Phase Separation of Br-Doped CsPbI: A Combined Cluster Expansion, Monte Carlo, and DFT Study.

Cluster expansion, which is a method that describes the concentration-dependent thermodynamic properties of materials while maintaining density functional theory accuracy, was used to predict new (CsPbIBr) structures. The cluster-expansion method generated 42 new stable (CsPb)IBr (where x = 1 to 3 and y and z = 1 to 8) structures and these were ranked as meta-stable structures based on their enthalpies of formation. Monte Carlo calculations showed that CsPbIBr composition separates into different phases at 300 K, but changes to a homogeneous phase at 700 K, suggesting that a different phase of CsPbI may exist at higher temperatures.