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Clathrate hydrates are crystalline compounds in which guest molecules are encaged within an ice-like lattice. They occur naturally and possess properties of significant interest for energy and storage applications. Here, we report the thermal conductivity κ of structure I CO₂ clathrate hydrate across a broad temperature range (90–265 K) and at pressures up to 1.2 GPa. Similar to structure II clathrate hydrates, κ decreases with decreasing temperature, displaying almost identical temperature dependence. However, the absolute values are 10–30% lower. Notably, κ of CO₂ clathrate hydrate is among the lowest observed for structure I clathrate hydrates, with κ = (426 ± 8) mW m^–1 K^–1 under stable conditions at 270 K and 1 MPa. Furthermore, the isothermal dependencies of κ on density ρ and pressure p─parameters crucial for thermal modeling at elevated pressures─are relatively weak, with (d ln κ/d ln ρ) = 1.2 ± 0.2 and (d ln κ/dp) = (12 ± 1) % GPa^–1 . The measurements show significantly lower κ values and a different temperature dependence compared with previously reported simulation results. Nevertheless, the experimental data confirm the simulation prediction that κ for CO₂ clathrate hydrate is significantly lower than for other structure I clathrates. Our findings further indicate that κ in both structures I and II clathrate hydrates tends to decrease with increasing van der Waals radius of the guest molecules, as reviewed here. This trend may arise from enhanced distortion and anharmonicity within the ice framework. We tentatively propose that the pronounced anharmonicity of the clathrate hydrate lattice leads to frequent phonon–phonon scattering, effectively suppressing phonon-mediated heat transport and resulting in predominantly diffusive thermal conduction.

The thermal conductivity κ of solid CO2 was studied in the temperature T range of 100–220 K and at pressures up to 200 MPa using the transient hot-wire method. The results are consistent with those expected for a polycrystal composed of small molecules, with κ increasing significantly as the temperature decreases and as pressure and density increase. The variation in κ with temperature is primarily attributed to changes in phonon–phonon scattering and density. The thermal conductivity behaviour is described using a two-basis model, where heat is transported by both phonons and diffuse modes. The density ρ dependence of the thermal conductivity, represented by the Bridgman parameter g = (d ln κ/d ln ρ)T, was found to be g = 6.7 at 190 K, increasing to 9.4 at 110 K as the temperature decreases. This increase is attributed to an enhanced phonon contribution to the total κ.

The thermal conductivity κ of cyclopentane clathrate hydrate (CP CH) of type II was measured at temperatures down to 100 K and at pressures up to 1.3 GPa. The results show that CP CH displays amorphous-like κ characteristic of many crystalline clathrate hydrates, e.g., tetrahydrofuran (THF) CH. The magnitude of κ is 0.47 W m⁻¹ K⁻¹ near the melting point of 280 K at atmospheric pressure, and it is almost independent of pressure and temperature T: ln κ = −0.621−40.1/T at atmospheric pressure (in SI-units). This is slightly less than κ of type II CHs of water-miscible solvents such as THF. Intriguingly, unlike other water-rich type II clathrate hydrates of water-miscible molecules M (M·17 H₂O), CP CH does not amorphize at pressures up to 1.3 GPa at 130 K and also remains stable up to 0.5 GPa at 240 K. This shows that CP CH is mechanically more stable than the previously studied water-rich type II CHs, and suggests that repulsive forces between CP and the H2O cages increase the mechanical stability of crystalline CP CH. Moreover, we show that κ of an ice-CH mixture, which often arises for CHs that form naturally, is described by the average of the parallel and series heat conduction models to within 5% for ice contents up to 22 wt%. The findings provide a better understanding of the thermal and stability properties of clathrate hydrates for their applications such as gas storage compounds.

