High-pressure synthesis and neutron scattering study of tantalum hydride TaH1.23(5) and a tantalum polymorph with A15-type structure

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 580∘C using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group Pm-3n, Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a=5.510(5)Å at T=85 K. The hydrogen content determined by hot extraction was H/Ta=1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps - first, ΔH/Ta=0.2 was desorbed at around -70∘C, and then the rest of the hydrogen was desorbed between 100∘C and 390∘C. The A15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A15-TaH1.23(5) demonstrated that hydrogen atoms occupy the 24k and 16i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24k and an increase of the 16i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A15-TaH1.23(5) at 72, 135, 145, and 166meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24k and 16i sites, respectively. No superconductivity was found in A15-TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.