Pressure effect on the hydrogen vibrations in γ-TiH and γ-ZrH

The inelastic neutron scattering spectra of gamma-TiH and gamma-ZrH were measured under high hydrostatic pressure. It was found that the energies of the hydrogen optical bands and bound multiphonon features (biphonons and triphonons) increased under pressure in linear proportion to their values at ambient pressure with the scaling factors of 1.02 for gamma-TiH at P = 15 kbar and 1.03 for gamma-ZrH at P = 17.5 kbar. The observed behaviour of the hydrogen-related spectral features is indicative of a uniform increase of the atomic force constants and the anharmonicity parameters in the model Hamiltonians representing the strength of two- and three-phonon interactions under pressure.