Publication date: 22nd July 2026
A thorough understanding of the temperature dependence of semiconductor band gaps is essential for optimizing optoelectronic devices. In this respect, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium-methylammonium (FA-MA) lead iodide perovskites [1] has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on a series of FAxMA1-xPbI3 mixed-cation single crystals with eleven compositions from 0 to 1, we unravel the origin of this bowing. Both thermal expansion as well as electron-phonon interaction effects are responsible [2,3]. However, the latter is the leading term, driven by the activation of an anomalous electron-phonon coupling mechanism linked to mixed vibrational modes [4], which combine inorganic-cage phonons involving octahedral tilting with low-frequency FA librations, i.e., FA rattler modes. This occurs mainly in a low-temperature (pseudo)tetragonal phase, presumably featuring stripe domains with alternating octahedral tilt patterns for FA concentrations between 20% and 90% [5]. In this way, we have shed light on an intriguing behavior of lead halide perovskites that directly affects their optoelectronic properties.
The Spanish "Ministerio de Ciencia, Innovación y Universidades" (MICIU) through the Agencia Estatal de Investigación (AEI) and "European Union NextGenerationEU/PRTR" are gratefully acknowledged for its support through grant CEX2023-001263-S (MATRANS42) in the framework of the Spanish Severo Ochoa Centre of Excellence program and the AEI/FEDER(UE) grants PID2021-128924OB-I00 (ISOSCELLES) and PID2024-163010OB-I00 (PV-MENU). The authors also thank the Catalan agency AGAUR for grant 2021-SGR-00444 and the National Network "Red Perovskitas" (MICIU funded). A.F.L. acknowledges a FPI grant BES-2016-076913 from MICIU, co-financed by the European Social Fund, and the PhD programme in Materials Science from Universitat Autònoma de Barcelona in which he was enrolled. K.X. acknowledges a fellowship (CSC201806950006) from China Scholarship Council and the PhD programme in Materials Science from Universitat Aut\`{o}noma de Barcelona in which he was enrolled. B.C. thanks the EPSRC for PhD studentship funding via the University of Bath, CSCT CDT (EP/G03768X/1).
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