Features of the Defect Structure of the Compositionally Homogeneous Crystal LiNbO3:Er3+(3.1 wt%) and the Gradient Crystal LiNbO3:Er3+ and Their Manifestation in the IR Transmission Spectra in the Region of Stretching Vibrations of Hydrogen Atoms of O

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 MeO₆ clusters (Me-Li⁺, Nb⁵⁺, vacant octahedron V, impurity ion) of the structure of the compositionally homogeneous crystal LiNbO₃:Er³⁺(3.1 wt%) and the gradient crystal LiNbO₃:Er³⁺(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_Li 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 LiNbO₃:Er³⁺ is almost an order of magnitude lower than in the compositionally homogeneous LiNbO₃:Er³⁺(3.1 wt%) crystal. This fact explains the lower hydrogen conductivity of the gradient crystal LiNbO₃:Er³⁺ and the lower photorefraction effect compared to the compositionally homogeneous LiNbO₃:Er³⁺(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.