Pressure-Induced Disappearance of the Raman Anomaly in CuCl

Raman spectra of CuCl were measured at T0ex0ex=0ex0ex5K under hydrostatic pressures up to 3.3GPa. The anomalous line shape of the transverse-optic (TO) scattering, 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 anharmonic coupling of the TO mode to acoustic two-phonon states (Fermi resonance). The results demonstrate that the ambient pressure TO anomaly does not arise from local vibrational modes of Cu atoms in off-center positions.