Linear birefringence behavior of (NH₄)₂ZnCl₄ and [N(CH1)₄]₂CuCl₄ near the normal-incommensurate phase transition

Linear birefringence (LB) behavior of (NH₄)₂ZnCl₄ and [N(CH₃)₄]₂CuCl₄ was studied in the critical region of normal-incommensurate phase transition. By scaling analysis of the linear birefringence behavior, we can determine critical exponent 2β = 0.73 and 0.76 for (NH₄)₂ZnCl₄ and [N(CH₃)₄]₂CuCl₄, respectively. These results are consistent with our previous studies of other A₂BX₄-type crystals, and can be well explained by 3-d XY model. In addition, our results allude that there is no symmetry breaking effect in the birefringence behavior. The existence of LB tail, which can be deduced from fluctuation effect, was observed above the transition temperature.