    van Wuellen, Leo
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Faessler, Thomas F.
    Stabilizing the Phase Li15Si4 through Lithium-Aluminum Substitution in Li15-xAlxSi4 (0.4 <= x <= 0.8)-Single Crystal X-ray Structure Determination of Li15Si4 and Li14.37Al0.63Si42013In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 25, no 20, p. 4113-4121Article in journal (Refereed)
    Abstract [en]

    Single crystals of Li15Si4 and Li15-xAlxSi4 (x = 0.63(1)) were obtained from equilibrated melts with compositions Li100-xSix (x = 10, 15) and Li83Al13Si4, respectively, and isolated by isothermal centrifugation. Li15Si4 and Li14.37(1)Al0.63(1)Si4 crystallize with the Cu15Si4 structure type (I (4) over bar 3d, a(x=0) = 10.6322(9) angstrom, a(x=0.63(1)) = 10.6172(4) angstrom, Z = 4, T = 123 K). The incorporation of Al equally affects both crystallographically distinguished Li positions in the Li15Si4 structure. The replacement of about 4% of Li is firmly established by the refinement of single crystal diffraction data and NMR spectroscopy. The homogeneity range of Li15-xAlxSi4 was assessed as 0.4 < x < 0.8 from synthesis experiments using stoichiometric proportions of the elements. Differential scanning calorimetry studies confirm the metastable character of Li15Si4, decomposing exothermally at temperatures around 200 degrees C. However, the decomposition process of Li15Si4, is sluggish and appreciable rates are not observed before temperatures reach 400 degrees C. In contrast Li15-xAlxSi4 is thermodynamically stable. The decomposition temperature is at about 700 degrees C. It is speculated that the thermodynamic stability of Li15-xAlxSi4 is a consequence of the increased electron concentration, shifting the Fermi level to a pseudo-gap in the electronic density of states. Since metastable Li15Si4 plays an important role during electrochemical lithiation of a silicon anode, thermodynamically stable Li15-xAlxSi4 may have interesting properties as anode material in lithium ion batteries.

  • 45. Zeilinger, Michael
    et al.
    Jantke, Laura-Alice
    Scherf, Lavinia M.
    Kiefer, Florian J.
    Neubueser, Gero
    Kienle, Lorenz
    Karttunen, Antti J.
    Konar, Sumit
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Faessler, Thomas F.
    Alkali Metals Extraction Reactions with the Silicides Li15Si4 and Li3NaSi6: Amorphous Si versus allo-Si2014In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 26, no 22, p. 6603-6612Article in journal (Refereed)
    Abstract [en]

    The silicides Li15Si4 and Li3NaSi6 were subjected to chemical extraction of the alkali metal component by liquid ammonia and ethanol, respectively, which after washing yielded black powders of amorphous silicon. The investigated reactions are interesting with respect to both the formation of novel Si modifications and the delithiation process in Si anode materials. The products termed a-Si (from Li15Si4) and a-allo-Si (from Li3NaSi6) were characterized by a combination of methodologies (X-ray diffraction, transmission electron microscopy, differential thermal analysis, Raman, atomic absorption, and energy-dispersive X-ray spectroscopy) which revealed (i) a porous microstructure for a-Si built from spherically shaped particles with sizes around 10 nm, (ii) partial surface oxidation of both materials and (iii) the presence of nanocrystalline Si in both materials. The result of the protic oxidation of Li3NaSi6 is at variance with earlier findings reporting the formation of a crystalline bulk allotrope of silicon (allo-Si) from the topotactic combination of silicon layers present as polyanions in Li3NaSi6. Additionally, quantum chemical calculations show that silicon layers in Li3NaSi6 cannot combine to energetically favorable allotropic forms of Si. This is different from Li7Ge12, where polyanionic germanium layers topotactically convert to the germanium allotrope m-allo-Ge upon oxidation.

