Co-substituted BiFeO₃: thermodynamic, electronic and ferroelectric properties from first principles

The ferroelectric character of BiFeO₃ might enhance carrier separation in photocatalytic applications, but its efficiency is limited by a wide electronic bandgap. We have investigated the thermodynamic, electronic, and ferroelectric properties of BiCo_xFe_1‑xO solid solutions, 0<x<0.13, using density functional theory. The bandgap can be reduced from 2.9 eV to 2.1 eV upon Co substitution, while simultaneously enhancing ferroelectric behavior: a large spontaneous polarization is predicted for the Co-substituted system, due to an anomalously large Born effective charge of Co compared to Fe cations. We discuss the interaction between Co impurities, which is strongly attractive and would drive the aggregation of Co at temperatures of interest, as evidenced by Monte Carlo simulations. Phase separation into a Co-rich phase is therefore predicted to be thermodynamically preferred, but a homogeneous solid solution with this composition can probably exist in metastable form, protected by slow cation diffusion kinetics. Finally, we discuss the band alignment of pure and Co-substituted BiFeO₃ with relevant redox potentials, in the context of its applications in photocatalysis.