Origin of the Temperature-Induced Gap Bowing of Formamidinium–Methylammonium Lead Iodide Perovskites: Role of Cationic Rattlers

Understanding the temperature dependence of semiconductor band gaps is essential for optimizing optoelectronic devices. However, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium–methylammonium (FA-MA) lead iodide perovskites has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on FA_xMA_1–xPbI₃ single crystals with x ∈ [0, 1], we unravel the origin of this bowing. Both thermal expansion and electron–phonon interaction effects are responsible. The latter is the leading term, driven by activation of an anomalous electron–phonon coupling mechanism linked to mixed vibrational modes, which combine inorganic-cage phonons involving octahedral tilting with low-frequency FA librations, called FA rattlers. As shown in the revised composition–temperature phase diagram, this occurs in the orthorhombic and (pseudo)tetragonal low-temperature phases, presumably featuring stripe domains with alternating octahedral tilt-axis patterns for x between 0.2 and 0.9. This sheds light on an intriguing behavior of lead halide perovskites that directly affects their optoelectronic properties.