  • 46. Zeilinger, Michael
    et al.
    Kurylyshyn, Iryna M.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Faessler, Thomas F.
    Revision of the Li-Si Phase Diagram: Discovery and Single-Crystal X-ray Structure Determination of the High-Temperature Phase Li4.11Si2013In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 25, no 22, p. 4623-4632Article in journal (Refereed)
    Abstract [en]

    Silicon has been regarded as a promising anode material for future lithium-ion batteries, and Li-Si phases play an important role. A detailed reinvestigation of the Li-rich part of the binary Li-Si phase diagram revealed the existence of a new phase, Li4.106(2)Si (Li16.42Si4). Li16.42Si4 forms through the peritectic decomposition of the Li-richest phase Li17Si4 at 481-486 degrees C and was characterized by single-crystal X-ray diffraction (a = 4.5246(2) angstrom, b = 21.944(1) angstrom, c = 13.2001(6) angstrom, space group Cmcm, Z = 16), differential scanning calorimetry, and theoretical calculations. Li16.42Si4 represents a high-temperature phase that is thermodynamically stable above similar to 480 degrees C and decomposes peritectically at 618 +/- 2 degrees C to Li13Si4 and a melt. Li16.42Si4 can be retained at room temperature. The structure consists of 3 and 10 different kinds of Si and Li atoms, respectively. Two Li positions show occupational disorder. Si atoms are well-separated from each other and have only Li atoms as nearest neighbors. This is similar to Li17Si4 and Li15Si4 compositionally embracing Li16.42Si4. The SiLin coordination polyhedra in the series Li15Si4, Li16.42Si4, and Li17Si4 are compared. Li15Si4 exclusively features coordination numbers of 12, Li16.42Si4 of 12 and 13, and Li17Si4 reveals 13- and 14-coordinated Si atoms. The band structure and density of states of Li16.42Si4 were calculated on the basis of two ordered model structures with nominal compositions Li16Si4 (a hypothetical Zintl phase) and Li16.5Si4. Both reveal a metallic character that is analogous to Li17Si4. In contrast, the electronic structure of Li15Si4 is characteristic of a p-doped semiconductor.

  • 47. Zeilinger, Michael
    et al.
    van Wüllen, Leo
    Benson, Daryn
    Kranak, Verina F.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Konar, Sumit
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fässler, Thomas F.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    LiBSi2: A Tetrahedral Semiconductor Framework from Boron and Silicon Atoms Bearing Lithium Atoms in the Channels2013In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 52, no 23, p. 5978-5982Article in journal (Refereed)
  • 48.
    Ångström, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jenei, István Zoltán
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Spektor, Kristina
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Formation of Hydrous, Pyroxene-Related Phases from LiAlSiO4 Glass in High-Pressure Hydrothermal Environments2019In: ACS Earth and Space Chemistry, ISSN 2472-3452, Vol. 3, no 1, p. 8-16Article in journal (Refereed)
    Abstract [en]

    Hydrous Al-bearing pyroxene-related phases were synthesized by subjecting LiAlSiO4 glass to hydrothermal environments at pressures of 5-10 GPa and temperatures of 400-600 degrees C. LiAlSiO3(OH)(2) formed at 5 GPa, whereas at 10 GPa, product mixtures of LiAlSiO3(OH)(2) and Li3Al4(Si2O7)(SiO3)(2)(OH)(5) were obtained. The monoclinic structure of LiAlSiO3(OH)(2) has been previously characterized from single-crystal X-ray diffraction data (Spektor, K.; Fischer, A.; Haussermann, U. Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures-Dense Hydrous Aluminosilicates. Inorg. Chem. 2016, 55 (16), 8048-8058, 10.1021/acs.inorgchem.6b01181). It resembles that of alpha-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edge-sharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. The structure solution of Li3Al4(Si2O7)(SiO3)(2)(OH)(5), as detailed here, was achieved by rotational electron diffraction analysis, and the model was refined against synchrotron powder X-ray diffraction data (space group C2/c, a = 4.921 angstrom, b = 25.849 angstrom, c = 9.170 angstrom, and beta = 99.42 degrees). The crystal structure of Li3Al4(Si2O7)(SiO3)(2)(OH)(5) features chains and pairs of corner-condensed SiO4 tetrahedra, with the Si atoms equally distributed among the two structural units, and thus Li3Al4(Si2O7)(SiO3)(2)(OH)(5) is a rare example of a mixed inosorosilicate. LiAlSiO3(OH)(2) and Li3Al4(Si2O7)(SiO3)(2)(OH)(5) are structurally closely related to recently discovered hydrous magnesium aluminosilicate phases (i.e., HAPY and HySo), which form at conditions similar to the hydrous lithium aluminosilicates. The conjecture is made that hydrothermal environments following chlorite but also lawsonite breakdown generally afford conditions for the formation of hydrous, pyroxene-related, aluminosilicate phases, with compositions of M2(1-m)M1TO(3+n)(OH)(2-o) (0 < m, n, and o < 1). These phases could be transients in breakdown reactions but also stable at cold slab conditions and, thus, may play an important role to water storage and transport to the transition zone.

