Work

High-pressure synthesis and neutron scattering study of tantalum hydride TaH 1.23 ( 5 ) and a tantalum polymorph with A15-type structure

Year: 2024

Type: article

Abstract: 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 P m − 3 n , Ta atoms at the 2 a and 6 c 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, 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 − TaH 1.23 ( 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 − TaH 1.23 ( 5 ) at 72, 135, 145, and 166 meV , 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

Source: Physical review. B./Physical review. B

DOI: 10.1103/physrevb.110.184113