LINESHAPE OF BRILLOUIN SPECTRA IN THE INCOMMENSURATE PHASE OF QUARTZ (SIO2) - ANALYSIS BY FANOS-FORMULA

Rayleigh-Brillouin spectra near the a-INC-P phase transition of quartz were measured. Near the INC-phase, a plateau appears between the LA-m ode and the Rayleigh line. In order to take account of the interaction between the continuum (two phonon scattering from the soft branch) an d the discrete LA-mode, Fano's formula was used in the analysis of the asymmetric spectra. The results are consistent with the interpretatio n of the low frequency Raman spectra observed by Berge et al. and they also agree with the ultrasonic data indicating that the single relaxa tion mechanism explains the temperature dependence of the sound wave n ear the beta --> INC phase transition.