Crystal structure and lattice dynamics of high-pressure scandium trihydride

Abstract

Powder samples of hcp $\mathrm{Sc}{\mathrm{H}}_{2.9}$ and $\mathrm{Sc}{\mathrm{D}}_{2.9}$ have been synthesized under a $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure of gaseous hydrogen or deuterium and studied at ambient pressure and $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by neutron diffraction. A profile analysis of the diffraction patterns indicated a crystal structure with $P{6}_{3}∕mmc$ symmetry that differs from the structures of hcp trihydrides of yttrium and rare-earth metals by the absence of long-range order in the displacements of hydrogen atoms from high-symmetry positions. The lattice dynamics of scandium trihydride and, for comparison, yttrium trihydride were studied by inelastic neutron scattering at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the range of energy transfers $4--250\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The spectra of optical H vibrations in these two trihydrides proved to be similar, suggesting similar arrangements of hydrogen atoms on a short-range scale.