Raman scattering in CuCl under pressure

Raman spectra of CuCl were measured under hydrostatic pressures up to 14 GP a at low temperatures (T = 5 K). The anomaly in the Raman lineshape of zinc blende CuCl at frequencies near the transverse-optic (TO) mode, which consi sts of a broad structure with several maxima, disappears at a pressure near 3 GPa. This effect is well reproduced by a model calculation of the anharm onic coupling of the TO mode to acoustic two-phonon states (Fermi resonance ). Alternative interpretations of the TO Raman anomaly in terms of local vi brational modes of Cu atoms in off-center positions are not supported by th e present results. Raman spectra indicate the existence of the phase CuCl-I Ia in a narrow pressure range (3.3 and 4.1 GPa). Based on calculated phonon dispersion relations we propose an assignment for the Raman modes observed for the binary BC8-analog phase CuCl-IV in the range from 4.1 to 14 GPa.