Ferroelastic phase transition in BiVO4I. Birefringence measurements using the rotating-analyser method

A brief review of methods of measuring birefringence is given. Apparatus for the accurate measurement of optical birefringence at high temperatures using a modulation (rotating-analyser) method is described in detail, together with suggestions for further applications of the equipment. The technique is applied to the particular case of the ferroelastic monoclinic–tetragonal phase transition in BiVO4. The birefringence was found to show mean-field behaviour over a wide temperature range with a mean-field transition temperature of 519.9 (3) K.