From crystalline tetrahydrofuran clathrate hydrate, THF–CH (THF·17H₂O, cubic structure II), three distinct polyamorphs can be derived. First, THF–CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF–CH and liquid THF·17H₂O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water–water correlations (less dense local water structure) and guest–water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest–host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H₂O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H₂O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H₂O. The hydration structure of THF in RA constitutes ∼18 H₂O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.

This thesis summarizes a study on the pressure-induced amorphization (PIA) and structures of amorphous states of clathrate hydrates (CHs).

PIA involves the transition of a crystalline material into an amorphous solid in response of mechanical compression at temperatures well below the melting point. The first material observed to undergo PIA was hexagonal ice. More recently it was shown that compounds of water undergo the same phenomenon without decomposition, despite the presence of solutes. CHs, which are crystalline inclusion compounds consisting of water molecules encaging small guest species, undergo PIA at ca. 1–4 GPa below 145 K. The obtained amorphous CH phase can be further densified on isobaric heating at high pressure. This annealing step enables to retain an amorphous material on pressure release. There has been a significant amount of studies into the understanding of the nature of PIA and transformations between amorphous phases of pure ice. The aim of this thesis has been the understanding of the PIA in CHs and its relation to pure ice. New information on the nature of PIA and subsequent amorphous-amorphous transitions in CH systems were gained from structural studies and in situ neutron diffraction played pivotal role due to the sensitivity of neutrons to the light element hydrogen. Here a generalized understanding of the PIA in CHs and a clear image of amorphous CH structures are presented.

Denna avhandling sammanfattar en studie om tryckinducerad amorfisering (TIA) av klatrathydrater (KH), samt strukturer av amorfa tillstånd där av.

TIA är övergången av ett kristallint material till ett amorft fast ämne som svar på mekanisk kompression vid temperaturer långt under smältpunkten. Det första materialet som observerades genomgå TIA var hexagonal is. Därefter har det visat sig att det finns strukturer av vatten som trots närvaron av lösta ämnen genomgår samma fenomen utan att strukturen bryts ned. KH:er är kristallina inneslutningskomplex som består av ett gitter av vattenmolekyler, vilka omsluter små gästmolekyler. Dessa strukturer genomgår TIA vid ca. 1–4 GPa vid temperaturer under 145 K. Den erhållna amorfa KH-fasen kan förtätas ytterligare vid isobarisk uppvärmning under högt tryck. Detta steg gör det möjligt att behålla ett amorft material vid tryckavlastning. Det har gjorts en betydande mängd studier av TIA:s natur och omvandlingar mellan amorfa faser av ren is. Syftet med denna avhandling har varit att förstå TIA i KH:er och dess relation till ren is. Ny information om karaktären hos TIA och efterföljande amorfa-amorfa övergångar i KH-system erhölls från strukturella studier, där in situ neutrondiffraktion spelade en avgörande roll tack vare neutronernas känslighet för det lätta elementet väte. Utifrån detta arbete presenteras här en generaliserad förståelse av TIA i KH samt en tydlig bild av amorfa KH-strukturer.

Type II clathrate hydrates (CHs) with tetrahydrofuran (THF), cyclobutanone (CB) or 1,3-dioxolane (DXL) guest molecules collapse to an amorphous state near 1 GPa on pressurization below 140 K. On subsequent heating in the 0.2–0.7 GPa range, thermal conductivity and heat capacity results of the homogeneous amorphous solid show two glass transitions, first a thermally weak glass transition, GT1, near 130 K; thereafter a thermally strong glass transition, GT2, which implies a transformation to an ultraviscous liquid on heating. Here we compare the GTs of normal and deuterated samples and samples with different guest molecules. The results show that GT1 and GT2 are unaffected by deuteration of the THF guest and exchange of THF with CB or DXL, whereas the glass transition temperatures (T_gs) shift to higher temperatures on deuteration of water; T_g of GT2 increases by 2.5 K. These results imply that both GTs are associated with the water network. This is corroborated by the fact that GT2 is detected only in the state which is the amorphized CH's counterpart of expanded high density amorphous ice. The results suggest a rare transition sequence of an orientational glass transition followed by a glass to liquid transition, i.e., kinetic unfreezing of H₂O reorientational and translational mobility in two distinct processes.

