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

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH₂]²⁺H₃O⁺Br^- ₃ (where [dabcoH₂]²⁺ = 1,4-di-aza-bicyclo-[2.2.2]octane dication) correspond to the general formula ABX ₃ 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.