Mixed-Halide Perovskite Alloys CsPb(I_1-xBr_x)₃ and CsPb(Br_1-xCl_x)₃: New Insight of Configurational Entropy Effect from First-Principles and Phase Diagrams

Stability is one of the key issues in mixed-halide perovskite alloys that are promising in emergent optoelectronics. Previous density functional theory (DFT) and machine learning studies indicate that the formation-energy convex hulls of these materials are very shallow, and stable alloy compositions are rare. In this work, we revisit this problem using DFT, with a special focus on the effects of configurational and vibrational entropies. Allowed by the 20-atomic models for the Formula Presented and Formula Presented series, the partition functions and therewith thermodynamic state functions are calculated by traversing all possible mixed-halide configurations. We can thus evaluate the temperature- and system-dependent configurational entropy, which largely corrects the conventional approach based on the ideal solution model. Finally, temperature-composition phase diagrams that include α, β, γ, and δ phases of both alloys are constructed based on the free energy data, for which the contribution of phonon vibrations is included.