The high pressure structural behavior of H₂ and Ne clathrate hydrates with approximate composition H₂/Ne·~4H₂O and featuring cubic structure II (CS-II) was investigated by neutron powder diffraction using the deuterated analogues at ~95 K. CS-II hydrogen hydrate transforms gradually to isocompositional C₁ phase (filled ice II) at around 1.1 GPa but may be metastably retained up to 2.2 GPa. Above 3 GPa a gradual decomposition into C₂ phase (H₂·H₂O, filled ice I_c) and ice VIII’ takes place. Upon heating to 200 K the CS-II to C₁ transition completes instantly whereas C₁ decomposition appears sluggish also at 200 K. C₁ was observed metastably up to 8 GPa. At 95 K C₁ and C₂ hydrogen hydrate can be retained below 1 GPa and yield ice II and ice I_c, respectively, upon complete release of pressure. In contrast, CS-II neon hydrate undergoes pressure-induced amorphization at 1.9 GPa, thus following the general trend for noble gas clathrate hydrates. Upon heating to 200 K amorphous Ne hydrate crystallizes as a mixture of previously unreported C₂ hydrate and ice VIII’.

An abundant source of CH₄ can be found in natural hydrate deposits. Recent demonstration of CH₄ recovery from hydrates via CO₂ exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH₄, CO₂, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO₂. Molecular vibrations are examined in CH₄, CO₂, and mixed CH₄-CO₂ hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH₄, as it is extremely sensitive to ¹H. The presence of CO₂ in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH₄ in mixed CH₄-CO₂ hydrate at 5 K. At 190 K, a large broadening of the CH₄ librational modes indicates disorder in the structure leading to dissociation.

The antiferromagnetic transition metal oxyhydride SrVO₂H is distinguished by its stoichiometric composition and an ordered arrangement of H atoms. The tetragonal structure is related to the cubic perovskite and consists of alternating layers of VO₂ and SrH. d² V(III) attains a sixfold coordination by four O and two H atoms. The latter are arranged in a trans fashion, which produces H–V–H chains along the tetragonal axis. Here, we investigate the vibrational properties of SrVO₂H by inelastic neutron scattering and infrared spectroscopy combined with phonon calculations based on density functional theory. The H-based vibrational modes divide into a degenerate bending motion perpendicular to the H–V–H chain direction and a highly dispersed stretching motion along the H–V–H chain direction. The bending motion, with a vibrational frequency of approximately 800 cm⁻¹, is split into two components separated by about 50 cm⁻¹, owing to the doubled unit cell from the antiferromagnetic structure. Interestingly, spin-phonon coupling stiffens the H-based modes by 50−100cm⁻¹ although super-exchange coupling via H is very small. Frequency shifts of the same order of magnitude also occur for V–O modes. It is inferred that SrVO₂H displays the hitherto largest recognized coupling between magnetism and phonons in a material.

The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates Ar center dot 6.5 H2O and Xe center dot 7.2 H2O featuring cubic structures II and I, respectively, was investigated by neutron powder diffraction (using the deuterated analogues) at 95 K. Both hydrates undergo pressure-induced amorphization (PIA), indicated by the disappearance of Bragg diffraction peaks, but at rather different pressures, at 1.4 and above 4.0 GPa, respectively. Amorphous Ar hydrate can be recovered to ambient pressure when annealed at >1.5 GPa and 170 K and is thermally stable up to 120 K. In contrast, it was impossible to retain amorphous Xe hydrate at pressures below 3 GPa. Molecular dynamics (MD) simulations were used to obtain general insight into PIA of Ng hydrates, from Ne to Xe. Without a guest species, both cubic clathrate structures amorphize at 1.2 GPa, which is very similar to hexagonal ice. Filling of large-sized H cages does not provide stability toward amorphization for structure II, whereas filled small-sized dodecahedral D cages shift PIA successively to higher pressures with increasing size of the Ng guest. For structure I, filling of both kinds of cages, large-sized T and small-sized D, acts to stabilize in a cooperative fashion. Xe hydrate represents a special case. In MD, disordering of the guest hydration structure is already seen at around 2.5 GPa. However, the different coordination numbers of the two types of guests in the crystalline cage structure are preserved, and the state is shown to produce a Bragg diffraction pattern. The experimentally observed diffraction up to 4 GPa is attributed to this semicrystalline state.

