Temperature Dependence of Photoelastic Effect in KTa1-xNbxO3 Crystals and Investigation of Its Origin

The temperature dependence of the photoelastic effect in KTa 1-x Nb x O 3 crystals was evaluated by measuring the elastic stiffness coefficient $c_{11}$ and the change in the refractive index with stress. The elastic stiffness coefficient $c_{11}$ was measured by using an ultrasonic thickness gauge and the change in the refractive index with the stress was measured by using a Mach--Zehnder interferometer. By considering the effect of the increased departure from the Curie--Weiss law with stress, we succeeded in obtaining a theoretical fit with the measured temperature dependence of the photoelastic effect. The temperature dependence of the photoelastic constant was described by combining the effects of the thermal fluctuation of the ionic and electronic polarizations and the effect of the local polarization.