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A large experimental body of literature on lithium niobate, a technologically important ferroelectric, suggests that nonstoichiometric defects dominate its physical behavior, from macroscale switching to nanoscale wall structure. The exact structure and energetics of such proposed intrinsic defects and defect clusters remains unverified by either first-principles calculations or experiments. Here, density functional theory (DFT) is used to determine the dominant intrinsic defects in Li Nb O 3 under various conditions. In particular, in an Nb 2 O 5-rich environment, a cluster consisting of a niobium antisite compensated by four lithium vacancies is predicted to be the most stable defect structure, thereby verifying what was thus far a conjecture in the literature. Under Li 2 O-rich conditions, the lithium Frenkel defect is predicted to be the most stable, with a positive defect formation energy (DFE). This is proposed as the underlying …