| Summary: | Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm<sup>−1</sup>. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A<sub>2</sub> symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C<sub>3V</sub><sup>6</sup> (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm<sup>−1</sup> definitely do not correspond to vibrations of A<sub>2</sub> symmetry type. We have paid special attention to these extra bands that appear in LiNbO<sub>3</sub> single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05−0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.
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