How Thermodynamically Accessible are Quaternary Mixed-Metal Chalcohalides?

Quaternary mixed-metal chalcohalides (MMCHs) are a promising mixed-anion semiconductor family, but their thermodynamic accessibility remains largely unexplored. We combine density functional theory with the Alexandria Materials Database to carry out a rapid and reliable evaluation of the thermodynamic accessibility of 54 M(II)${}_2$M(III)Ch${}_2$X${}_3$ compounds. All evaluated MMCHs are predicted to be thermodynamically unstable, but a considerable fraction of MMCHs lie within computational uncertainty of the convex hull. Their predicted decomposition follows five distinct reaction pathways, most commonly into M(III)${}_2$Ch${}_3$, M(II)X${}_2$, and M(II)Ch. Solution processing of six Sn-based MMCHs supports our analysis. Notably, this includes the first synthesis of the previously unreported Sn${}_2$InS${}_2$Br${}_3$ in a multiphase film. Our approach identifies convex hull proximity and competing phase formation as key constraints on MMCH synthetic accessibility during solution processing, thereby guiding future MMCH exploration.