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

Abstract

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 Au^1+/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 × 10⁴ 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.