Fig. 5. Structural characterization through high-resolution XRD.

Crystal structure of black perovskite (α-phase) with a cubic structure (Pm3m space group), yellow FAPbI₃ nonperovskite (δ-phase or 2H polytype) with a hexagonal structure (P63mc space group), and lead iodide (PbI₂) with a hexagonal structure (Pm1 space group). The high-resolution diffractogram (A) of the CsFAMA sample confirms the α-phase perovskite and lead iodide in smaller quantity. The intensity of the PbI₂ diffraction peak (B) of the CsFAMA degraded sample increased. The complete XRD of the CsFAMA degraded sample is available in the Supplementary Materials (fig. S4). The high-resolution XRD measurements were performed at 7 keV.