High-pressure synthesis of tantalum dihydride

Abstract

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching H/Ta = 0.92(5) at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at P = 2.2 GPa. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-N 2 temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with a = 3.224(3) and c = 5.140(5)Å at T = 85 K. The hydrogen content determined by thermal desorption was H/Ta = 2.2(1).

Synthesis and stability of tantalum hydride at high pressures

“…Upon compression, Ta already reacted with hydrogen to form hcp-TaH ∼2 at our first pressure point (45 GPa), consistent with the previous result [18]. The structure of bcc-Ta and hcp-TaH 2 are shown in Figure . 1(a) and (b).…”

“…In the previous experiment, various tantalum hydrides were shown to be formed at ambient pressure with the hydrogen content less than one [17]. Tantalum dihydride can be formed at relatively low pressures (5 GPa) and can be quenched to ambient pressure at low temperature [18], however, other tantalum hydrides with higher hydrogen content were not found in previous experiments. Earlier theoretical calculations have predicted several stable hydrogen-rich tantalum hydrides at high pressure, which could be potential high-T c superconductors with T c 's exceeding 100 K [16], but these predictions still need an experimental confirmation.…”

High pressure synthesis and stability of cobalt hydrides

“…This transition and associated volume change are in agreement with previous observations of the formation of CoH and other transition metal monohydrides. 7,8,18 One sample of CoH was compressed up to 54 GPa, into the predicted stability range of the cobalt polyhydrides. Over a timescale of 2 hr, no further transformation to a hydride with H/Co ≥ was observed.…”

“…Similar behaviour is observed in the Ta-H and Rh-H systems, however this should be further explored with high-pressure neutron diffraction. 3,8 It was previously thought that the Rh-H 2 system was a unique example of a two-step hydrogen absorptiondesorption from metal (M ) to M H to M H 2 by the stepwise filling of octahedral and tetrahedral sites whilst maintaining the fcc lattice. 3 Our study shows that this is not the case, and synthesis of Co-CoH-CoH 2 can be achieved on both compression and the application of high temperature.…”

“…2 High pressure has been an effective and clean route to producing stoichiometric and non-stoichiometric hydrides with extremely high hydrogen contents. [3][4][5][6][7][8][9][10] This is due to the chemical potential of hydrogen rising steeply with pressure, resulting in the ability to synthesize novel metal hydrides.…”