Thiocyanate-Stabilized Pseudo-cubic Perovskite CH(NH₂)₂PbI₃ from Coincident Columnar Defect Lattices

α-FAPbI₃ (FA⁺ = CH(NH₂)₂⁺) with a cubic perovskite structure is promising for photophysical applications. However, α-FAPbI₃ is metastable at room temperature, and it transforms to the δ-phase at a certain period of time at room temperature. Herein, we report a thiocyanate-stabilized pseudo-cubic perovskite FAPbI₃ with ordered columnar defects (α'-phase). This compound has a √5a_p × √5a_p × a_p tetragonal unit cell (a_p: cell parameter of primitive perovskite cell) with a band gap of 1.91 eV. It is stable at room temperature in a dry atmosphere. Furthermore, the presence of the α'-phase in a mixed sample with the δ-phase drastically reduces the δ-to-α transition temperature measured on heating, suggesting the reduction of the nucleation energy of the α-phase or thermodynamic stabilization of the α-phase through epitaxy. The defect-ordered pattern in the α'-phase forms a coincidence-site lattice at the twinned boundary of the single crystals, thus hinting at an epitaxy- or strain-based mechanism of α-phase formation and/or stabilization. In this study, we developed a new strategy to control defects in halide perovskites and provided new insight into the stabilization of α-FAPbI₃ by pseudo-halide and grain boundary engineering.