Neutron scattering study of tantalum dihydride

Physical Review B: Condensed Matter and Materials Physics. 2020. Vol. 102. No. 2. Article 024113.

Kuzovnikov M., Antonov Vladimir E., Ivanov A., Hansen T., Savvin S., Kulakov V., Tkacz M., Kolesnikov A., Gurev V.

A single-phase sample of TaH2.2(1) with a hexagonal close-packed metal lattice (hcp; space group P63/mmc)
was synthesized under a hydrogen pressure of 9 GPa and a temperature of 100 °C; quenched to the liquid
nitrogen temperature; recovered to ambient pressure and studied by neutron diffraction (ND) and inelastic
neutron scattering (INS). The ND study showed that hydrogen atoms occupied one half of the tetrahedral (T) and
all octahedral (O) interstitial sites in the hcp lattice of Ta atoms. The arrangement of the H atoms over the T sites
was proven to be ordered, which lowered the symmetry of the full crystal structure of the dihydride to P63mc.
Due to the resulting asymmetry in the local environment of the O sites, the H atoms were considerably displaced
from the centers of these sites along the z axis, away from the H atoms occupying the neighboring T sites. The
INS study demonstrated that the potential wells for H atoms at both the T and O sites are highly anharmonic and
anisotropic. The potential wells at the O sites are softer along the z axis than in the x, y plane, while the T sites
show opposite anisotropy.

Keywords: hydrides, Neutron diffraction, Inelastic neutron scattering