Optical Studies on the Effect of Electric Fields on the Transitions of Barium Titanate

Abstract

The effect of electric fields on the three transition points of barium titanate was studied experimentally by observing the shift of the curve of the thermal transition of transparency with varying biasing fields. Transparent electrodes, thin films of stannic oxide, were used in order to observe the intensity of light transmitted through crystals to which electric fields were applied. The Curie point is raised up to a critical temperature of 132°C, the required strength of the field being 9 kv/cm, and above this point the transition of the first order no longer takes place. From this fact, the coefficient of P_z⁴ in the Devonshire expression of the free-energy function is obtained, which supports the value measured by Drougard. The observed shifts of the Curie point and the 5°C transition point are in good agreement with the values calculated by using the Devonshire function of the sixth order. At the −80°C transition, the experimental value is explained by adding the terms of eighth and tenth powers of polarization to the usual Devonshire function.
The relationship between the polarization of a crystal and the intensity of the transmitted light is pointed out in the case when the crystal is placed between crossed Nichols.
Remarks are also given of the effects which the coating of electrodes has upon the shape of the curve of the thermal transition of transparency at the −80°C transition.