Critical ultrasonic behavior near phase transitions in BCCD crystals

Abstract The ultrasonic velocity and attenuation of longitudinal waves have been measured along the a, b and c directions as a function of frequency (10 – 70 MHz) and temperature (110 – 300 K) in single - crystal betaine calcium chloride dihydrate. The ultrasonic anomalies were observed at incomplete devil's staircase transition temperatures. The largest anomaly appeared at the normal - incommensurate phase transition at Ti ≈ 164 K Landau theory has been used to explain the experimental data below Ti. From attenuation data, the relaxation time of the amplitudon in the incommensurate phase has been estimated to be τ = (1.3 · 10−13) f 1, where t = (Ti-T)/Ti . In addition, the critical ultrasonic velocity behavior in the hydrodynamic region (ωτ≪ 1) was analyzed in terms of a scaling model. The critical - exponent value was found to be μ ≈ 0.3.