Second-order Raman spectroscopic study of lithium hydride and lithium deuteride at high pressure

Second-order Raman spectra of LiH and LiD have been measured as functions of pressure up to 15 GPa (ΔV/(Formula presented)=-0.25) at ambient temperature. The assignments of the many features in these spectra have been thoroughly studied. The features have been compared with the two-phonon dispersion curves derived from neutron-scattering data and with isotope effect scaling. In addition, an internal self-consistency procedure is employed to validate the phonon assignments. Most of the features are assigned to combinations of zone-boundary phonons. The pressure and volume dependence of many zone-boundary phonons have been determined. These include all of the phonons at the X point as well as some at the L and the W points. Most of these phonons have constant mode Grüneisen parameters, while others exhibit volume-dependent mode Grüneisen parameters. There is significant variation in the magnitude of these parameters between the various modes. The results are shown to be in good agreement with our ab initio calculations.