Facile strategy to synthesize cesium gold-based bromide perovskites: an integrated experimental and theoretical approach to study temperature-dependent structural and optical properties

All-inorganic perovskites CsAuBr4, CsAuBr3, and Cs2Au2Br6 are developed with a systematic addition of Br ions in CsAuCl4. The phase changes in a controlled manner from CsAuBr4 to CsAuBr3 to Cs2Au2Br6 with varying concentrations of Br. The subtle change in structures is associated with a change in the valence state of Au3+ to Au2+ to Au1+/3+, as seen in XPS studies. Optical absorption of the compound shows a bandgap variation from 2.3 eV to 1.4 eV when the phase varies from a wide bandgap to metallic to optically suitable for visible light absorption. A sharp absorption edge associated with a high absorption coefficient (14.01 x 10(4) cm(-1)) for Cs2Au2Br6 and a low Urbach energy of 75 meV indicates low disorders in Cs2Au2Br6. The compounds CsAuBr4/CsAuBr3 are not robust; however, they end up forming Cs2Au2Br6 at a shelf-life of four-five months. Similarly, annealing of CsAuBr4/CsAuBr3 renders Cs2Au2Br6 phase only. Ab initio calculations correctly confirm the metallic and semiconducting nature of the three perovskites. The compositional phase diagram establishes the stability of only CsAuBr4 and Cs2Au2Br6 among the three phases. Further theoretical analysis confirms a clear CsAuBr3 to Cs2Au2Br6 phase transition, marked by a change in volume, which agrees fairly well with the experiment.