<p>Optical microscopy studies have demonstrated that the chemical composition of the Li₂O–Nb₂O₅–Tb₂O₃ melt has a significant effect on the macro- and microdomain configuration of the growth defect structure in the bulk of the nonlinear optical LiNbO₃ single crystal doped with subthreshold concentrations of the Tb³⁺ ions, which is promising as an active nonlinear laser medium for continuous-wave generation in the blue spectral range. In the frequency range of 1000–2000&#xa0;cm⁻¹, second-order Raman lines were detected in LiNbO₃:Tb³⁺(0.1, 0.48, 2.21 wt%) crystals, the intensity and frequencies of which depend on the concentration of the Tb³⁺ ions. These lines are interpreted as manifestations of bound states of fundamental optical vibrations formed as a result of strong anharmonicity of vibrations and the presence of various types of structural defects in the crystal. It is shown that the presence of lines with frequencies of 3470, 3483, and 3486&#xa0;cm⁻¹ in the IR absorption spectra of LiNbO₃:Tb³⁺(0.1, 0.48, 2.21 wt%) crystals in the region of OH-group stretching vibrations is due to the violation of the stoichiometry (Li/Nb values) of the crystal and the formation of complex defects (V_Li-OH). It is established that in the studied LiNbO₃:Tb³⁺ crystals, there are two times fewer V_Li point defects than in the congruent LiNbO₃ crystal.</p>