Crystal structure, defect chemistry and oxygen ion transport of the ferroelectric perovskite, Na0.5Bi0.5TiO3: insights from first-principles calculations

Recent experimental studies have shown how A-site nonstoichiometry in Na0.5Bi0.5TiO3 (NBT) can dramatically alter its electrical properties and conduction mechanisms. In nominal NBT and Bi-deficient NBi0.49T, electrical conductivity primarily comes from oxygen ion conduction and not electronic conduction. This is contrary to the behaviour of traditional titanates and could potentially give NBT a role in the design of new intermediate-temperature solid oxide fuel cells (SOFCs). In this study, we use density functional theory (DFT) with the Hubbard U correction to investigate the much debated local structure and defect chemistry of NBT, with the primary focus on oxygen vacancy formation and oxygen ion transport. We confirm significant cation and oxygen displacement in both the Cc and R3c structures. Small ordering energies confirm an essentially random distribution of A-site ions. Oxygen vacancies are shown to preferentially form in the vicinity of Bi ions, confirming that weak Bi-O bonds do indeed promote oxygen ion migration. Nudged elastic band (NEB) calculations predict oxygen migration energies that are in excellent agreement with experiment. Oxygen ion migration to Bi-rich chemical environments is generally shown to be unfavourable, while migration to Na-rich chemical environments produces lower migration energies. Our calculations confirm many of the assumptions recently reported in experimental studies and provide a far greater understanding into the unique and complex defect chemistry of NBT than is currently available.