Improper ferroelectric polarization in a perovskite driven by intersite charge transfer and ordering

It is of great interest to design and make materials in which ferroelectric polarization is coupled to other order parameters such as lattice, magnetic, and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realized. Here we report detailed crystallographic studies of a perovskite (HgMn3Mn4O12)-Mn-A-Mn-A'-O-B that is found to exhibit a polar ground state on account of such couplings that arise from charge and orbital ordering on both the A' - and B-sites, which are themselves driven by a highly unusual Mn-A' - Mn-B intersite charge transfer. The inherent coupling of polar, charge, orbital, and hence magnetic degrees of freedom make this a system of great fundamental interest, and demonstrating ferroelectric switching in this and a host of recently reported hybrid improper ferroelectrics remains a substantial challenge.