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 FAxMA1-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.