Defect Structure Model of MgO-Doped LiNbO 3

Abstract

To study the defect structure of MgO-doped lithium niobate, single crystals of lithium niobates (LiNbO 3, "LN") with varying MgO content were characterized by chemical analysis, lattice parameters, and density measurements. An Mg-incorporation mechanism was assumed on the basis of the chemical formulae derived from the data and in light of our recently proposed defect model of nondoped LN. At first, Mg would replace the Nb ion at the Li site and complete replacement would take place at 3% MgO doping keeping the molar ratio Li/Nb = 0.94. This corresponds to the formula [Li 0.94Mg 0.03□ 0.03] [Nb 1.0]O 3. Further Mg ions are incorporated into the Li site, replacing Li ions, with accompanying vacancy creation, down to Li/Nb = 0.84, which corresponds to the Nb-rich side limit of the LN solid solution range. The number of vacancies would reach a maximum at this composition and the formula would be [Li 0.84Mg 0.08□ 0.08] [Nb 1.0]O 3. Beyond this point, Mg ions enter the Nb and Li sites simultaneously, maintaining the Li/Nb ratio, leading to a decrease in vacancies. Two thresholds in the change of composition and properties reported so far in the literature can be interpreted by this model. Improved optical damage resistance due to MgO-doping was attributed to the increase in vacancies, and not by its decrease as was generally supposed.