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

There has recently been renewed interest in improper ferroelectricity due to the discovery of a ``hybrid'' mechanism by which two nonpolar lattice modes couple to a third polar mode. Here, the authors demonstrate that the polar ground state of perovskite HgMn₃Mn₄O₁₂ emerges due to electronic instabilities that drive charge and orbital ordering on the A- and B-sites. These order parameters couple to polar displacements (P) in a manner conceptually similar to hybrid improper ferroelectricity. The results open up a possible route for electronic control of orbital ordering - an appealing prospect given the technologically important properties dependent on such states.