Observation of unusual temperature-dependent stripes in LiTaO₃ and LiTa_xNb_1-xO₃ crystals with near-zero birefringence

A birefringence imaging microscopy study of LiTaO₃ and some mixed LiTa_xNb_1-xO₃ crystals is reported. The initially congruent LiTaO₃ crystals have been subject to vapour transport equilibration in order to increase the Li content and at the same time to reduce the optical birefringence close to zero at room temperature. In both types of crystal, the presence of unusual periodic self-organized stripes, existing over large distances, has been observed, which correlate with the well documented cleavage and twin planes in this material. Furthermore, the stripes become narrower as the temperature is changed away from the zero-birefringence temperature, but become temperature-independent above the ferroelectric-paraelectric phase-transition temperature.