Columnar Shifts as Symmetry-Breaking Degrees of Freedom in Molecular Perovskites

TL;DR

This paper introduces columnar shifts as a new structural degree of freedom in molecular perovskites, influencing phase behavior and symmetry breaking, with potential applications in functional materials like ferroelectrics.

Contribution

It proposes a novel concept of columnar shifts, develops a generalized notation for their characterization, and applies this framework to interpret various molecular perovskite structures.

Findings

Columnar shifts affect phase transitions in molecular perovskites.

A generalized notation for shift distortions is introduced.

Implications for negative thermal expansion and ferroelectricity are discussed.

Abstract

We introduce columnar shifts---collective rigid-body translations---as a structural degree of freedom relevant to the phase behaviour of molecular perovskites ABX$_{\textrm3}$ (X = molecular anion). Like the well-known octahedral tilts of conventional perovskites, shifts also preserve the octahedral coordination geometry of the B-site cation in molecular perovskites, and so are predisposed to influencing the low-energy dynamics and displacive phase transitions of these topical systems. We present a qualitative overview of the interplay between shift activation and crystal symmetry breaking, and introduce a generalised terminology to allow characterisation of simple shift distortions, drawing analogy to the "Glazer notation" for octahedral tilts. We apply our approach to the interpretation of a representative selection of azide and formate perovskite structures, and discuss the implications for functional exploitation of shift degrees of freedom in negative thermal expansion materials and hybrid ferroelectrics.