Metal-free enantiomorphic perovskite [dabcoH2]2+[H3O]+Br−3 and its one-dimensional polar polymorph

Abstract

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH2]2+H3O+Br−3 (where [dabcoH2]2+ = 1,4-diazabicyclo[2.2.2]octane dication) correspond to the general formula ABX3 characteristic of perovskites. In enantiomorphic trigonal polymorph α of [dabcoH2]2+H3O+Br−3, the corner-sharing [H3O]Br6 octahedra combine into a 3D framework embedding [dabcoH2]2+ dications in pseudo-cubic cages. In the more dense polymorph β, the face-sharing [H3O]Br6 octahedra form 1D polyanionic columns separated by [dabcoH2]2+ dications. These different topologies correlate with different crystal fields around the cations and their different disorder types: orientational disorders of [dabcoH2]2+ dications and H3O+ cations in polymorph α and positional disorder of [H3O]+ cations in polymorph β. The orientational disorder increases the lengths of OH...Br hydrogen bonds in polymorph α, but NH...Br distances of ordered dabcoH2 dications are longer in polymorph β. The presence of polar [H3O]+ cations in [dabcoH2]2+H3O+Br−3 polymorphs offers additional polarizability of the centres compared with analogous metal-free [dabcoH2]2+[NH4]+Br−3 perovskite.

Keywords

• metal-free perovskite
• polymorphism
• disorder
• ionic crystals