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Linear and nonlinear optical investigations of LiNbO3 and LiTaO3 from room temperature to temperatures above their ferroelectric phase transitions are described. The measurements include studies of second harmonic generation with an Nd laser (1.06 μ) and determinations of the refractive indices as well as the birefringence at 0.6328 μ. All the optical data exhibit distinct changes at the Curie temperatures, Tc = 1195°C ± 15°C for LiNbO3, and Tc = 610°C ± 10°C for LiTaO3. In each case the phase change has the characteristics of a second-order ferroelectric transition. The uniaxial paraelectric phases are concluded to be centrosymmetric — probably space group R3̄c.
Miller et al. (Mon,) studied this question.