Role of d electrons in multifunctional materials

Multifunctional materials are of today’s quest. A class of multifunctional materials display simultaneous ordering in different dimensions like magnetism, polarization etc. They are more known as multiferroics. Quite a significant number of them are based on perovskite structures having a transition metal ion in the body centre. The electronic configuration of this central transition element play a key role in the behavior of those multiferroics. For example, those with d⁰ configuration display ferroelectricity while those with dⁿ (where n is non zero) generally display magnetic behavior. The d⁰ configuration leads to distortion of metal-oxygen bond in these perovskites and breaking of special inversion symmetry causing a local dipole moment and ferroelectricity. Often this distortion in metal-oxygen bond is caused by lone pair of electrons of  non transitional cations in the corners of the perovskite structure.BiFeO₃ (BFO) is a shining example of this kind of distortion, where the central ion Fe(III) causes magnetism but Bi(III) ions in the corner cause the polarization leading to the multiferroicity in BFO displaying both ferroelectric and antiferromagntic ordering. The symmetry breaking manifests in different kinds of ordering. Moreover, the variable valency of the central transition metal ion with d electrons manifests in redox behavior in these materials leading to various uses. We have observed such various functions in the multiferroic BFO like ferroelectric, magnetic and high specific capacitance rendering these materials useful in green energy storage materials. Moreover these redox behavior of these transition metal ion also leads to antimicrobial behavior in BFO.