Exploring Structural Order Parameters and Energy Landscape in a Thiophosphate Ferroelectric Sn₂P₂S₆

Recently ferroelectric materials from the thiophosphate family have attracted considerable attention due to their rich functional behavior and potential for applications. For Sn₂P₂S₆, a 3D ferroelectric, a triple-well potential was predicted where both a polar and a non-polar state coexist below Curie temperature [1]. In this work we explore local structural features of Sn₂P₂S₆ to identify the relevant descriptors that correlate with polar order and observe their evolution across the phase transition using high-resolution scanning transmission electron microscopy (STEM) and density functional theory (DFT). Sn₂P₂S₆ has a monoclinic structure that alters in complex ways as a function of strain/pressure and polar state. However, Sn₂ dimers are easily identifiable on high resolution images for multiple zone axis orientations. DFT calculations conducted for both polar and non-polar states show that Sn₂ dimer rotation with respect to lattice direction correlates with changing isostatic pressure on the system and lattice polarization, and that the rotation angles are measurably distinct for polar and non-polar states. STEM imaging across the phase transition yields a complex picture of Sn₂ rotations below the Curie temperature, while above it the rotations corresponded closely to those expected for the non-polar state, suggesting coexistence of different states below the transition.