Guest-Activated Forbidden Tilts in a Molecular Perovskite Analogue

Abstract

The manipulation of distortions in perovskite structures is critical to tailoring the properties of these materials for a variety of applications. Here we demonstrate a violation of established octahedral tilt rules in the double perovskite analogue (NH4)2SrFe(CN)6·2H2O. The forbidden tilt pattern we observe arises through coupling to hydration-driven Jahn-Teller-like distortions of the Sr coordination environment. Access to novel distortion mechanisms and the ability to switch these distortions on and off through chemical modification fundamentally expands the toolbox of techniques available for engineering symmetry-breaking processes in solid materials.

Columnar shifts as symmetry-breaking degrees of freedom in molecular perovskites

“…Our key point in covering all these different possibilities is to demonstrate that activation of different shift modes results in different symmetry-breaking processes that can be fundamentally distinct from those accessible via tilt degrees of freedom-whether conventional 43 or forbidden. 20,[24][25][26] Notation Given the complexity of shift distortions and their combinationseven in 2D-it is clearly desirable to develop a concise notation to represent the particular set of shift modes active in a given structure. In the case of tilt distortions, the most widely-used notation is that of Glazer; 5 we first review this notation with the view of subsequently extending the approach to shifts.…”

Crystal structure of guanidinium hexafluoridovanadate(III), (CN3H6)3[VF6]: an unusual hybrid compound related to perovskite

“…The rigid connectivity in conventional perovskites means that tilting of one octahedron dictates tilting, in the opposite sense, of all directly linked octahedra within a plane perpendicular to the tilt axis. However, new families of "hybrid" perovskites have recently been described, which are based on extended polyatomic (rather than monatomic) bridging ligands, such as azide, cyanide or dicyanometallate (Du et al, 2014;Duycker et al, 2016;Hill et al, 2016). This provides enhanced flexibility of the BX 6 framework, and allows tilt patterns that are "forbidden" in conventional perovskites.…”

Crossover between Tilt Families and Zero Area Thermal Expansion in Hybrid Prussian Blue Analogues

“…One important consequence of the polyatomic linker anions in the hybrid perovskite analogues is the possibility of activating “forbidden” distortion modes, which occur at Brillouin zone points where it would be impossible to distort an inorganic perovskite with monoatomic linker . Neither of the modes discussed here is forbidden in this sense.…”