Synthesis, crystallographic study and solid-state properties of metal-free perovskites with P-atom containing A-site cations†

A P-atom was introduced at the A-site in the metal-free perovskite A(NH₄)X₃ using diprotonated 1,3,5-triaza-7-phosphaadamantane (pta), where X are Br⁻, I⁻, and BF₄⁻ anions, labeled as p-Br, p-I, and p-BF₄, respectively. The pta is analogous to hexamethylenetetramine (hmta) with one N-atom replaced by a P-atom. The p-Br and p-BF₄ perovskites were not isostructural with the corresponding perovskite structure with hmta as the A-site cation. p-Br undergoes a phase transition from an ordered phase to a thermally disordered plastic crystalline phase, whereas the hmta-analog h-Br does not. p-I exhibited greenish photoluminescence, in contrast to the orangish photoluminescence observed in the dabco-based metal-free perovskites. Introduction of the P-atom at the A-site molecule led to differences in phase transition and photoluminescence, providing a new molecular design for metal-free perovskites.