Formamidinium [FA, HC(NH₂)₂⁺] lead iodide and its cation mixture have attracted interests as potential applications for efficient solar cell superior to well-known methy-lammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA_1−xCs_xPbI₃ 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 cubic Pm−3m structure with orientationally disordered FA molecules confined in the non-distorted 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 previously known single β−γ transition. After doping with 10% Cs, the multiple first-order transitions disappear, 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.