TEMPERATURE DEPENDENCE OF THE OPTICAL PROPERTIES OF FERROELECTRIC LiNbO3 AND LiTaO3

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, T_c = 1195°C ± 15°C for LiNbO₃, and T_c = 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 $\text{R3̄c}$ .