Manifestation of defects and imperfections of lithium niobate crystals in Raman spectra

We discuss the main reasons for the appearance of low-intensity ‘extra’ lines (not corresponding to fundamental lattice vibrations) in the Raman spectrum of nonstoichiometric nonlinear optical $\rm LiNbO_3$ crystals. These lines arise due to the formation of two-particle states of acoustic crystal vibrations with a zero total wave vector, strong anharmonicity of fundamental vibrations, the presence of microstructures (clusters) whose structure differs from that of the crystal matrix, the presence of microinclusions of impurity phases of other lithium niobates, and the photorefraction effect. It is shown that ‘extra’ Raman spectral lines can serve as a new highly sensitive tool for studying the structural perfection of $\rm LiNbO_3$ crystals.