By combining hydrogen and sulfur within diamond-anvil cells we synthesize (H 2 S) 2 H 2 at 5 GPa and 373 K. Through a series of Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction experiments we have constrained the phase diagram of (H 2 S) 2 H 2 within a wide P-T range. On compression we observe the phase transition sequence of I-II-II-III, where II is a previously unreported phase; at room temperature this sequence spans from 5 to 47 GPa, while the application of low temperatures stabilizes this sequence to 127 GPa (< 80 K). Above these pressures we propose that phase III of (H 2 S) 2 H 2 transforms to a nonmolecular H 3 S network. Our Raman and infrared measurements indicate that the transition from (H 2 S) 2 H 2 to H 3 S is reversible at room temperature. X-ray diffraction reveals that the symmetry of the underlying S lattice of (H 2 S) 2 H 2 and H 3 S is retained along this compression path up to at least 135 GPa.

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H–H bond is caused by the charge transfer from the metal to and by the steric effect of the metal host on the H–H bond. This effect is most prominent for BaH 4 , where the precompression of units at 50 GPa results in bond lengths comparable to that of pure H 2 above 275 GPa.

Transition metal nitrides have applications in a range of technological elds. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K the elemental copper reacts with nitrogen forming copper diazenide (CuN 2 ). Through a combination of synchrotron X-ray diraction and rst-principles calculations we have explored the stability and electronic structure of CuN 2 . We nd that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN 2 is metallic and exhibits partially lled N 2 antibonding orbitals, leading to an ambiguous electronic structure between Cu + /Cu 2+ . This leads to weak Cu-N bonds and the lowest bulk modulus observed for any transition metal nitride. Graphical TOC Entry

The observation of high-temperature superconductivity in hydride sulfide (HS) at high pressures has generated considerable interest in compressed hydrogen-rich compounds. High-pressure hydrogen selenide (HSe) has also been predicted to be superconducting at high temperatures; however, its behaviour and stability upon compression remains unknown. In this study, we synthesize HSe in situ from elemental Se and molecular H at pressures of 0.4 GPa and temperatures of 473 K. On compression at 300 K, we observe the high-pressure solid phase sequence (I-I'-IV) of HSe through Raman spectroscopy and x-ray diffraction measurements, before dissociation into its constituent elements. Through the compression of HSe in H media, we also observe the formation of a host-guest structure, (HSe)H, which is stable at the same conditions as HSe, with respect to decomposition. These measurements show that the behaviour of HSe is remarkably similar to that of HS and provides further understanding of the hydrogen chalcogenides under pressure.

This paper assesses the performance of plane-wave density functional theory calculations at returning reliable structural information for molecular crystal structures where the primary intermolecular interactions are either hydrogen bonding or dispersion interactions. The computed structures are compared with input structures obtained from the Cambridge Structural Database, and assessed in terms of crystal packing similarities, unit-cell volume and shape, short contact distances and hydrogen-bond distances. The results demonstrate that the PBE functional [Perdew, Burke & Ernzerhof (1996). Phys. Rev. Lett. 77, 3865-3868] with Tkatchenko and Scheffler's `TS' dispersion correction [Tkatchenko & Scheffler (2009). Phys. Rev. Lett. 102, 073005] is capable of returning reliable full structural optimizations, in which both atomic positions and unit-cell vectors are free to optimize simultaneously.

The structure of the primary amino acid L-leucine has been determined for the first time by neutron diffraction. This was made possible by the use of modern neutron Laue diffraction to overcome the previously prohibitive effects of crystal size and quality. The packing of the structure into hydrophobic and hydrophilic layers is explained by the intermolecular interaction energies calculated using the PIXEL method. Variable-temperature data collections confirmed the absence of phase transitions between 120 and 300 K in the single-crystal form.

High-pressure single-crystal neutron Laue diffraction yields data suitable for fully anisotropic structure refinement, allowing joint X-ray and neutron studies of exactly the same sample. Remarkably, data completeness is similar to ambient-pressure measurements, despite the presence of a pressure cell.

By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, Ba8H46, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and first-principles calculations to determine that this compound adopts a highly symmetric structure with an unusual 5 :1 hydrogen-to-barium ratio. This singular stoichiometry corresponds to the well-defined type-I clathrate geometry. This clathrate consists of a Weaire–Phelan hydrogen structure with the barium atoms forming a topologically close-packed phase. In particular, the structure is formed by H20 and H24 clathrate cages showing substantially weakened H–H interactions. Density functional theory (DFT) demonstrates that cubic Ba8H46 requires dynamical effects to stabilize the H20 and H24 clathrate cages.