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

It is found that in LiTaO₃ crystals with compositions between congruent and stoichiometric and in mixed LiNb_xTa_1-xO₃ crystals self-organized stripes appear whose widths vary with temperature. They are broad and indistinct when the birefringence is close to zero and become narrower as the temperature is decreased or increased.