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Abstract 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− 5x Nb x□ 4x][Nb] O 3 and the latter [Li 1− 5x Nb 5x][Nb 1− 4x□ 4x] 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 …