Pressure dependence of the Raman scattering by copper halides

Abstract

We report Raman scattering in CuI, CuBr, and CuCl at 40, 100, and 295 K under hydrostatic pressures from 1 to 7 kbar. The TO- and LO-mode Gr\"uneisen parameters are determined for cases in which these modes are directly observed experimentally. By combining the experimental pressure dependence of the phonon frequencies with the experimental temperature dependence one can separate the anharmonic contributions into volume effects and multiphonon effects. In most cases the major contribution to the isobaric temperature dependence of the phonon frequencies in these crystals is the temperature dependence of the anharmonic coupling of the phonons. The logarithmic derivative of the Szigeti effective charge is calculated from mode Gr\"uneisen parameters; in CuBr at 40 K the value of this derivative is anomalous. A new design for sapphire windows for use in pressure cells is described.