Ultrasonic and Piezoelectric Investigations of Phase Transitions in Ferroelastic Cu6PS5(I,Br) Mixed Crystals

The ultrasonic velocity and attenuation anomalies were observed near the elastic phase transitions (PT) in the vicinity of 270 K in Cu6PS5(I, Br) mixed superionic crystals, where I was partially substituted by Br. We have shown that elastic anomalies at phase transition gradually decrease with increasing of iodine concentration in the material. The critical anomalies were described by relaxational Landau theory, which implies the increase of order parameter relaxation time: tau = tau(o)/(T-c-T). The prefactor tau(o) increases with increasing iodine content in crystal. The ultrasonic anomalies near low temperature superionic phase transitions were weak. These transitions were observed using piezoelectric detection of ultrasonic waves by surface barrier. The temperature dependencies of detected by surface barrier signal were measured and it was shown that this method is very sensitive for indication of phase transitions in such superionic crystals.