Structural Stability, Vibrational Properties, and Photoluminescence in CsSnI₃ Perovskite upon the Addition of SnF₂

The CsSnI3 perovskite and the corresponding SnF2-containing material with nominal composition CsSnI2.95F0.05 were synthesized by solid-state reactions and structurally characterized by powder X-ray diffraction. Both materials undergo rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI3) to the yellow orthorhombic phase (Y-CsSnI3), followed by irreversible oxidation into Cs2SnI6 within several hours. The phase transition occurs at a significantly lower rate in the SnF2-containing material rather than in the pure perovskite. The high hole-carrier concentration of the materials prohibits the detection of Raman signals for B-γ-CsSnI3 and induces a very strong plasmonic reflectance in the far-IR. In contrast, far-IR phonon bands and a rich Raman spectrum are observed for the Y-CsSnI3 modification below 140 cm-1 with weak frequency shift gradients versus temperatures between -95 and +170 °C. Above 170 °C, the signal is lost due to B-α-CsSnI3 re-formation. The photoluminescence spectra exhibit residual blue shifts and broadening as a sign of structural transformation initiation.