LiTaO₃ crystals with near-zero birefringence

A study of vapour transport equilibrated crystals of LiTaO₃ shows periodic fringes that change with temperature. The origin of these fringes and their behaviour is explained in terms of domains with slanted walls. Micro-Raman measurements have been used to determine the Li₂O concentration through the crystals, and by comparison with the observed fringe pattern the variation of birefringence with Li₂O content has been determined.