It is found that the speckle structure of the photoinduced light scattering indicatrix of the LiNbO3:Y3+(0.46 wt%) crystal and its behavior with the time of crystal irradiation with a laser undergo an atypical behavior caused by the features of the dissipation processes of laser-induced defects in the crystal. In the frequency range of 100–4000 cm−1, the Raman spectra of the LiNbO3:Y3+(0.46 wt%) single crystal were recorded upon excitation by visible (532 nm) and near-IR (785 nm) laser radiation. Five second-order Raman scattering lines were detected in the frequency range of 1000–2100 cm−1, with the frequencies of two of them (of about 1790 and 1940 cm−1) somewhat exceeding the doubled value of the frequencies of fundamental vibrations of the 4A1(z)LO (longitudinal optical) and 9E(x,y) symmetry types, which allows us to attribute these lines to the overtones of the fundamental vibrations of 4A1(z)LO and 9E(x,y). It is found that only one Raman scattering line is observed in the region of stretching vibrations of OH-groups (3200–3800 cm−1). The frequency of this line is found to depend on the scattering geometry, varied within 3431–3438 cm−1, and to be shifted to the low-frequency region by about 30–50 cm−1 relative to the frequencies in the IR absorption spectrum. This finding may be due to the alternative prohibition rule due to the presence of the center of symmetry of the oxygen octahedra O6 of the crystal structure.
Sidorov et al. (Thu,) studied this question.