Low temperature, high pressure phase transition of the Improper Ferroelectric HgMn7O12

We have found that the 134-perovskite, HgMn7O12 (HgAMnA’3MnB4O12) exhibits improper ferroelectric coupling due to an unusual orbital ordering of the manganese cations on both the A’ and B-sites of the perovskite structure. This is at odds with the isostructural CaMn7O12, which undergoes a different sequence of phase transitions with decreasing temperature, resulting in a 3:1 charge ordered, non-polar ground state structure, whereas the Hg perovskite undergoes a further phase transition below 240 K to afford a 1:1 charge ordering and a polar ground state structure. We expect that by applying pressure to the HgMn7O12 low-temperature structure, we will force a reversal in the charge transfer process, which will give us insight into how the unusual charge and orbital physics surrounding the ferroelectric state in HgMn7O12 arises, and also how it could be induced in other related perovskites.