Ferroelastic phase transition in BiVO₄ II. Birefringence at simultaneous high pressure and temperature

The accurate measurement of optical birefringence at simultaneous high pressures and temperatures using a diamond-anvil pressure cell is described. Data are presented for BiVO₄ which show that the optical birefringence may be described by mean-field behaviour. An applied hydrostatic pressure of only 8 × 10⁸ Pa reduces the mean-field temperature of the ferroelastic monoclinic tetragonal phase transition from 519.7 to 393 K. A birefringence-pressure-temperature phase diagram is presented.