Muence of hydrostatic pressure on the temperature dependences of ! he optical buehingence and ulhasonic velocities is studied in the vicinity of Ihe phase transition tempenhues of csZHgBr4, Cs2CdBn and Cs,HgCI~ crystals. The new polycritical niple points, which separate the normal. inmmmeusurate and prop f m l a s t i c phases, have teen found in the P-T phase diagram of all h e wmpounds af applied pressures of 140 Mpa, 100 Mpa and 140 Mpa, respectively. The origin of the triple points and the acoustic and optical pmperties near the phase msitions are discussed within the framework of phenomenological uleory.
The optical activity, birefringence and the indicatrix rotation of a (001) sample of (N(CH3)4)2ZnCl4 are measured in the high-temperature, incommensurate and partly lock-in phases with a new high-accuracy null polarimeter. The optical properties are shown to be sensitive to the annealing and X-irradiation of a sample via the defect concentration changes. The results can be explained by consideration of a modulated gyration tensor in the incommensurate phase, using the idea of modulation wave distortions imposed by defects.
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