LINEAR BIREFRINGENCE BEHAVIOR OF A(2)BX(4)-TYPE FERROIC CRYSTALS IN THE NORMAL-INCOMMENSURATE PHASE-TRANSITION

Critical behavior of linear birefringence was studied in the normal-in commensurate phase transition of various A(2)BX(4)-type crystals. We c onsidered the fluctuation effect in the scaling analysis so that we co uld obtain a precise value of the critical exponent beta from linear b irefringence measurements. The numerical values of 2 beta obtained fro m our experiment were 0.72 similar to 0.75 (+/-0.02). These results ar e in agreement with the previous theoretical prediction which suggests that the normal-incommensurate phase transition of A(2)BX(4)-type cry stals belongs to the universality class of the three-dimensional (3D) XY model. Beyond the critical region, we observed crossover from 3D XY -like behavior into classical Landau-like behavior, 2 beta=1, in Rb2Zn Cl4 and K2ZnCl4.