Giant thermally induced band-gap renormalization in anharmonic silver chalcohalide antiperovskites

Silver chalcohalide antiperovskites (CAP), Ag₃XY (X = S, Se; Y = Br, I), are a family of highly anharmonic inorganic compounds with great potential for energy applications. However, a substantial and unresolved discrepancy exists between the optoelectronic properties predicted by theoretical first-principles methods and those measured experimentally at room temperature, hindering the fundamental understanding and rational engineering of CAP. In this work, we employ density functional theory, tight-binding calculations, and anharmonic Fröhlich theory to investigate the optoelectronic properties of CAP at finite temperatures. Near room temperature, we observe a giant band-gap (<i>E</i> _g) reduction of approximately 20-60% relative to the value calculated at <i>T</i> = 0 K, bringing the estimated <i>E</i> _g into excellent agreement with experimental measurements. This relative <i>T</i>-induced band-gap renormalization is roughly twice the largest value previously reported in the literature for similar temperature ranges. Low-energy optical polar phonon modes, which break inversion symmetry and enhance the overlap between silver and chalcogen s electronic orbitals in the conduction band, are identified as the primary drivers of this significant <i>E</i> _g reduction. Furthermore, when temperature effects are considered, the optical absorption coefficient of CAP increases by nearly an order of magnitude in the visible light spectrum. These findings not only bridge a critical gap between theory and experiment but also pave the way for future technologies where temperature, electric fields, and light dynamically modulate optoelectronic properties, establishing CAP as a versatile platform for energy and photonic applications.