Electronic structure of multiferroic BiFeO3 and related compounds: Electron energy loss spectroscopy and density functional study

The electronic structure of complex oxides is important for the understanding of their functional properties. In this paper, the electronic structures of the multiferroic perovskite bismuth ferrite (BiFeO3) and the related isostructural compounds Bi0.9La0.1FeO3 and BiFe0.7Mn0.3O3 are investigated through experiments and modeling. Using electron energy loss spectroscopy the oxygen K edge, i.e., the unoccupied O p density of states, is probed. As these states participate in covalent bonding with both Bi and Fe states, insight into the bonding in the materials is obtained. By substituting on both cation sites, it is possible to connect features in the spectrum to chemical bonds to the cations. We compare the experimental results of substituted and unsubstituted BiFeO3 and apply a multiple-scattering approach as well as density functional theory to interpret the differences in terms of changes in electronic structure and density of states. Specifically, we show that although mainly ionic, both Bi-O and Fe-O bonds have some covalent character, and that Mn substitution on Fe sites is found to alter the Bi-O bonds and reduce the anisotropy of the system. Upon introduction of La on Bi sites, the covalent character of the material is reduced and the ionic interaction increases as the La-O bond is higher in energy and mediated through other cation orbitals (La d orbitals) than the Bi-O bond (Bi p orbitals). Also, La substitution is found to influence the Fe electronic structure, showing that the A and B site cations are more coupled than commonly recognized. Thus, we use the electronic structure to confirm that B site cation substitution can influence the ferroelectricity, which is usually almost exclusively attributed to A site cation anisotropy.