Crystal structure and lattice dynamics of high-pressure scandium trihydride

Powder samples of hcp ScH2.9 and ScD2.9 have been synthesized under a 1 GPa pressure of gaseous hydrogen or deuterium and studied at ambient pressure and 95 K by neutron diffraction. A profile analysis of the diffraction patterns indicated a crystal structure with P6(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 K in the range of energy transfers 4-250 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.