Temperature investigations of Raman spectra of stoichiometric and congruent lithium niobate crystals

In the temperature range 100–450 K, we have investigated Raman spectra of congruent and stoichiometric LiNbO3 crystals. We have found that, in this temperature range, frequencies and widths of all the spectral lines depend linearly on temperature. However, the width of the line that corresponds to vibrations of the A1(TO) symmetry of Li+ ions depends on temperature much more weakly than the width of the line that corresponds to vibrations of the A1(TO) symmetry of Nb5+ ions. This fact indicates that the anharmonicity of vibrations of Nb5+ ions along the polar axis is much stronger compared to vibrations of Li+ ions. It is likely that this anharmonicity is noticeably contributed by O2− ions, which are characterized by an anharmonic potential, vibrations of which, according to calculations from first principles, are mixed with vibrations of Nb5+ ions. The anharmonicity of vibrations of O2− ions is evidenced by a strong temperature dependence of the width of the line that corresponds to vibrations of the A1(TO) symmetry of O2− ions perpendicularly to the polar axis. We have found that the temperature dependence of the intensity of lines that correspond to fundamental vibrations is nonmonotonic. At the same time, the temperature dependence of the intensity of “superfluous lines” is strictly linear. It is likely that this behavior of the intensities of lines of fundamental vibrations is related to the occurrence of clusters and microstructures in the crystal structure.