Features of the Defect Structure of the Compositionally Homogeneous Crystal LiNbO:Er(3.1 wt%) and the Gradient Crystal LiNbO:Er and Their Manifestation in the IR Transmission Spectra in the Region of Stretching Vibrations of Hydrogen Atoms of OH-Groups.

Based on the analysis of the IR transmission spectra in the region of stretching vibrations of hydrogen atoms of OH-groups, it was established that the oxygen-octahedral MeO clusters (Me-Li, Nb, vacant octahedron V, impurity ion) of the structure of the compositionally homogeneous crystal LiNbO:Er(3.1 wt%) and the gradient crystal LiNbO:Er(congruent composition by the main components, Er gradient of 0.55 at%/cm) have a shape close to the regular one. In this case, the value of R = [Li]/[Nb] ≈ 1, and in the structure of both crystals, there are practically no point defects in Nb responsible for the photorefraction effect. By using the IR transmission spectra and Klauer's method, it was found that the volume concentration of OH-groups in the gradient crystal LiNbO:Er is almost an order of magnitude lower than in the compositionally homogeneous LiNbO:Er(3.1 wt%) crystal. This fact explains the lower hydrogen conductivity of the gradient crystal LiNbO:Er and the lower photorefraction effect compared to the compositionally homogeneous LiNbO:Er(3.1 wt%) crystal. The results obtained are important for the development of materials for active nonlinear laser media and for the conversion of laser radiation.