Stability of intrinsic defects and defect clusters in LiNbO3 from density functional theory calculations

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 LiNbO3 under various conditions. In particular, in an Nb2O5-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 Li2O-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 reason that the vapor-transport equilibration (VTE) method can grow stoichiometric LiNbO3. The effects of temperature and oxygen partial pressure are also explored by combining the DFT results with thermodynamic calculations. These predictions provide a picture of a very rich defect structure in lithium niobate, which has important effects on its physical behavior at the macroscale.