  • 49. Ångström, Jonas
    et al.
    Johansson, Robert
    Sarkar, Tapati
    Sörby, Magnus H.
    Zlotea, Claudia
    Andersson, Mikael S.
    Nordblad, Per
    Scheicher, Ralph H.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Sahlberg, Martin
    Hydrogenation-Induced Structure and Property Changes in the Rare-Earth Metal Gallide NdGa: Evolution of a [GaH](2-) Polyanion Containing Peierls-like Ga-H Chains2016In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 1, p. 345-352Article in journal (Refereed)
    Abstract [en]

    The hydride NdGaH1+x (x approximate to 0.66) and its deuterized analogue were obtained by sintering the Zintl phase NdGa with the CrB structure in a hydrogen atmosphere at pressures of 10-20 bar and temperatures near 300 degrees C. The system NdGa/NdGaH1+x exhibits reversible H storage capability. H uptake and release were investigated by kinetic absorption measurements and thermal desorption mass spectroscopy, which showed a maximum H concentration corresponding to NdGaH2 (0.93 wt % H) and a two-step desorption process, respectively. The crystal structure of NdGaH1+x was characterized by neutron diffraction (P2(1)/m, a = 4.1103(7), b = 4.1662(7), c = 6.464(1) angstrom, beta = 108.61(1)degrees Z = 2). H incorporates in NdGa by occupying two distinct positions, H1 and H2. HI is coordinated in a tetrahedral fashion by Nd atoms. The H2 position displays flexible occupancy, and H2 atoms attain a trigonal bipyramidal coordination by centering a triangle of Nd atoms and bridging two Ga atoms. The phase stability and electronic structure of NdGaH1+x, were analyzed by first-principles DFT calculations. NdGaH1H2 (NdGaH2) may be expressed as Nd3+(H1(-)[GaH2](2-). The two-dimensional polyanion [GaH](2-) features linear -H-Ga-H-Ga- chains with alternating short (1.8 A) and long (2.4 angstrom) Ga-H distances, which resembles a Peierls distortion. H2 deficiency (x < 1) results in the fragmentation of chains. For x = 0.66 arrangements with five-atom moieties, Ga-H-Ga-H-Ga are energetically most favorable. From magnetic measurements, the Curie-Weiss constant and effective magnetic moment of NdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, and the latter attains a value of similar to 3.6 mu(B), which is typical for compounds containing Nd3 ions.

  • 50. Österberg, Carin
    et al.
    Fahlquist, Henrik
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Brown, Craig M.
    Udovic, Terrence J.
    Karlsson, Maths
    Dynamics of Pyramidal SiH3- Ions in ASiH(3) (A = K and Rb) Investigated with Quasielastic Neutron Scattering2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 12, p. 6369-6376Article in journal (Refereed)
    Abstract [en]

    The two alkali silanides ASiH(3) (A = K and Rb) were investigated by means of quasielastic neutron scattering, both below and above the order-disorder phase transition occurring at around 275-300 K. Measurements upon heating show that there is a large change in the dynamics on going through the phase transition, whereas measurements upon cooling reveal a strong hysteresis due to undercooling of the disordered phase. The results show that the dynamics is associated with rotational diffusion of SiH3- anions, adequately modeled by H-jumps among 24 different jump locations radially distributed around the Si atom. The average relaxation time between successive jumps is of the order of subpicoseconds and exhibits a weak temperature dependence with a small difference in activation energy between the two materials, 39(1) meV for KSiH3 and 33(1) meV for RbSiH3. The pronounced SiH3- dynamics explains the high entropy observed in the disordered phase resulting in the low entropy variation for hydrogen absorption/desorption and hence the origin of these materials' favorable hydrogen storage properties.

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