Ultrasonic Study of the Li-induced Structural Phase Transition in K 1− x Li x TaO 3

In order to clarify the nature of the possible structural changes taking place in K 1− x Li x TaO 3 (KLT), we have performed an ultrasonic study of three KLT crystals with approximate Li concentrations, 1.5%, 3.5% and 5%. A step increase in the velocity and a corresponding attenuation anomaly clearly indicate structural changes occurring at 30 K, 55 K and 75 K, respectively. In the two higher-concentration crystals, the large magnitude of this change and the observation of thermal hysteresis signal a macroscopic structural transition with a first-order character. In the lower-concentration crystal, a 3 to 5 times smaller effect and the absence of a hysteresis are attributed to local structural distortions without long-range order. This interpretation agrees with the conclusion of a recent Raman study of the same crystals. It provides a direct physical test of this conclusion which was derived from selection rules.