Recipes for improper ferroelectricity in molecular perovskites

Abstract

The central goal of crystal engineering is to control material function via rational design of structure. A particularly successful realisation of this paradigm is hybrid improper ferroelectricity in layered perovskite materials, where layering and cooperative octahedral tilts combine to break inversion symmetry. However, in the parent family of inorganic ABX3 perovskites, symmetry prevents hybrid coupling to polar distortions. Here, we use group-theoretical analysis to uncover a profound enhancement of the number of improper ferroelectric coupling schemes available to molecular perovskites. This enhancement arises because molecular substitution diversifies the range of distortions possible. Not only do our insights rationalise the emergence of polarisation in previously studied materials, but we identify the fundamental importance of molecular degrees of freedom that are straightforwardly controlled from a synthetic viewpoint. We envisage that the crystal design principles we develop here will enable targeted synthesis of a large family of new acentric functional materials.

Two Rare‐Earth Molecular Ferroelectrics with High Curie Temperatures, Large Spontaneous Polarization, Switchable Second Harmonic Generation Effects, and Strong Photoluminescence

“…It is widely known that symmetry breaking usually accompanies a para‐ ferroelectric reversible phase transition . Before the phase transition, a material has lower symmetry and belongs to an acentric polar point group, whereas after the phase transition it possesses higher symmetry and possibly crystallizes in a nonpolar space group.…”

First-principles study of the structural, electronic, magnetic, and ferroelectric properties of a charge-ordered iron(ii)-iron(iii) formate framework

“…While the hybrid improper ferroelectricity was initially discussed in the context of inorganic multiferroics, 27 it has become clear that in hybrid materials it may have much more interesting applications and possibility for increasing the functionality of hybrid perovskite. 28 (DMAFeFe) is the first example exhibiting negative magnetization assigned as Neel N-Type ferrimagnet in a 3D molecule-based magnet. 7 Recently, the magnetoelectric coupling effect in the mixed-valence formate framework was experimentally observed at 3.5 K. 35 According to previous studies, 36 DMAFeFe crystallizes in the trigonal P-31c space group at room temperature.…”

Structure and thermal expansion of the distorted Prussian blue analogue RbCuCo(CN)6

“…Coexistence of distortions is important, as it provides a potential avenue for the development of polar structures by improper ferroelectricity, as reported for other molecular perovskites. [20][21][22] Yet, some multiply distorted PBAs are known and they often show interesting features. To illustrate, the charge-transfer system RbMnFe(CN) 6 contains both alternating occupational Rb order and Jahn-Teller distortions, which reduces the symmetry to the piezoelectric space group I% 4m2.…”

“…the development of hybrid improper ferroelectricity. 20,22 Group-theoretical analysis highlights the coupling of the three distortions, where the presence of two induces the third. The potential problems associated with the determination of tilt systems for Prussian blue analogues highlighted here have implications for research on both PBAs and other systems in a large number of fields.…”