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

In recent years a number of imaging techniques to determine the optical pro perties of materials, either in reflection or in transmission, have been de veloped. Here the use of an imaging version of the so-called rotating-polar izer method in the study of phase transformations in crystals is demonstrat ed. This method creates false-coloured images representing the light transm ission I-0, the extinction angle phi (orientation of the optical indicatrix ) and |sin delta, a function of the retardation resulting from the birefrin gence (and a measure of the magnitude of optical anisotropy). When combined with a computer-controlled heating stage, this method provides an opportun ity to create separate moving images of orientation and magnitude of optica l anisotropy, showing the dynamics of twinning and domain-wall behaviour du ring temperature changes. It is believed that this is the first time that q uantitative imaging of changes in birefringence has been used in this way t o describe phase transitions. Two-phase transitions in a crystal of Na0.5Bi 0.5TiO3 (NBT) are presented as examples of the use of the system.