Ultrasonic investigation of elastic phase transitions in betaine compounds

The results of ultrasonic velocity and attenuation measurements in deuterat ed betaine phosphite (DBPI) and its compounds near the antiferrodistortive phase transition (PT) are presented for X, Y and Z crystallographic directi ons. For longitudinal ultrasonic modes propagating along X and Z axes in DB PI the critical slowing down in ultrasonic velocity and attenuation maxima have been observed at PT. Instead, we have not found the critical elastic a nomalies for longitudinal Y mode. In DBP crystals the elastic anomalies app eared at PT for all X, Y and Z modes. In solid solution DBP/DBPI the orient ation dependencies of elastic anomalies varied from those of DBPI to those of DBP. The phase diagram has been obtained from ultrasonic data. The exper imental data in the antiferrodistortive phase have been interpreted in term s of the mean field Landau theory. It is shown that the phase transitions a re very close to the tricritical ones. (C) 2000 Elsevier Science S.A. All r ights reserved.