Diverse Polymorphism in Ruddlesden-Popper Chalcogenides

Ruddlesden-Popper (RP) chalcogenides are an emerging class of layered semiconductors with tunable properties and chemical stability, making them promising candidates for a wide range of functional applications. Over the past four decades, the structural diversity of RP oxides has been exploited to realize advanced functionalities; however, similar strategies have not yet been applied to RP chalcogenides, whose structural behavior remains poorly understood. In this study, we develop a high-accuracy machine-learned interatomic potential to perform large-scale molecular dynamics simulations of the homologous RP series Ba𝑛+1⁢Zr𝑛⁢S3⁢𝑛+1. We identify new polymorphs for each 𝑛 value, predict the corresponding phase transition temperatures, and validate our approach through comparison with existing experimental data. We find that the 𝑛 =1 phase exhibits in-plane negative thermal expansion, that the 𝑛 =1 and 𝑛 =3 phases undergo unusual ascending symmetry breaking, and that phases with 𝑛 ≥3 develop layer-dependent tilt patterns not previously observed in inorganic RP compounds. This distinctive behavior arises from the interplay between ZrS6 octahedral rotations and BaS rumpling at the perovskite-rocksalt interface, suggesting new strategies for realizing advanced functionalities and tuning properties in RP chalcogenides.