Olga Degtyareva

Formation of transition metal hydrides at high pressures

Abstract Silane (SiH 4 ) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H 2 . The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane and reaction of hydrogen with Re gasket. From the data of a previous experimental report [M.I. Eremets, I.A. Trojan, S.A. Medvedev, J.S. Tse, Y. Yao, Science 319 (2008) 1506], the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.

High-pressure structural studies of group-15 elements

Recent advances in high-pressure experimental techniques have yielded high-quality x-ray diffraction data for the high-pressure phases of the group-15 elements Bi, Sb and As, and have made it possible to solve several long-standing problems in their structures. In particular, several complex incommensurate host–guest structures have been identified. This paper reviews the present state of knowledge of the structural transition sequences for these elements at high pressure and room temperature, including a summary of previous work, a detailed presentation of the new structures, and revised equations of state.

Crystal structure of the superconducting phases of S and Se

Compressed S and Se are studied by x-ray diffraction with synchrotron radiation up to 160 GPa. The S-IV phase is shown to have a body-centered monoclinic structure and to be stable between 83 and 150 GPa on pressure increase, where it transform to S-V with a primitive rhombohedral

Crystal structure of simple metals at high pressures

\beta

Structure stability in the simple element sodium under pressure

-Po structure. Observation of the modulation reflections in S-IV up to 135 GPa shows that its crystal structure is incommensurately modulated, as recently reported for Se and Te. The modulated body-centered monoclinic phase of Se, Se-IV, is shown to transform to the

Crystal structure of sulfur and selenium at pressures up to 160 GPa

\beta

Phase transitions under high pressure in binary Sn alloys (with In, Hg and Ga)

-Po phase Se-V at around 80 GPa. Se-V transforms to a body-centered cubic phase at 140 GPa in accordance with previous studies. Pressure dependence of the structural parameters of these high-pressure phases in S and Se are discussed in relation to their superconducting behaviour.

On the effects of high temperature and high pressure on the hydrogen solubility in rhenium

The effects of pressure on the crystal structure of simple (or sp-) elements are analysed in terms of changes in coordination number, packing density, and interatomic distances, and general rules are established. In the polyvalent elements from groups 14–17, the covalently bonded structures tend to transform to metallic phases with a gradual increase in coordination number and packing density, a behaviour normally expected under pressure. Group 1 and 2 metallic elements, however, show a reverse trend towards structures with low packing density due to intricate changes in their electronic structure. Complex crystal structures such as host–guest and incommensurately modulated structures found in these elements are given special attention in this review in an attempt to determine their role in the observed phase-transition sequences.

Simple Metals at High Pressures

The simple alkali metal Na that crystallizes in a body-centred cubic structure at ambient pressure exhibits a wealth of complex phases at extreme conditions as found by experimental studies. The analysis of the mechanism of stabilization of some of these phases, namely, the low-temperature Sm-type phase and the high-pressure cI16 and oP8 phases, shows that they satisfy the criteria for the Hume-Rothery mechanism. These phases appear to be stabilized due to a formation of numerous planes in a Brillouin–Jones zone in the vicinity of the Fermi sphere of Na, which leads to the reduction of the overall electronic energy. For the oP8 phase, this mechanism seems to work if one assumes that Na becomes a divalent metal at this density. The oP8 phase of Na is analysed in comparison with the MnP-type oP8 phases known in binary compounds, as well as in relation to the NiAs-type hP4 structure.

NEW RESULTS ON OLD PROBLEMS: THE USE OF SINGLE-CRYSTALS IN HIGH-PRESSURE STRUCTURAL STUDIES

Using advanced in situ X-ray diffraction techniques at high pressures and temperatures, we have resolved the long-standing problem of the phase transition sequence of sulfur in its non-metallic state. Our data show that there are only two phases of sulfur stable between 1.5 GPa and pressure of metallization of 86 GPa, S-II with triangular chain structure and S-III with novel squared chain structure. The same squared chain structure is formed in the heavier group-VI element Se at pressures of 20 GPa and temperatures of 450 K. Our X-ray diffraction data on metallic phases of sulfur above 83 GPa show that the S-IV phase has an incommensurately modulated monoclinic structure, the same as recently reported modulated structures of Te-III and Se-IV. S-IV is shown to transform to primitive rhombohedral β-Po phase at 153(3) GPa, the same transition is found in Se at pressure of around 80 GPa.