The effect of stoichiometry and Mg doping on the Raman spectra of LiNbO3:Mg crystals

Abstract

LiNbO3:Mg crystals doped with 0–8 mol. % Mg with stoichiometric, intermediate and congruent compositions were systematically investigated by Raman spectroscopy in backscattering y(zx)y, y(zz)y and z(xx)z geometries. The damping was found to be a very sensitive parameter for the characterization of the crystal composition. The half-widths of E(TO3)–E(TO9) and A 1(TO1)–A 1(TO4) bands having significant composition dependence for the undoped LiNbO3 crystals show only a weak Mg concentration dependence below the photorefractive threshold, which is a consequence of the counteracting effect of the decreasing NbLi and increasing MgLi contents. The half-widths of the bands, however, increase linearly with growing Mg content for samples above the threshold, irrespective of the Li/Nb ratio. The change in the Mg concentration dependence at a given Li/Nb ratio determines the same threshold value as that concluded from IR and UV spectroscopic measurements. The half-width of the main A 1(LO4) band at 873 cm-1 increases linearly with growing Mg concentration, but no threshold effect is observed. However, the ratio of the area of the main band and the high-frequency sideband shows a threshold effect that can be interpreted by the existing defect incorporation models. The small Raman band at about 740 cm-1 attributed earlier to NbLi vibration is also detected in above-threshold LiNbO3:Mg crystals, which can be explained by the vibration of Nb ions in Mg4Nb2O9 defect clusters appearing at high Mg concentrations.