Ab initio energetics of lanthanum substitution in ferroelectric bismuth titanate

Abstract

Using first principles calculations and atomistic thermodynamicsthe bulk and defect properties of orthorhombic bismuth titanate(Bi4Ti3O12) and bismuthlanthanum titanate (Bi3.25La0.75Ti3O12) have been investigated. Heats of formation, valid chemical conditions for synthesis,lanthanum substitution energies and oxygen and bismuth vacancy formation energies havebeen computed. The study improves our understanding of how native point defects andsubstitutional impurities influence the ferroelectric properties of these layered perovskitematerials. It is found that lanthanum incorporation could occur on either of the twodistinct bismuth sites in the structure and that the effect of substitution is to increase theformation energy of nearby native oxygen vacancies. The results provide directatomistic evidence over a range of chemical conditions supporting the suggestionthat lanthanum incorporation reduces the oxygen vacancy concentration. Oxygenvacancies contribute to ferroelectric fatigue by interacting strongly with domainwalls, and therefore a decrease in their concentration is beneficial. The conditionsthat favor the greatest reduction in oxygen vacancy concentration are described.