Three amorphous forms of Ar hydrate were produced using the crystalline clathrate hydrate Ar·6.5H₂O (structure II, Fdm, a ≈ 17.1 Å) as a precursor and structurally characterized by a combination of isotope substitution (³⁶Ar) neutron diffraction and molecular dynamics (MD) simulations. The first form followed from the pressure-induced amorphization of the precursor at 1.5 GPa at 95 K and the second from isobaric annealing at 2 GPa and subsequent cooling back to 95 K. In analogy to amorphous ice, these amorphs are termed high-density amorphous (HDA) and very-high-density amorphous (VHDA), respectively. The third amorph (recovered amorphous, RA) was obtained when recovering VHDA to ambient pressure (at 95 K). The three amorphs have distinctly different structures. In HDA the distinction of the original two crystallographically different Ar guests is maintained as differently dense Ar–water hydration structures, which expresses itself in a split first diffraction peak in the neutron structure factor function. Relaxation of the local water structure during annealing produces a homogeneous hydration environment around Ar, which is accompanied with a densification by about 3%. Upon pressure release the homogeneous amorphous structure undergoes expansion by about 21%. Both VHDA and RA can be considered frozen solutions of immiscible Ar and water in which in average 15 and 11 water molecules, respectively, coordinate Ar out to 4 Å. The local water structures of HDA and VHDA Ar hydrates show some analogy to those of the corresponding amorphous ices, featuring H₂O molecules in 5- and 6-fold coordination with neighboring molecules. However, they are considerably less dense. Most similarity is seen between RA and low density amorphous ice (LDA), which both feature strictly 4-coordinated H₂O networks. It is inferred that, depending on the kind of clathrate structure and occupancy of cages, amorphous states produced from clathrate hydrates display variable local water structures.

Clathrate hydrates with the cubic structure II (CS-II) form typically with large guest molecules, such as tetrahydrofuran, trimethylamine oxide, or propane. However, CS-II is also realized for argon hydrate despite the comparatively small van der Waals diameter of the guest (around 3.8 angstrom). Here, the structure of deuterated argon hydrate was studied at ambient pressure in the temperature range 20-95 K using neutron diffraction and comparing natural Ar with Ar-36, which scatters neutrons more than 13 times more efficiently. The procedure allowed to unambiguously establish the positional disorder within the large cages of CS-H, while simultaneously refining host and guest structures. These cages are singly occupied and off-centered argon atoms distribute on two tetrahedron-shaped split positions with a ratio 3:1. Molecular dynamics (MD) simulations revealed that the crystallographic positional disorder structure is due to mobile argon atoms even at 20 K. The MD potential energy distribution confirmed the diffraction model. It is noted that the unit cell volumes of argon hydrate in the investigated temperature range are virtually identical to N-2 hydrate, which has a similar composition at ambient pressure, indicating a very similar (slightly attractive) host-guest interaction.

Layered zinc hydroxides (LZHs) with the general formula (Zn2+)(x)(OH-)(2x-my ),(A(m-))(y)center dot nH(2)O (A(m-) = Cl- , NO3- , ac(-) , SO42-, etc) are considered as useful precursors for the fabrication of functional ZnO nanostructures. Here, we report the synthesis and structure characterization of the hitherto unknown binary representative of the LZH compound family, Zn-5(OH)(10)center dot 2H(2)O, with A(m-) = OH- , x = 5, y = 2, and n = 2. Zn-5(OH)(10)center dot 2H(2)O was afforded quantitatively by pressurizing mixtures of epsilon-Zn(OH)(2) (wulfingite) and water to 1-2 GPa and applying slightly elevated temperatures, 100-200 degrees C. The monoclinic crystal structure was characterized from powder X-ray diffraction data (space group C2/c, a = 15.342(7) angstrom, b = 6.244(6) angstrom, c = 10.989(7) angstrom, beta = 100.86(1)degrees). It features neutral zinc hydroxide layers, composed of octahedrally and tetrahedrally coordinated Zn ions with a 3:2 ratio, in which H2O is intercalated. The interlayer d(200) distance is 7.53 angstrom. The H-bond structure of Zn-5(OH)(10)center dot 2H(2)O was analyzed by a combination of infrared/Raman spectroscopy, computational modeling, and neutron powder diffraction. Interlayer H2O molecules are strongly H-bonded to five surrounding OH groups and appear orientationally disordered. The decomposition of Zn-5(OH)(10)center dot 2H(2)O, which occurs thermally between 70 and 100 degrees C, was followed in an in situ transmission electron microscopy study and ex situ annealing experiments. It yields initially 5-15 nm sized hexagonal w-ZnO crystals, which, depending on the conditions, may intergrow to several hundred nm-large two-dimensional, flakelike crystals within the boundary of original Zn-5(OH)(10)center dot 2H(2)O particles.