Study of the Ferroelastic Phase Transition of TlH2PO4Acoustic Phonon Softening

Abstract

Elastic, thermal and dielectric measurements of TlH 2 PO 4 were carried out in a temperature range covering the upper phase transition point at 357 K. The specific heat shows a λ-type anomaly associated with the upper phase transition of second order and yields the transition entropy Δ S =1.46 J/mol·K. The elastic stiffness c 55 among c i i ( i =1, 2,···, 6) determined from the ultrasonic velocity data at a frequency of 10 MHz shows a large anomaly with a tendency to approach zero in value at the upper phase transition point. It was shown that the upper, ferroelastic phase transition is caused by an instability of the B 2g transverse acoustic mode, and the elastic, thermal and dielectric behaviours near the ferroelastic phase transition point can be reasonably explained by a phenomenological theory.