Perovskite-type oxhydrides such as BaTiO_3−xH_y exhibit mixed hydride ion and electron conduction and are an attractive class of materials for developing energy storage devices. However, the underlying mechanism of electric conductivity and its relation to the composition of the material remains unclear. Here we report detailed insights into the hydride local environment, the electronic structure and hydride conduction dynamics of barium titanium oxyhydride. We demonstrate that DFT-assisted solid-state NMR is an excellent tool for differentiating between the different feasible electronic structures in these solids. Our results indicate that upon reduction of BaTiO₃ the introduced electrons are delocalized among all Ti atoms forming a bandstate. Furthermore, each vacated anion site is reoccupied by at most a single hydride, or else remains vacant. This single occupied bandstate structure persists at different hydrogen concentrations (y = 0.13–0.31) and a wide range of temperatures (∼100–300 K).

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.

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.

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.

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.

This PhD thesis presents investigations of hydrogen incorporation in Zintl phases and transition metal oxides. Hydrogenous Zintl phases can serve as important model systems for fundamental studies of hydrogen-metal interactions, while at the same time hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science. Hydrogen incorporation in transition metal oxides leads to oxyhydride systems in which O and H together form an anionic substructure. The H species in transition metal oxides may be highly mobile, making these materials interesting precursors toward other mixed anion systems. 

Zintl phases consist of an active metal, M (alkali, alkaline earth or rare earth) and a more electronegative p-block metal or semimetal component, E (Al, Ga, Si, Ge, etc.). When Zintl phases react with hydrogen, they can either form polyanionic hydrides or interstitial hydrides, undergo full hydrogenations to complex hydrides, or oxidative decomposition to more E-rich Zintl phases. The Zintl phases investigated here comprised the CaSi₂, Eu₃Si₄, ASi (A= K, Rb) and GdGa systems which were hydrogenated at various temperature, H₂ pressure, and dwelling time conditions. For CaSi₂, a regular phase transition from the conventional 6R to the rare 3R took place and no hydride formation was observed. In contrast, GdGa and Eu₃Si₄ were very susceptible to hydrogen uptake. Already at temperatures below 100 ºC the formation of hydrides GdGaH_2-x and Eu₃Si₄H_2+x was observed. The magnetic properties of the hydrides (antiferromagnetic) differ radically from that of the Zintl phase precursor (ferromagnetic). Upon hydrogenating ASi at temperatures around 100 ^oC, silanides ASiH₃ formed which contain discrete complex ion units SiH₃^-. The much complicated β – α order-disorder phase transition in ASiH₃ was evaluated with neutron powder diffraction (NPD), ²H NMR and heat capacity measurements. 

A systematic study of the hydride reduction of BaTiO₃ leading to perovskite oxyhydrides BaTiO_3-xH_x was done. A broad range of reducing agents including NaH, MgH₂, CaH₂, LiAlH₄ and NaBH₄ was employed and temperature and dwelling conditions for hydride reduction examined. Samples were characterized by X-ray powder diffraction (XRPD), thermal gravimetric analysis and ¹H NMR. The concentration of H that can be incorporated in BaTiO_3-xH_x was found to be very low, which is in contrast with earlier reports. Instead hydride reduction leads to a high concentration of O vacancies in the reduced BaTiO₃. The highly O-deficient, disordered, phases - BaTiO_3-xH_y□_(x-y) with x up to 0.6 and y in a range 0.05 – 0.2 and (x-y) > y – are cubic and may represent interesting materials with respect to electron and ion transport as well as catalysis.

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 (C_p), 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 C_p(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 C_p 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.

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.

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.

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 BaTiO_3-xH_x - 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 BaTiO_3-xH_x 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 BaTiO_3-xH_y□_(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 BaTiO₃H_x. 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 BaTiO_3-xH_y□_(x-y). Sintering of BaTiO_3-xH_y□_(x-y) in a gaseous H₂ atmosphere resulted in more ordered materials as indicated by considerably sharper ¹H resonances.