In order to study the defect structure of MgO-doped samples, single crystals with various MgO contents were characterized by means of chemical analyses, and lattice parameter and density measurements. A Mg incorporation mechanism was proposed on the basis of the chemical formula that was deduced from the data, and account was taken of a recently proposed defect model for the undoped niobate. It was suggested that, at first, Mg replaced the Nb at Li sites; with complete replacement taking place at a dopant content of 3%MgO, while keeping the Li/Nb molar ratio constant at 0.94. This corresponded to the formula, [Li0.94Mg0.030.03][Nb1.0]O3. Further Mg ions were incorporated at the Li site, thus replacing Li ions (with associated vacancy creation), down to a Li/Nb ratio of 0.84. This corresponded to the Nb-rich limit of the niobate solid-solution range. The number of vacancies was expected to reach a maximum value at this composition, and the formula here was: [Li0.84Mg0.080.08][Nb1.0]O3. Beyond this point, the Mg ions entered the Nb and Li sites simultaneously. This maintained the Li/Nb ratio, while leading to a decrease in the number of vacancies. Two reported thresholds in composition and properties could be explained by this model. An improved optical damage resistance, due to MgO doping, was attributed to an increase in the number of vacancies, rather than to a decrease; as usually supposed.

N.Iyi, K.Kitamura, Y.Yajima, S.Kimura, Y.Furukawa, M.Sato: Journal of Solid State Chemistry, 1995, 118[1], 148-52