Atomically Unveiling the Phase Evolution in Weakly Coupled Layered Transition-Metal Phosphorus Trichalcogenide by Chalcogen Doping

The stacking configuration significantly influences the properties of van der Waals (vdW) layered magnets by dictating crystallographic and magnetic symmetries. Transition-metal phosphorus trichalcogenides (MPX₃, X = S, Se) intrinsically exhibit diverse stacking polytypes, being an optimal platform for magnetic phase engineering. Unlike MX₂, where chalcogen doping has a minimal impact on stacking, MPX₃ allows stacking control via elemental substitution. However, the atomic-scale mechanisms governing stacking variations remain unclear. Using scanning transmission electron microscopy (STEM) and density functional theory (DFT) calculations, we reveal that in <i>3d</i> transition metal MPX₃, tuning the S/Se ratio induces a transition from the <i>C</i>2/<i>m</i> to <i>R</i>3̅ phase due to modified interlayer S-S/Se-Se and P-P interactions. In contrast, stacking control becomes challenging for <i>4d</i> CdPX₃, due to relatively weak interlayer coupling. These insights provide a stacking basis for stacking polytypes in MPX₃, paving the way for tuning magnetic couplings via stackingtronics.