Elastic Anomaly of the Ferroelectric Phase Transition of Li₂Ge₇O₁₅ Studied by the Brillouin Scattering

Abstract

Temperature dependence of the Brillouin scattering spectra of lithium heptagermanate Li₂Ge₇O₁₅ has been observed near the paraelectric-ferroelectric phase transition temperature T_C. A longitudinal acoustic mode propagating along the b-axis shows an elastic anomaly in the frequency shift and also in the spectral width. An order-disorder type phase transition mechanism describes the temperature dependence of the elastic anomaly in terms of a piezoelectric coupling below T_C. The depolarization field effect is confirmed by comparison with both results on the transverse and longitudinal acoustic modes each other.