Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides

Formamidinium [FA, HC(NH₂)₂⁺] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA_1-CsPbI₃ for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α-FA_0.9Cs_0.1PbI₃ as well as α-FAPbI₃ crystallize in a cubic Pm3̅m structure with orientationally disordered FA molecules confined in the nondistorted Pb-I framework. In FAPbI₃, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β- and γ-phases instead of the previously known single β-γ transition. After doping with 10% Cs, the multiple first-order transitions disappeared, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.