Co-Substituted BiFeO3: Electronic, Ferroelectric, and Thermodynamic Properties from First Principles

Abstract

Bismuth ferrite, BiFeO3, is a multiferroic solid that is attracting increasing attention as a potential photocatalytic material, because the ferroelectric polarization enhances the separation of photogenerated carriers. With the motivation of finding routes to engineer the band gap and the band alignment, while conserving or enhancing the ferroelectric properties, the thermodynamic, electronic, and ferroelectric properties of BiCoxFe1−xO3 solid solutions (x = 0, 0.0625, 0.125) using density functional theory are investigated. It is shown that the band gap can be reduced from 2.9 to 2.1 eV by cobalt substitution, while simultaneously increasing the spontaneous polarization, which is associated with a notably larger Born effective charge of Co compared to Fe cations. The interaction between Co impurities is discussed, which is strongly attractive and can drive the aggregation of Co, as evidenced by Monte Carlo simulations. Phase separation into a Co-rich phase is therefore predicted to be thermodynamically preferred, and the homogenous solid solution can only exist in metastable form, protected by slow cation diffusion kinetics. Finally, the band alignment of pure and Co-substituted BiFeO3 with relevant redox potentials, in the context of its applicability in photocatalysis, is discussed.