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Gas hydrates are ice-like solids, in which guest molecules or atoms are trapped inside cages formed within a crystalline host framework (clathrate) of hydrogen-bonded water molecules (1). They are naturally present in large quantities on the deep ocean floor and as permafrost, can form in and block gas pipelines, and are thought to occur widely on Earth and beyond. A natural point of reference for this large and ubiquitous family of inclusion compounds is the empty hydrate lattice (1-6), which is usually regarded as experimentally inaccessible because the guest species stabilize the host framework. However, it has been suggested that sufficiently small guests may be removed to leave behind metastable empty clathrates (7,8), and guest-free Si- and Ge-clathrates have indeed been obtained (9,10). Here we show that this strategy can also be applied to water-based clathrates: five days of continuous vacuum pumping on small particles of neon hydrate (of structure sII) removes all guests, allowing us to determine the crystal structure, thermal expansivity and limit of metastability of the empty hydrate. It is the seventeenth experimentally established crystalline ice phase (11), ice XVI according to the current ice nomenclature, has a density of 0.81 grams per cubic centimetre (making it the least dense of all known crystalline water phases) and is expected (7,12) to be the stable low-temperature phase of water at negative pressures (that is, under tension). We find that the empty hydrate structure exhibits negative thermal expansion below about 55 kelvin, and that it is mechanically more stable and has at low temperatures larger lattice constants than the filled hydrate. These observations attest to the importance of kinetic effects and host-guest interactions in clathrate hydrates, with further characterization of the empty hydrate expected to improve our understanding of the structure, properties and behaviour of these unique materials., The two main gas hydrate structure types (1), sI and sII, both have a cubic symmetry. Topologically, they are related to the Si[O.sub.2] phases of melanophlogite and dodecasil-3C, respectively (13), [...]

Gas hydrates grown at gas-ice interfaces were examined by electron microscopy and found to have a sub-micrometer porous structure. In situ observations of the formation of porous C[H.sub.4]--and C[O.sub.2]--hydrates from deuterated ice Ih powders were made at different pressures and temperatures, using time-resolved neutron diffraction data from the high-flux D20 diffractometer (ILL, Grenoble) as well as in-house gas consumption measurements. The C[O.sub.2] experiments conducted at low temperatures are particularly important for settling the open question of the existence of C[O.sub.2] hydrates on Mars. We found that at similar excess fugacities, the reaction of C[O.sub.2] was distinctly faster than that of C[H.sub.4]. A phenomenological model for the kinetics of the gas hydrate formation from powders of spherical ice particles is developed with emphasis on ice-grain fracturing and sample-consolidation effects due to the outward growth of gas hydrate. It describes (1) the initial stage of fast crack-filling and hydrate film spreading over the ice surface and the two subsequent stages which are limited by (2) the clathration reaction at the ice-hydrate interface and/or by (3) the diffusive gas and water transport through the hydrate shells surrounding the shrinking ice cores. In the case of C[O.sub.2]--hydrate, the activation energies of the ice-surface coating in stag are estimated to be 5.5 kJ/mol at low temperatures and 31.5 kJ/mol above 220 K, indicating that water molecule mobility at the ice surface plays a considerable role in the clathration reaction. Comparable activation energies of 42.3 and 54.6 kJ/mol are observed in the high temperature range for the reaction and diffusion-limited stages 2 and 3, respectively.

A study of gas hydrates grown at gas-ice interfaces which were examined by electron microscopy and found to have a submicron porous structure, is presented. Diffraction experiments were also performed with methane in hydrogenated samples and the isotopic differences were found to be insignificant.

