  It is of great interest to design and make materials in which ferroelectric
polarisation 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 realised. Here we report detailed crystallographic
studies of a novel perovskite
Hg$^{\textbf{A}}$Mn$^{\textbf{A'}}_{3}$Mn$^{\textbf{B}}_{4}$O$_{12}$ 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$^B$ inter-site 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.
