High-pressure synthesis of tantalum dihydride

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 $\mathrm{H}/\mathrm{Ta}=0.92(5)$ 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 $P=2.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. 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-${\mathrm{N}}_{2}$ temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with $a=3.224(3)$ and $c=5.140(5)\phantom{\rule{0.16em}{0ex}}\AA{}$ at $T=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The hydrogen content determined by thermal desorption was $\mathrm{H}/\mathrm{Ta}=2.2(1)$.