Ultrasonic anomalies in betaine phosphate/betaine phosphite mixed crystals Free

Measurements of the longitudinal ultrasonic velocity and the attenuation along the x, y, and z axes have been carried out in deuterated betaine phosphite (DBPI) single crystals in the vicinity of the ferroelectric and antiferrodistortive phase transitions. For ultrasonic modes propagating along the x and z axes the critical slowing down has been observed, but along the y axis critical slowing down was absent for both phase transitions. Regardless of the weakness of elastic anomalies at ferroelectric phase transition the ultrasonic behavior in DBPI can be explained by the Landau–Khalatnikov theory for uniaxial ferroelectrics. The ultrasonic behavior at the antiferrodistortive phase transition indicates that this phase transition is very close to the tricritical one. The relaxation time of the order parameter in the vicinity of the transition to the antiferodistortive phase for DBPI crystal was estimated. Similar results have been obtained for the mixed crystals betaine phosphate/betaine phosphite in the region of the antiferrodistortive phase transition. The temperature behavior of the ultrasound velocity and attenuation indicates the existence of the ionic conductivity in these cystals. It should be noted that conductivity phenomena is highly anisotropic in these crystals.