Interplay of static and dynamic disorder in the mixed-metal chalcohalide Sn₂SbS₂I₃

Chalcohalide mixed-anion crystals have seen a rise in interest as ‘perovskite-inspired materials’ with the goal of combining the ambient stability of metal chalcogenides with the exceptional optoelectronic performance of metal halides. Sn₂SbS₂I₃is a promising candidate, having achieved a photovoltaic power conversion efficiency above 4%. However, there is uncertainty over the crystal structure and physical properties of this crystal family. Using a first-principles cluster expansion approach, we predict a disordered room temperature structure, comprising both static and dynamic cation disorder on different crystallographic sites. These predictions are confirmed using single-crystal X-ray diffraction. Disorder leads to a lowering of the bandgap from 1.8 eV at low temperature to 1.5 eV at the experimental annealing temperature of 573 K. Cation disorder tailors the bandgap, allowing for targeted application for this class of materials in optoelectronics.