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 degrees C and 250 degrees 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 degrees C to 250 degrees 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.