A series of six LiNbO3:Tb ([Tb] = 0.1 — 2.89 wt%) crystals and seven LiNbO3:Er ([Er] = 0.08÷2.71 wt%) crystals were grown according to a single methodology. Optical uniformity and optical resistance were compared in LiNbO3:Tb and LiNbO3:Er of various chemical composition. The periods of the crystal lattice were determined by the methods of a full -profile analysis of the XRD patterns of polycrystals; models of the atomic structure of LiNbO3:Tb and LiNbO3:Er crystals were analyzed with a change in the dopant concentration. In a series of LiNbO3:Tb crystals, a concentration threshold near the concentration of terbium ~ 2.2 — 2.3 wt% was discovered for the first time. A concentration threshold near the concentration of erbium ~ 2.4 — 2.5 wt% was discovered in LiNbO3:Er. In the area of the concentration threshold, pronounced anomalies of physicochemical, optical and structural characteristics are observed. Growth irregular and regular domain microstructures were revealed in as-grown LiNbO3:Tb and LiNbO3:Er crystals by optical and atomic force microscopy. Structural characteristics and threshold effects are studied in LiNbO3:Gd and LiNbO3:Gd,Cu crystals by Raman spectroscopy, PILS, laser conoscopy and optical microscopy. In LiNbO3:Gd crystals the photorefractive effect is suppressed at as low concentration as [Gd] = 0.05 wt%. For LiNbO3:Gd,Cu crystals a distinct photorefractive response is observed, it increases with increasing Cu concentration. The effect of the association of defects (carriers) is experimentally confirmed and thermodynamically justified by the example of oxygen-octahedron structures such as perovskite and pseudoi:lmental during ion conduction in a certain temperature interval. The discovered phenomenon is extremely important for creating and evaluating the temperature range of operability of ion current sources.

The values of the photovoltaic (Epv) and diffusion (ED) fields were calculated for LiNbO3stoich, LiNbO3cong, LiNbO3 : Zn (0.04–2.01 mol. % ZnO in the crystal) and LiNbO3 : B (0.55 and 0.83 mol. % B2O3 in the charge) crystals according to the parameters of the indicatrix of photoinduced light scattering. It is shown that the ED contribution to the optical damage is maximum for LiNbO3stoich crystal. This indicates an increased concentration of shallow electron traps in LiNbO3stoich as compared to other studied crystals.