Comparative study of defect structures in lithium niobate with different compositions

Structure refinements were conducted on LiNbO 3 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 LiNbO 3 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 [Li 1−5 x Nb x □ 4 x ][Nb]O 3 and the latter [Li 1−5 x Nb 5 x ] [Nb 1−4 x □ 4 x ]O 3, 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 Nb 5+ 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.