Defect structure dependence on composition in lithium niobate.

X-ray diffraction studies show full atomic order in stoichiometric LiNbO3. In the congruent composition [Li0.941NbO2.971], 6% Li is missing from all Li-sites. Each missing Li(+) ion is replaced by a Nb(5+) ion, with compensating vacancies at both Nb and O sites maintaining charge neutrality. Lithium vapor phase equilibration at 1373K in congruent material is postulated to involve Nb ion migration by about 3.1angstrom through an oxygen atom octahedral face. The Nb ion displacement of 0. 2786angstrom in stoichiometric LiNbO3 [0.2692angstrom in congruent material] along the polar axis from the midposition between oxygen atom planes allows the Curie temperature to be predicted as 1532 [1449]K; the experimental temperature is 1471 [1402]K.