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

alpha-FAPbI(3) (FA(+) = CH(NH2)(2)(+)) with a cubic perovskite structure is promising for photophysical applications. However, alpha-FAPbI(3) is metastable at room temperature, and it transforms to the delta-phase at a certain period of time at room temperature. Herein, we report a thiocyanate-stabilized pseudo-cubic perovskite FAPbI(3) with ordered columnar defects (alpha'-phase). This compound has a root 5a(p) x root 5a(p) x 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 alpha'-phase in a mixed sample with the delta-phase drastically reduces the delta- to-alpha transition temperature measured on heating, suggesting the reduction of the nucleation energy of the alpha-phase or thermodynamic stabilization of the alpha-phase through epitaxy. The defect-ordered pattern in the alpha'-phase forms a coincidence-site lattice at the twinned boundary of the single crystals, thus hinting at an epitaxy- or strain-based mechanism of alpha-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 alpha-FAPbI(3) by pseudo-halide and grain boundary engineering.