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

, 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.