A precise and accurate measurement of the crystal structure of ice-Ih is hindered by its disordered H-bond network. In this work, we carried out first-principle calculations to study the effects of H-bond topology on the structure of ice-Ih with emphasis on the molecular geometry of water and the distortion in oxygen lattice. An analytic algorithm based on group and graph theory is employed to enumerate all possible configurations in a given unit cell and to select a set of structures for detailed examinations. In total we have studied more than 60 ice-Ih structures in a hexagonal unit cell of 48 water molecules by quantum-chemical methods and found a significant amount of static distortion in the oxygen positions from their crystallographic positions which is in good agreements with highly significant higher-order terms obtained from both x-ray and neutron-diffraction data. Much debated structural information such as H–O–H angle and O–H bond length is found to be 106.34±0.36° and 0.9997±0.0008 Å, compared to experimental value of 106.6±1.5° and 0.986±0.005 Å. Detailed benchmarking calculations were carried out to gauge the influence of using different exchange and correlation functionals, pseudopotentials, and unit-cell sizes. Our results have proven that first-principle methods are useful complementary tools to experiments, especially for cases in which experimental accuracy is limited by intrinsic orientational disorder. [ABSTRACT FROM AUTHOR]

The crystal structures of type II N[sub 2-], O[sub 2-], and air-clathrate hydrates have been studied under their thermodynamic conditions of stability by neutron powder diffraction. The isothermal compressibilities were deduced from the refined lattice constants and found to be significantly different for N[sub 2] and O[sub 2] clathrate hydrates. The water host lattice structure rearranges as a function of pressure with a resulting change of the volume ratio of small and large cages. The cage fillings were established on an absolute scale for the first time. The large cages were found to be partly occupied by two molecules. The results are compared with predictions based on the statistical thermodynamic theory of van der Waals and Platteeuw. While the filling follows the predicted trend significant differences exist in detail. [ABSTRACT FROM AUTHOR]

Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are performed to evaluate the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples. The permeation coefficient D is close to the energy of breaking hydrogen bonds in ice Ih and indicates that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.

The structure of ice-VI by density functional methods is studied and the intrinsic disorder is investigated by studying all ice-rule-allowed ice configurations in its primary unit cell. Result shows that there are two interpenetrating but independent hydrogen-bonded sublattices in ice-VI, however, the molecular geometry of water molecules in ice-VI is closely related to other intermediate pressure phases, which are characterized by considerable distortions in bond angles and distances.

Neutron powder diffraction experiments show that, in the presence of ice and at helium-gas pressures exceeding 0.28 GPa, a gas hydrate with the composition He(6+x)H2O is formed. Its host lattice closely resembles that of ice II. However, the stability range of the helium hydrate is considerably more extended as compared to that of ice II: helium hydrate melts and recrystallizes from the liquid and is more stable than ice III/IX or ice V, at least up to 0.5 GPa. The helium content increases with increasing-pressure/decreasing-temperature, but does not appear to follow an ideal solution behavior, as do the guests in some heavier noble-gas clathrates. The lattice parameters of helium hydrate are significantly different from those of ice II, while the atomic arrangement of the host lattice is very similar, showing full orientational order of the water molecules. [ABSTRACT FROM AUTHOR]

A novel X-ray diffraction based method is presented, capable of determining volume-based crystal size distribution ( CSD) of polycrystalline materials and crystalline powders with unprecedented sampling statistics; the method is named fast X-ray diffraction crystal size distribution analysis ( FXD- CSD). FXD- CSD can be performed with standard laboratory X-ray diffractometers equipped with a position sensitive detector and uses a software package written in Python for the data analysis. FXD- CSD is a destruction-free and generally applicable method to establish CSDs of polycrystalline materials as well as powders for sizes well below 1 μm up to about 100 μm; it even allows for studies of samples enclosed in complex environments, e.g., for in situ measurements in a furnace or in a pressure cell. To show the capability of the method the microstructural evolution of four alumina substrates with different time-spans of sintering (4, 8, 16, and 24 hour at 1600°C) is investigated via FXD- CSD and SEM imaging. The corresponding CSDs and average grain sizes are determined, results obtained by FXD- CSD and the line-intersection methods are compared and clear evidence for the presence of abnormal grain growth ( AGG) during sintering is shown. From three tested probability density functions ( PDF) describing the CSDs a log-normal PDF fits best to the volume based CSDs; the method provides size distributions with unprecedented precision opening the way to a systematic and meaningful comparison between theoretically predicted and observed CSDs. [ABSTRACT FROM AUTHOR]