Stability of the Halide Double Perovskite Cs₂AgInBr₆

Cs₂AgInBr₆ is among the lead-free halide perovskites of interest, predicted by first-principles calculations to be stable with a direct band gap, but there has been only one report of its synthesis. Herein we report the formation of Cs₂AgInBr₆ thin films through thermal evaporation of CsBr, AgBr, and InBr₃ and subsequent annealing between 130 °C and 250 °C. Cs₂AgInBr₆ appears stable in this temperature range. However, Cs₂AgInBr₆ thin films are thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices. Cs₂AgInBr₆ decomposed into Cs₂AgBr₃, Cs₃In₂Br₉, AgBr, and InBr₃ upon cooling from 130 °C to 250 °C to room temperature. This conclusion did not depend on illumination, film thickness, annealing environment, or details of the film formation, pointing to an intrinsic thermodynamic instability of the material. Optical absorption measurements may be interpreted as Cs₂AgInBr₆ having a direct band gap of 1.57 ± 0.1 eV.