Third-Order Elastic Constants in a Critical Region of Temperature

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Single crystals of perovskite-type crystal CsPbCl_3 were grown by the modified Stockbarger method. Two cubic samples of about 1 cm^3 were obtained in the present study. Ultrasonic wave velocities of CsPbCl_3 have been measured above the cubic-tetragonal phase trnsition temperature under hydrostatic pressure by the pulse-echo overlap method. Ultrasonic waves of the longitudinal or the transverse mode were generated by quartz transducers of fundamental frequency of 10 MHz.The combinations of third-order elastic constants C_<111>+2C_<112>'C_<111>-C_<123> and C_<144>+2C_<166> were deduced from pressure derivatives of the sound velocity over the temperature interval 47-100ﾟC.Critical anomalies were found in the temperature dependence of C_<111>+2C_<112> and C_<111>-C_<123>. It has been revealed that the order-parameter fluctuation is responsible for the observed anomaly in the third-order elastic constants .