BIREFRINGENCE STUDY OF THE FERROELASTIC PHASE-TRANSITION OF K2ZNCL4

The linear birefringence of a K2ZnCl4 single crystal was measured accurately. We observed a discontinuity at 150 K as well as a deviation from linear behavior at 180 K. The discontinuity at 150 K agreed with the first-order ferroclastic phase transition as previously reported. However, temperature dependence curve of the linear birefringence began to deviate gradually from the regular linear thermo-optic behavior at 180 K. This suggests that a spontaneous strain has appeared at that temperature and crisis even above the first-order phase transition temperature.