Recipes for improper ferroelectricity in hybrid perovskites

TL;DR

This paper uses group-theoretical analysis to reveal how molecular substitution in inorganic perovskites enhances improper ferroelectric coupling, guiding the design of new polar materials.

Contribution

It uncovers the role of molecular degrees of freedom in enabling improper ferroelectricity in inorganic perovskites, expanding design strategies beyond traditional lattice distortions.

Findings

Molecular substitution diversifies possible distortions in ABX3 perovskites.

Enhanced improper ferroelectric coupling schemes are identified.

Insights explain polarization in previously studied materials.

Abstract

The central goal of crystal engineering is to develop precise control over material function \emph{via} rational design of structure. A particularly successful realisation of this paradigm is the example of hybrid improper ferroelectricity in layered perovskite materials, where cation vacancies and cooperative octahedral tilts combine to break inversion symmetry. However, in the more chemically diverse and technologically relevant family of inorganic ABX$_3$ perovskites, symmetry conspires to render hybrid coupling to polar distortions impossible. In this study, we use group-theoretical analysis to uncover a profound enhancement of the number of improper ferroelectric coupling schemes available to \emph{molecular} perovskites. This enhancement arises because molecular substitution on the A- and/or X-site significantly diversifies the range of distortions possible. Not only do our insights rationalise the emergence of polarisation in a number of previously-studied materials, but we identify the fundamental importance of molecular degrees of freedom at the A-site, which are much more straightforwardly controlled from a synthetic viewpoint than are lattice, charge and orbital degrees of freedom. We envisage that the crystal design principles we develop here will be enable targeted synthesis of a large family of new acentric functional materials.