2ND-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 functi ons of pressure up to 15 Gpa (Delta V/V-0= - 0.25) at ambient temperat ure. The assignments of the many features in these spectra have been t horoughly studied. The features have been compared with the two-phonon dispersion curves derived from neutron-scattering data and with isoto pe 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 an d 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 t he L and the W points, Most of these phonons have constant mode Grunei sen parameters, while others exhibit volume-dependent mode Gruneisen p arameters. There is significant variation in the magnitude of these pa rameters between the various modes. The results are shown to be in goo d agreement with our ab initio calculations.