Improper Ferroelectric Polarisation in a Perovskite driven by Inter-site Charge Transfer and Ordering

TL;DR

This paper reports a novel perovskite material exhibiting improper ferroelectric polarization driven by inter-site charge transfer and ordering, revealing a new coupling mechanism between electronic and polar instabilities.

Contribution

It demonstrates a compact lattice coupling between electronic and ferroelectric instabilities through charge and orbital ordering in a new perovskite, advancing understanding of improper ferroelectric mechanisms.

Findings

Polar ground state due to charge and orbital ordering

Inter-site Mn-Mn charge transfer drives polarization

Coupling of charge, orbital, and magnetic degrees of freedom

Abstract

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.