Optical and elastic anomalies near the phase transition in

Abstract

shows a structural phase transition near . The temperature dependence of the optical birefringence , of the elastic stiffness constants and of the effective electro-optical coefficients is determined. At the derivative changes its sign. Elastic properties are studied by recording the acoustic resonances of a rectangular parallelepiped. We discuss those features of the method which are important in the investigation of phase transitions. No signature of elastic softening near is observed in . Below and increase continuously. The elastic hardening is accompanied by the onset of a small electro-optical effect. Its existence demonstrates the loss of the centre of symmetry, its anisotropy is consistent with a triclinic symmetry, and its magnitude suggests a spontaneous polarization of about in the low-temperature phase. Although is reminiscent of a weak ferroelectric material there are some observations which are uncommon to a paraelectric - ferroelectric phase transition.