Pressure-induced disappearance of the Raman anomaly in CuCl

Raman spectra of CuCl were measured at T = 5 K under hydrostatic pressures up to 3.3 GPa. The anomalous line shape of the transverse-optic (TO) scatte ring, which consists of a broad structure with several maxima, undergoes a drastic change under pressure. The anomaly disappears completely at 3 GPa. These pressure effects are well reproduced by a model calculation of the an harmonic coupling of the TO mode to acoustic two-phonon states (Fermi reson ance). The results demonstrate that the ambient pressure TO anomaly does no t arise from local vibrational modes of Cu atoms in off-center positions.