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

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