91����

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 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 , Ta atoms at the and Wyckoff positions) and a lattice parameter of at K. The hydrogen content determined by hot extraction was . Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, Δ⁢H⁡/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 A⁢15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A⁢15−TaH1.23⁢(5) demonstrated that hydrogen atoms occupy the 24⁢𝑘 and 16⁢𝑖 Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24⁢𝑘 and an increase of the 16⁢𝑖 site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A⁢15−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 24⁢𝑘 and 16⁢𝑖 sites, respectively. No superconductivity was found in A⁢15−TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.