Optical Properties and Defects of Single-Crystal LiNbO₃:Tb³⁺ Plates with a Post-Threshold Concentration of Terbium...

The macro- and microstructures of LiNbO₃:Tb³⁺(2.25 wt%) and LiNbO₃:Tb³⁺(2.9 wt%) crystals were studied. It was shown that a slight increase in terbium content dramatically alters the growth domain structure and enhances fractal self-organization. The defect structure features of LiNbO₃:Tb³⁺(2.25 wt%) and LiNbO₃:Tb³⁺(2.9 wt%) single-crystal plates with an after-threshold terbium concentration were studied using optical absorption spectra, Raman scattering spectra in the frequency range of 65-4000 cm^-1, and IR absorption spectra in the frequency range of 1000-8000 cm^-1. It was shown that the LiNbO₃:Tb³⁺(2.25 wt%) plate has a lower concentration of Nb_Li point defects and a higher stoichiometry compared to the LiNbO₃:Tb³⁺(2.9 wt%) plate. A good match was found between the Raman spectra of the LiNbO₃:Tb³⁺(2.25 wt%) and LiNbO₃:Tb³⁺(2.9 wt%) plates throughout the entire studied range. A significant difference was found in the frequencies of the lines corresponding to stretching vibrations of OH^--groups in the vibrational spectra of the studied plates. This observation is explained by the fact that OH^--groups in the LiNbO₃ crystal structure are defects, and, therefore, the stretching vibrations of OH^--groups are not fundamental lattice vibrations. The existing selection rules for fundamental lattice vibrations, developed on the basis of group theory, do not apply to the defect sublattice.