Stability of the Halide Double Perovskite Cs 2 AgInBr 6 .

Cs 2 AgInBr 6 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 Cs 2 AgInBr 6 thin films through thermal evaporation of CsBr, AgBr, and InBr 3 and subsequent annealing between 130 °C and 250 °C. Cs 2 AgInBr 6 appears stable in this temperature range. However, Cs 2 AgInBr 6 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. Cs 2 AgInBr 6 decomposed into Cs 2 AgBr 3 , Cs 3 In 2 Br 9 , AgBr, and InBr 3 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 Cs 2 AgInBr 6 having a direct band gap of 1.57 ± 0.1 eV.