Raman spectra of lithium niobate crystals heavily doped with zinc and magnesium

Resumo

We have examined the Raman spectra of heavily doped lithium niobate single crystals (at close-to-threshold concentrations of doping cations): LiNbO₃:Zn (4.5 mol % ZnO), LiNbO₃:Mg (5.01 mol %):Fe (0.005 mol %), LiNbO₃:Mg (5.1 mol %), and LiNbO₃:Mg (5.3 mol % MgO). Low-intensity lines with frequencies at 209, 230, 298, 694, and 880 cm^–1 have been revealed for the first time. Analysis of the data from the literature on lattice dynamics calculations from first principles (ab initio) does not make it possible to unambiguously state that these lines correspond to fundamental vibrations of the А₂ symmetry species, which are forbidden for the С_3V⁶ (R3c) space group. At the same time, ab initio calculations unambiguously indicate that the experimentally observed low-intensity “superfluous” lines with the frequencies at 104 and 119 cm^–1 cannot correspond to vibrations of the А₂ symmetry species. It is most likely that they correspond to two-particle states of acoustic phonons with a total wave vector equal to zero.