Birefringence imaging of phase transitions: application to Na0.5Bi0.5TiO3

Abstract

In recent years a number of imaging techniques to determine the optical properties of materials, either in reflection or in transmission, have been developed. Here the use of an imaging version of the so-called rotating-polarizer method in the study of phase transformations in crystals is demonstrated. This method creates false-coloured images representing the light transmissionI0, the extinction angle φ (orientationof the optical indicatrix) and |sin δ|, a function of the retardation resulting from the birefringence (and a measure of themagnitudeof optical anisotropy). When combined with a computer-controlled heating stage, this method provides an opportunity to createseparatemoving images oforientationandmagnitudeof optical anisotropy, showing the dynamics of twinning and domain-wall behaviour during temperature changes. It is believed that this is the first time that quantitative imaging of changes in birefringence has been used in this way to describe phase transitions. Two-phase transitions in a crystal of Na0.5Bi0.5TiO3(NBT) are presented as examples of the use of the system.