  Bismuth ferrite, BiFeO3, is a multiferroic solid that is attracting
increasing attention as a potential photocatalytic material, because the
ferroelectric polarisation 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, we have
investigated the thermodynamic, electronic and ferroelectric properties of
BiCoxFe1 xO3 solid solutions, with 0 < x < 0.13, using density functional
theory. We show that the band gap can be reduced from 2.9 eV to 2.1 eV by
cobalt substitution, while simultaneously increasing the spontaneous
polarisation, which is associated with a notably larger 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, as
evidenced by Monte Carlo simulations. Phase separation into a Co-rich phase is
therefore predicted to be thermodynamically preferred, and the homogeneous
solid solution can only 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
applicability in photocatalysis.
