Pressure and temperature dependence of the Raman phonons in isotopic γ-CuI

The transverse-optic (TO) phonons of zinc-blende-type CuCl $(\gamma$-CuCl) and the longitudinal-optic (LO) phonons of $\gamma$-CuBr exhibit striking anharmonic self-energy anomalies at the center of the Brillouin zone. We investigate here by Raman spectroscopy the dependence of the LO and TO phonons of $\gamma$-CuI on pressure, temperature, and isotopic mass of copper, in the search for related effects. We find that the pressure dependence of the TO phonon linewidth is qualitatively different at low temperature and close to room temperature. A model is developed to interpret these differences based on anharmonic decay into two and three phonons (two different channels). In order to make semiquantitative predictions we used the experimental pressure dependence of the TO phonons and that of phonons at the edge of the Brillouin zone that we obtained by linear-response ab initio local-density approximation calculations. We have also calculated the pressure dependence of the zone-edge phonons of $\gamma$-CuCl and $\gamma$-CuBr and compared it with the few existing experimental results. In this process, interesting anomalies concerning the transverse-acoustic phonons at the zone edge have been found.