Two-phonon Raman spectra of LiH and LiD crystals

The experimental two-phonon Raman spectra of LiD and LiH are reported here. A deformation dipole model formulated elsewhere for LiD is used to compute the lattice dynamics of LiD and LiH required in the Raman intensity calculations. The use of the same model for LiH is justified by the good agreement between the peak positions of the temperature-weighted two-phonon density of states and the various experimental spectra. The Raman intensity calculations are carried out by treating the second-order expansion coefficients in the polarisability tensor as adjustable parameters. The need for the parameters associated with the next-nearest-neighbor ions is clearly demonstrated for all the spectra. A single set of six parameters for the T₂₈ spectra and thirteen parameters for the E₈ (or A₁₈) spectra is found to explain all the experimental spectra of LiD and LiH quite well. Since the polarisability of Li⁺ is very small, the need for the second-neighbor positive-positive parameters reflects on the extended and highly polarisable nature of the H^? or D^? ion.