LINEAR BIREFRINGENCE STUDIES IN [(CH3)(2)NH2]5CD3CL11 CRYSTAL

Linear birefringence measurements have been performed for the [(CH3)(2 )NH2]5Cd3Cl11-(DMACC) crystal. A diffused anomaly of the linear birefr ingence in a temperature range 250-320K and a very distinct anomaly ty pical for a second order phase transition was found at 178.5 K. A sequ ence of phase transitions: of a second order at a T-2 = 178.5 K, and o f a first order at a T-3 = 127 K, characteristic for crystals with inc ommensurate phases was confirmed. Critical index values and a hysteret ic behaviour of the linear birefringence also indicated a possibility of the incommensurate phase existence in the DMACC crystal.