Using first principles calculations and atomistic thermodynamics the bulk and defect properties of orthorhombic bismuth titanate (Bi4Ti3O12) and bismuth lanthanum titanate (Bi3.25La0.75Ti3O12) were investigated. Heats of formation, valid chemical conditions for synthesis, lanthanum substitution energies and oxygen and bismuth vacancy formation energies were computed. The study improved the understanding of how native point defects and substitutional impurities influenced the ferroelectric properties of these layered perovskite materials. It was found that lanthanum incorporation could occur on either of the two distinct bismuth sites in the structure and that the effect of substitution was to increase the formation energy of nearby native oxygen vacancies. The results provide direct atomistic evidence over a range of chemical conditions supporting the suggestion that lanthanum incorporation reduces the oxygen vacancy concentration. Oxygen vacancies contribute to ferroelectric fatigue by interacting strongly with domain walls, and therefore a decrease in their concentration was beneficial. The conditions that favour the greatest reduction in oxygen vacancy concentration were described.

Ab initio Energetics of Lanthanum Substitution in Ferroelectric Bismuth Titanate. S.H.Shah, P.D.Bristowe: Journal of Physics - Condensed Matter, 2011, 23[15], 155902