WH_nunder pressure

Abstract

An initial observation of the formation of WH under pressure from W gaskets surrounding hydrogen in diamond anvil cells led to a theoretical study of tungsten hydride phases. At P = 1 atm no stoichiometry is found to be stable with respect to separation into the elements, but as the pressure is raised WH(n) (n = 1-6, 8) stoichiometries are metastable or stable. WH and WH(4) are calculated to be stable at P > 15 GPa, WH(2) becomes stable at P > 100 GPa and WH(6) at P > 150 GPa. In agreement with experiment, the structure computed for WH is anti-NiAs. WH(2) shares with WH a hexagonal arrangement of tungsten atoms, with hydrogen atoms occupying octahedral and tetrahedral holes. For WH(4) the W atoms are in a distorted fcc arrangement. As the number of hydrogens rises, the coordination of W by H increases correspondingly, leading to a twelve-coordinated W in WH(6). In WH(8) H(2) units also develop. All of the hydrides considered should be metallic at high pressure, though the Fermi levels of WH(4) and WH(6) lie in a deep pseudogap. Prodded by these theoretical studies, experiments were then undertaken to seek phases other than WH, exploring a variety of experimental conditions that would favor further reaction. Though a better preparation and characterization of WH resulted, no higher hydrides have as yet been found.

Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

“…In our study 9 of the ground-state tungsten hydrides, there were no hydrogen pairs when the number of hydrogen atoms in WH n fell below six. However, we found H pairs in WH 8 .…”

“…However, we found H pairs in WH 8 . 9 Accordingly, we examined in considerable detail the geometrical and electronic structure of niobium hydrides with even higher concentrations of hydrogen, and indeed we find molecular units in the high-pressure structures of NbH 12 , with unusual intermediate H-H separations. However, none of these compositions lie on the convex hull (see below).…”

“…Also the quest for metallic hydrogen under pressure has also involved several theoretical groups in the examination of metal hydrides; particularly, we mention here investigations of alkali and alkaline earth hydrides, 2-5 group 14 hydrides, [6][7][8] and the tungsten hydrides, 9 some of these carried out by our own research groups. We were led to look at the niobium hydrides in particular by the following additional observation: it is well known that the transition metal Nb holds the record for the highest superconducting transition temperature T c (9.3 K) of an element at normal pressure, 10 and compounds of Nb, such as Nb 3 Ge (23 K), 11 NbC (11 K), 12 and NbN (16 K), 13 also have the highest T c values for their respective classes.…”

Potential high- T c superconducting lanthanum and yttrium hydrides at high pressure

“…For instance, one might reason from the fact that Y (54) itself has a distorted fcc structure that it may be possible to produce sodalite-like YH 10 at a metal-hydrogen interface. The exploration, theoretical and experimental, of high hydrides of heavy metals is thus a promising field (55)(56)(57)(58).…”

High-Pressure Synthesis and Characterization of Iridium Trihydride

“…The formation of a hydride phase is readily observed in x-ray diffraction (XRD) measurements as an expansion of the unit cell (see, e.g., in rhenium [12]), or a structural phase transition with increased volume per metal atom compared to the parent metal (e.g., bcc to hcp in tungsten [7,8,19] or fcc to hcp in platinum [9]). It has been found that, due to contributing its electron to the valence band of the surrounding metal, the presence of one hydrogen atom expands the host lattice by 2-3 # A 3 (depending on the material, see Refs.…”