Curie Temperature and Birefringence Variation in Ferroelectric Lithium Metatantalate as a Function of Melt Stoichiometry

The ferroelectric Curie temperature of lithium metatantalate is linearly dependent on the melt stoichiometry. Curie temperatures ranging from 700° to 540°C were measured on single crystals and correspond to a melt composition variation of approximately 6 mole % on each side of apparent stoichiometry. High Curie temperatures occur at high lithium oxide concentrations. The Curie temperature of LiTaO3 grown from a nominally stoichiometric melt is 618°± 5°C. The birefringence of lithium metatantalate is also a linear function of the melt composition. Birefringence values from +0.006 to –0.004 were measured and correspond to the melt composition variation described above with positive birefringence associated with low Curie temperatures.