Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

/pdf/high-pressure-phases-of-group-iv-iii-v-and-ii-vi-compounds-4wptdc31z8.pdf

High-pressure phases of group-IV, III–V, and II–VI compounds

High-pressure Raman, infrared, x-ray, and neutron studies show that H2 and H2O mixtures crystallize into the sII clathrate structure with an approximate H2/H2O molar ratio of 1:2. The clathrate cages are multiply occupied, with a cluster of two H2 molecules in the small cage and four in the large cage. Substantial softening and splitting of hydrogen vibrons indicate increased intermolecular interactions. The quenched clathrate is stable up to 145 kelvin at ambient pressure. Retention of hydrogen at such high temperatures could help its condensation in planetary nebulae and may play a key role in the evolution of icy bodies.

https://science.sciencemag.org/content/sci/297/5590/2247.full.pdf

Hydrogen clusters in clathrate hydrate.

A modern definition for whole powder pattern decomposition WPPD methods would be that they simultaneously have to refine the unit-cell parameters and extract the best estimations of the Bragg peak intensities from a complete diffractogram. This is done very fast nowadays, irrespective of the number of Bragg peaks present in a powder diffraction pattern, but we did not attain this comfortable situation without some past efforts. The WPPD methods’ introduction occurred slowly and progressively thanks to the increase in computer power, the improvements in graphical user interfaces, the diffractometer data digitalization, the availability of synchrotron and neutron radiation, and last but not least, the proposition of new algorithms. Innovations were not instantly accepted this being true for all the whole powder pattern fitting methods including the Rietveld and the decomposition methods or could not be applied immediately to every radiation source or diffractometer the hardware before adaptations made by an essential category of crystallographers being conceivers and developers of the software. Ancestors of the WPPD methods extracted peak intensities without the cell restraint, so that each peak position was a parameter to be refined as well as the peak intensity, the peak shape and its width. This is still useful if the aim is the search for the peak positions for indexing, though derivative methods can make that peak-position-hunting job faster. Taking advantage of the indexing see a recent review paper by Bergmann et al. 2004, new WPPD methods, applying cell restraint to the peak position, opened the door to a long list of new possibilities and applications including first indexing confirmation which are detailed in this paper. However, only some selected application references will be provided because the number of papers involved is quite high and increasing more than 2000 texts specify the use of WPPD methods. Contributions from Rennes by Louer’s group from 1987 to 1993 will be especially enlightened, not forgetting the other players during that same time, restraining generally to the structure determinations by powder diffractometry SDPD applications published in the early stages of this retrospection because the subsequent activity increased too considerably, by more than 850 SDPDs in the last ten years. If only a partial review of applications can be given, the evolution of the methods will be discussed as completely as possible.

/pdf/whole-powder-pattern-decomposition-methods-and-applications-597bm0bp60.pdf

Whole powder pattern decomposition methods and applications: A retrospection

Materials for hydrogen storage: current research trends and perspectives

Gas hydrates have received considerable attention due to their important role in flow assurance for the oil and gas industry, their extensive natural occurrence on Earth and extraterrestrial planets, and their significant applications in sustainable technologies including but not limited to gas and energy storage, gas separation, and water desalination Given not only their inherent structural flexibility depending on the type of guest gas molecules and formation conditions, but also the synthetic effects of a wide range of chemical additives on their properties, these variabilities could be exploited to optimise the role of gas hydrates This includes increasing their industrial applications, understanding and utilising their role in Nature, identifying potential methods for safely extracting natural gases stored in naturally occurring hydrates within the Earth, and for developing green technologies This review summarizes the different properties of gas hydrates as well as their formation and dissociation kinetics and then reviews the fast-growing literature reporting their role and applications in the aforementioned fields, mainly concentrating on advances during the last decade Challenges, limitations, and future perspectives of each field are briefly discussed The overall objective of this review is to provide readers with an extensive overview of gas hydrates that we hope will stimulate further work on this riveting field

Gas hydrates in sustainable chemistry

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

The effect of high pressure on the crystal structures of α-, β-, and γ-glycine has been investigated. A new polymorph, δ-glycine, is obtained from β glycine. δ-Glycine is monoclinic, P21/a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) A, β = 125.83(4)° at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of β-glycine to a single crystal of δ-glycine, resulting in an equal number of NH···O hydrogen bonds but an increase in the number and strength of CH···O hydrogen bonds, which act to close-up “holes” that are formed within the layers of β-glycine in the centers of R-type hydrogen-bonded motifs. Trigonal γ-glycine begins to undergo a transition to another high-pressure phase, e-glycine, at 1.9 GPa, but the transformation is destructive; it is essentially complete at 4.3 GPa. The structure is monoclinic Pn, a = 4.8887(10), b = 5.7541(11), c = 5.4419(11) A, β = 116.682(10)° at 4.3 GPa. The structure consists of layers similar to those observed in α-glycine with inte...

https://www.pure.ed.ac.uk/ws/files/10603298/Effect_of_High_Pressure_on_the_Crystal_Structures_of_Polymorphs_of_Glycine.pdf

Effect of high pressure on the crystal structures of polymorphs of glycine

The structure of phase IV of barium has been solved from single-crystal and powder diffraction x-ray data. It is composed of a tetragonal ``host,'' with ``guest'' chains in channels along the $c$ axis of the host. These chains form two different structures, one well crystallized and the other highly disordered and giving rise to strong diffuse scattering. The guest structures are incommensurate with the host.

Self-hosting incommensurate structure of barium iv

/pdf/new-structural-systematics-in-the-ii-vi-iii-v-and-group-iv-4vjdncwybi.pdf

New structural systematics in the II–VI, III–V and group IV semiconductors at high pressure

Nonexistence of the diatomic beta -tin structure

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