Optical birefringence and acoustic properties near the phase transitions and triple point in incommensurate proper ferroelastic Cs₂HgBr₄, Cs₂CdBr₄ and Cs₂HgCl₄ crystals

The influence of hydrostatic pressure on the temperature dependence of the optical birefringence and ultrasonic velocities is studied in the vicinity of the phase transition temperatures of Cs₂HgBr₄, Cs₂CdBr₄ and Cs₂HgCl₄ crystals. The new polycritical triple points which separate the normal, incommensurate and proper ferroelastic phases have been found in the P-T phase diagrams of all the compounds at applied pressures of 140 MPa, 100 MPa and 140 MPa, respectively. The origin of the triple points and the acoustic and optical properties near the phase transitions are discussed within the framework of phenomenological theory.