TiO₂-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 ​nm sized) titania particles with an energy resolution of about 0.2 ​eV. Electronic structure features revealed from titanium L_3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO₂-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO₆ units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO₂-II was determined to be indirect and with a magnitude of ∼3.18 ​eV, which is very similar to anatase (indirect, ∼3.2 ​eV), and distinctly different from rutile (direct, ∼3.05 ​eV) and brookite (direct, ∼3.45 ​eV).

A multianvil cell assembly with octahedral edge length 25 mm has been adapted for high pressure investigations involving water-rich environments up to 6.5 GPa and 400°C. Water-rich samples are confined in Teflon containers with a volume up to 300 mm3. Applicability tests were performed between 250 and 400°C by investigating the transformation of amorphous titania particles close to the rutile–TiO2-II (∼5 GPa) phase boundary, and the transformation of amorphous silica particles close to the quartz–coesite (∼2.5 GPa) and coesite–stishovite (∼7 GPa) phase boundaries. The performed experiments employed 25.4 mm tungsten carbide anvils with a truncation edge length of 15 mm. The sample pressure at loads approaching 820 t was estimated to be around 6.5 GPa. The large volume multianvil cell is expected to have broad and varied application areas, ranging from the simulation of geofluids to hydrothermal synthesis and conversion/crystal growth in aqueous environments at gigapascal pressures.

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.

Properties of a crystal are used to determine the point defects concentration, n, the self diffusion coefficient, D and variation of n and D with tempertaure, T. Also, spontaneous change in the properties of a crystal’s non-equilibrium state is used to determine the decrease in n with time. Both n and D decrease with increase in the pressure, P, until the pressurising-rate dependent, kinetic-freezing pressure for defects disorder, P_D-F, is reached. At P > P_D-F, a crystal is in a non-equilibrium state. We consider such configurationally-frozen states of a crystal produced by using three unusual P-T protocols: (i) pressurising a crystal to P > P_D-F, cooling to a low T and depressurising, (ii) pressurising to P < P_D-F, cooling through the defects freezing temperature, T_D-F, and depressurising, and (iii) cooling a crystal at 1 bar to T <T_D-F pressurising and maintaining at high P. The non-equilibrium state of the crystal would have defect concentration that kinetically froze at P = P_D-F or T = T_D-F, but its volume and phonon properties would not correspond to the kinetically-frozen state at P = P_D-F or at T = T_D-F. On aging, their properties would change differently than those of a non-equilibrium state of a crystal produced by quenching at a fixed P. We relate n, D and the electrical resistivity to thermal conductivity, κ, by the Wiedemann-Franz equation, and discuss how κ would change on aging of a crystal. The above-given effects alter the properties of metallic and non-metallic, metastable materials during their commercial use.

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’.

Cellulose is a crystalline polymer with intriguing, amorphous-like, temperature dependence of thermal conductivity κ. To determine its origin, we have studied κ of cellulose nanocrystals (CNCs) derived from cotton by sulfuric acid hydrolysis, in both porous and nonporous states by pressure densification; κ increases weakly with increasing temperature and density, like in a fully amorphous material, and it is remarkably similar to that of cellulose fibers (CFs) and cellulose nanofibers (CNFs). For a powder derived from a natural material, like cellulose, amorphous-like κ may originate from poor thermal contact between particles or a high amorphous content, but the latter is not the case for CNCs. Moreover, the amorphous-like behavior is unaffected by densification and, therefore, improved thermal contacts. Instead, we attribute the behavior to CNCs' nanometer-sized fibrils, which limit the phonon mean free path to a few nanometers in a network of randomly oriented CNCs. This explains why κ is essentially the same in networks of CNCs, CFs, and CNFs, which are materials with the same structural unit-elementary fibrils of 3-5 nm in diameter. We obtain κ = (0.60 ± 0.01) W m-1 K-1 for a nonporous network of randomly oriented CNCs at 295 K and atmospheric pressure, and κ increases by only 14% GPa-1, which is unusually weak for a polymer. By using a model for such a network, we find κ = 1.9 W m-1 K-1 along a CNC and argue that this is a good estimate also along a CNF and a CF at room temperature.