Ultrasonic investigation of elastic phase transitions in betaine compounds

The results of ultrasonic velocity and attenuation measurements in deuterated betaine phosphite (DBPI) and its compounds near the antiferrodistortive phase transition (PT) are presented for X, Y and Z crystallographic directions. For longitudinal ultrasonic modes propagating along X and Z axes in DBPI the critical slowing down in ultrasonic velocity and attenuation maxima have been observed at PT. Instead, we have not found the critical elastic anomalies for longitudinal Y mode. In DBP crystals the elastic anomalies appeared at PT for all X, Y and Z modes. In solid solution DBP/DBPI the orientation dependencies of elastic anomalies varied from those of DBPI to those of DBP. The phase diagram has been obtained from ultrasonic data. The experimental data in the antiferrodistortive phase have been interpreted in terms of the mean field Landau theory. It is shown that the phase transitions are very close to the tricritical ones.