Optical gyration G, birefringence Δn, and their electric-field E dependences of transparent ceramics of Pb0.92La0.08(Zr0.70Ti0.30)0.98O3 were studied by using high accuracy universal polarimeter in a temperature range between -10 °C and 90°C. The high-temperature phase manifested neither spontaneous nor induced G by applying the biasing E, while the low-temperature phase did characteristic hysteresis loops of G, and Δn with respect to E. Group-theoretical consideration of gyro-optical properties of both phases led us to the conclusion that the high- and low-temperature phases are cubic Th and trigonal C3. They transform into each other at Tc=10.3 °C with essentially second but apparently first order. The free-energy function which drives the transition was determined by using electro-optic phenomena occurring in the phase-transition region. Calculated and observed values of G and Δn as a function of E were in excellent agreement in the transition region. However, they began to deviate from 50 °C and the deviations became serious above 80 °C. Brief comments are given on the anomalous dielectric behavior of lanthanum modified lead zirconate titanate ceramics.
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