Defect structures and the MgO-doping-level-threshold effect on the optical absorption of reduced MgO-doped lithium niobate

Congruent LiNbO₃ crystals have been doped with Mg impurity to levels of 1.0, 2.7, 5.0, and 6.0 mol %. With doping levels between 5.0 and 6.0 mol %, there was an abrupt change in the features of the optical absorption. This observation is consistent with our theoretical calculation, which independently predicted that the critical magnesium concentration, [Mg], for the threshold effect should occur at [Mg]c=5.3 mol %. The spectra exhibit anisotropy. Defect-structure models are proposed. We demonstrate that, under thermochemical reduction, the density of the crystal increases and oxygen vacancies are also formed. We propose that when congruent crystals are thermochemically reduced, the observed optical absorptions are due to defects comprising both oxygen vacancies and different types of dipolarons.