Prediction of Thermodynamic Stability of Double-Perovskite Halides: Density Functional Theory Calculations and the Role of Different Exchange-Correlation Functionals*

Recently, a set of double-perovskite halide compounds have attracted intensive interests as promising alternatives to CH₃NH₃PbI₃ because they are Pb-free and may exhibit enhanced stability. The thermodynamic stability of lots of double-perovskite halides has been predicted based on density functional theory calculations of compound formation energies. We found that the stability prediction can be sensitive to the exchange-correlation functionals, e.g., the widely-used PBE functional predict that Cs₂AgBiBr₆ is thermodynamically unstable, obviously inconsistent with the good stability observed experimentally. The incorrect prediction by the PBE functional results from its failure to predict the correct ground-state structures of AgBr, AgCl and CsCl. In contrast, the DFT calculations based on LDA, optB86b-vdW and optB88-vdW functionals can correctly predict the ground-state structures of these binary halides. Furthermore, the optB88-vdW functional is found to give the most accurate description of the lattice constants of the double-perovskite halides and their competing phases. Hence, we suggest that the optB88-vdW functional should be used for predicting thermodynamic stability for new double-perovskite halides.