Manifestation of a photorefractive effect in Raman spectra of lithium niobate crystals of different compositions

The ordering of structural units in a cation sublattice and the photorefractive properties of lithium niobate single crystals of different compositions: nominally pure with different [Li]/[Nb] ratios and doped with non-photorefractive cations Mg²⁺, Gd³⁺, and Y³⁺, are studied by their Raman spectra. It is shown that at low concentrations of Mg²⁺, Gd³⁺, and Y³⁺, the magnitude of the photorefractive effect is determined by the ordering of the structural units of the cation sublattice. It is found for the first time that the intensity of a Raman line corresponding to the bridge valence vibrations of oxygen atoms in the NbO₆ octahedra is sensitive to the dipole ordering of the cation sublattice.