Defects-Induced Density of Acoustic States in Raman Spectra of Congruent LiNbO3

We investigate Raman spectra and quasi-elastic light scattering in stoichiometric and congruent lithium niobate crystals at 296 K. We carry out a quantitative analysis of the spectra in the frequency range 0 – 70 cm − 1 for samples with different values of Q factor in a model, where coupling of the low-frequency A 1 (TO)-symmetry optical mode with a relaxing self-energy describing stoichiometric defects is taken into account. We show that a wide band in the frequency range 60 – 120 cm belongs to the density of acoustic states. The results obtained by comparing the model calculations with the experiment allow us to draw the conclusion that spatially-extended defects play a significant role in the formation of a dynamic central peak during a structural phase transition in congruent lithium niobate crystals. At the same time, defects provided due to the substitution of lithium atoms by niobium atoms and impurity atoms induce violation of the selection rules for the wave vector in the Brillouin zone and registration of the acoustic density of states in the Raman spectrum.