Comparative study of the singularity in the optical properties of congruent doped LiNbO3 crystals

A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d33, of congruent LiNbO3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of no and ne are described by a generalised Sellmeier equation considering the different ions sites in the crystal.