Structural stability and polymorphic transitions in LnSI (Ln = lanthanides)

Mixed-anion compounds exhibit diverse crystal structures distinct from those of single-anion compounds due to heteroleptic coordination. In this study, we experimentally and theoretically investigate the structural relationships among three polymorphs of lanthanide sulfide iodides, LnSI (Ln = lanthanides). For Ln = Gd-Lu, the system adopts a two-dimensional (2D) FeOCl-type structure with six-fold coordinated Ln (LnS₄I₂). As the ionic radius of Ln³⁺ increases (Ln = Pr-Sm), the structure transitions to the 2D SmSI-type with seven-fold coordinated Ln (LnS₄I₃). Accommodation of a larger Ln (Ln = La and Ce) leads to the formation of a three-dimensional (3D) SrI₂-type structure. These structural transitions are discussed in terms of martensitic-like transformations involving the Ln-I bond creation and rearrangement. Furthermore, we discover that the cation-to-anion radius ratio, r_cation/r_anion, provides a unified descriptor of the Ln size-dependent and pressure-induced polymorphisms. This study offers fundamental insights into the structural control and phase transitions of mixed-anion compounds, paving the way for the design of new materials with tailored structures and properties.