Brillouin-scattering studies of the transverse acoustic modes of incommensurate K2SeO4

Brillouin scattering from the six pure transverse acoustic modes of K2SeO4 from 80 to 300 K was investigated. The temperature dependence of the hypersonic shear elastic constants C44, C55, and C66 and the corresponding damping constants were obtained. C44 and C66 show anomalies near Ti and C55 near Tc, closely resembling the results of ultrasonic experiments. The C44 anomaly was analyzed in terms of bilinear coupling to the lowest temperature-dependent B3g optical mode, which was observed in Raman scattering. The C55 anomaly near Tc was compared with the plane-wave-approximation result for the coupling of the strain to the phason. For the three pairs of transverse acoustic modes there was no asymmetry observed in the incommensurate and commensurate phases under interchange of the propagation and polarization directions.