DEFECT STRUCTURE MODEL OF MGO-DOPED LINBO3

To study the defect structure of MgO-doped lithium niobate, single cry stals of lithium niobates (LiNbO3, ''LN'') with varying MgO content we re 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 rec ently proposed defect model of nondoped LN. At first, Mg would replace the Nb ion at the Li site and complete replacement would take place a t 3% MgO doping keeping the molar ratio Li/Nb = 0.94. This corresponds to the formula [Li(0.94)Mg(0.03)square(0.03)][Nb-1.0]O-3. Further Mg ions are incorporated into the Li site, replacing Li ions, with accomp anying vacancy creation, down to Li/Nb = 0.84, which corresponds to th e Nb-rich side limit of the LN solid solution range. The number of vac ancies would reach a maximum at this composition and the formula would be [Li(0.84)Mg(0.08)square(0.08)][Nb-1.0]O-3. Beyond this point, Mg i ons enter the Nb and Li sites simultaneously, maintaining the Li/Nb ra tio, 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. (C) 1995 Academic Press, Inc.