Raman spectroscopy and X-Ray analysis of non-stoichiometric lithium niobate crystals

Lithium niobate single crystals of congruent composition and doped with various impurities are investigated using Raman spectroscopy and X-ray analysis. Raman scattering spectra and X-ray diffraction (XRD) patterns of the studied crystals contain weak reflections forbidden by the R3c space group of symmetry. These superstructure lines are not associated with a new crystalline phase. It is found that neither the increasing of nonstoichiometry of the grown crystals, nor increasing the concentration of dopants, nor the type of dopant, nor the doping method of batch affect the intensity and position of the additional reflections on the XRD patterns. The frequencies and the intensities of the superfluous lines in the Raman spectra are also independent of these parameters. It can be caused by the formation of a superstructural hexagonal sublattice with a double unit cell parameter due to the ordering of defects.