Anion Ordering and Phase Stability Govern Optical Band Gaps in BaZrS_3x Se_3-3x

Chalcogenide perovskites have emerged as promising lead-free materials for photovoltaic and thermoelectric applications. Among them, BaZrS₃ has attracted particular attention due to its thermal and chemical stability, favorable optoelectronic properties, and low thermal conductivity. Here, we combine molecular dynamics and Monte Carlo simulations based on a machine-learned interatomic potential with scanning transmission electron microscopy to investigate mixing thermodynamics and phase stability in the BaZrS_3x Se_3-3x system. We identify an unusual ordered structure, with signatures of anion ordering persisting at room temperature, most prominently at 33% S, where S and Se atoms form alternating layers within the crystal. Free-energy calculations yield the temperature-composition phase diagram, including a nonperovskite δ phase in the Se-rich limit and a perovskite phase in the S-rich limit, separated by a broad two-phase region. Analysis of the dielectric function and the absorption coefficient demonstrates that composition, crystal structure, and anion ordering jointly control the optical band gap. Selenium alloying enables tuning between approximately 1.5 and 1.9 eV, while anion ordering within a given composition reduces the gap by about 0.2 eV. Lastly, variations between structural polymorphs give rise to band gap differences of up to 0.5 eV.