Incommensurate phase of quartz. - II : Brillouin scattering studies

Abstract

The recently discovered incommensurate phase of quartz close to the α-β transition has been investigated using Brillouin scattering. The frequency and the damping of the longitudinal (LA) and transverse (TA) acoustic modes propagating along the X, Y and Z directions have been measured as a function of the temperature in the range 845 K-860 K with a temperature resolution better than 0.02 K. The frequency of the TA modes is practically temperature independent but the damping of the TA mode propagating along the Y direction and polarized along the X direction shows an increase near the incommensurate transition point T i . This anomalous damping which is not observed for TA modes propagating along the X axis, is interpreted as a consequence of the anisotropic coupling of the soft mode with TA modes which was invoked by Aslanyan and Levanyuk to explain the occurrence of the incommensurate phase. The temperature dependence of the LA modes is very strong and exhibits a change of slope at T i . This temperature dependence can be attributed to third order anharmonic interactions between the LA modes and the soft mode Mesure de la frequence et de l'amortissement des modes LA et TA se propageant selon les directions X, Y, Z, en fonction de la temperature (845-860 K) avec une resolution d'au moins 0,02 K: variation thermique pratiquement nulle pour la frequence des modes T-A, mais augmentation de l'amortissement du mode TA se propageant suivant Y et polarise suivant X a l'approche de la temperature de la transition incommensurable T i (couplage anisotrope entre mode mou et modes TA). Forte variation thermique des modes LA, avec rupture de pente a T i (termes anharmoniques du 3eme ordre)