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However, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium\u2013methylammonium (FA-MA) lead iodide perovskites has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on FAxMA1\u2013xPbI3 single crystals with x \u2208 [0, 1], we unravel the origin of this bowing. Both thermal expansion and electron\u2013phonon interaction effects are responsible. The latter is the leading term, driven by activation of an anomalous electron\u2013phonon 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\u2013temperature 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.<\/jats:p>","DOI":"10.1021\/acs.jpclett.6c01965","type":"journal-article","created":{"date-parts":[[2026,8,4]],"date-time":"2026-08-04T13:13:08Z","timestamp":1785849188000},"page":"9097-9106","source":"Crossref","is-referenced-by-count":1,"title":["Origin of the\nTemperature-Induced Gap Bowing of Formamidinium\u2013Methylammonium\nLead Iodide Perovskites: Role of Cationic Rattlers"],"prefix":"10.1021","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-6999-1904","authenticated-orcid":true,"given":"Kai","family":"Xu","sequence":"first","affiliation":[{"name":"Institut de Cie\u0300ncia de Materials de Barcelona (ICMAB-CSIC) , Carrer dels Til.lers , ,","place":["Cerdanyola del Valle\u0300s, Spain, 08193"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Adria\u0301n","family":"Francisco-Lo\u0301pez","sequence":"additional","affiliation":[{"name":"Institut de Cie\u0300ncia de Materials de Barcelona (ICMAB-CSIC) , Carrer dels Til.lers , ,","place":["Cerdanyola del Valle\u0300s, Spain, 08193"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Bethan L.","family":"Charles","sequence":"additional","affiliation":[{"name":"University of Bath , , Claverton Down , ,","place":["Bath, U.K., BA2 7AY"]},{"name":"University of Bristol , , ,","place":["Bristol, U.K., BS8 1TR"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7669-5871","authenticated-orcid":true,"given":"M. Isabel","family":"Alonso","sequence":"additional","affiliation":[{"name":"Institut de Cie\u0300ncia de Materials de Barcelona (ICMAB-CSIC) , Carrer dels Til.lers , ,","place":["Cerdanyola del Valle\u0300s, Spain, 08193"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Miquel","family":"Garriga","sequence":"additional","affiliation":[{"name":"Institut de Cie\u0300ncia de Materials de Barcelona (ICMAB-CSIC) , Carrer dels Til.lers , ,","place":["Cerdanyola del Valle\u0300s, Spain, 08193"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7475-0379","authenticated-orcid":true,"given":"Mark T.","family":"Weller","sequence":"additional","affiliation":[{"name":"University of Bath , , Claverton Down , ,","place":["Bath, U.K., BA2 7AY"]},{"name":"Cardiff University , , ,","place":["Wales, U.K., CF10 3AT"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1193-3063","authenticated-orcid":true,"given":"Alejandro R.","family":"Gon\u0303i","sequence":"additional","affiliation":[{"name":"Institut de Cie\u0300ncia de Materials de Barcelona (ICMAB-CSIC) , Carrer dels Til.lers , ,","place":["Cerdanyola del Valle\u0300s, Spain, 08193"]},{"name":"ICREA , Passeig Llui\u0301s Companys 23 , ,","place":["Barcelona, Spain, 08010"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,7,28]]},"reference":[{"key":"2026080609083816500_cit1","volume-title":"Best Research-Cell Efficiency\nChart"},{"key":"2026080609083816500_cit2","doi-asserted-by":"publisher","first-page":"245","DOI":"10.1038\/s41586-018-0575-3","article-title":"Perovskite\nLight-Emitting Diodes with External Quantum Efficiency Exceeding 20\nper Cent","volume":"562","author":"Lin","year":"2018","journal-title":"Nature"},{"key":"2026080609083816500_cit3","doi-asserted-by":"publisher","first-page":"2163","DOI":"10.1103\/PhysRevB.31.2163","article-title":"Temperature\nDependence of Band Gaps\nin Si and Ge","volume":"31","author":"Lautenschlager","year":"1985","journal-title":"Phys. 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