COMPARATIVE-STUDY OF DEFECT STRUCTURES IN LITHIUM-NIOBATE WITH DIFFERENT COMPOSITIONS

Structure refinements were conducted on LiNbO3 crystals with four different compositions, ranging from near stoichiometric ( Li (Li + Nb) = 0.498) to highly nonstoichiometric ( Li (Li + Nb) = 0.470), by the X-ray single-crystal diffraction and the TOF neutron powder diffraction methods to clarify the major defect mechanism of LiNbO3 governing its nonstoichiometry. Two models-"Li-site vacancy model" and "Nb-site vacancy model"-were chosen on the basis of density data and examined in the refinements. The former is expressed as [Li1-5xNbx□4x][Nb]O3 and the latter [Li1-5xNb5x] [Nb1-4x□4x]O3, where □ denotes a vacancy. The refinement results revealed that the amount of Nb occupancy was composition-independent and that Li+ ions were replaced by the Nb5+ ions, creating vacancies at the Li site. Rietveld analysis of the neutron diffraction data was consistent with the X-ray refinement. These results strongly support the Li-site vacancy model. © 1992.