Stability of SnSe S solid solutions revealed by first-principles cluster expansion.

The configurational thermodynamics of a pseudo-binary alloy SnSe S in the Pnma phase is studied using first-principles cluster-expansion method in combination with canonical Monte Carlo simulations. We find that, despite the alloy having a tendency toward a phase decomposition into SnSe and SnS at 0 K, the two constituent binaries readily mix with each other to form random SnSe S solid solutions even at a temperature below room temperature. The obtained isostructural phase diagram of SnSe S reveals that the alloy is thermodynamically stable as a single-phase random solid solution over a whole composition range above 200 K. These findings provide a fundamental understanding on the alloying behavior of SnSe S and bring clarity to the debated clustering tendency in this alloy system.