Site-Selective Fe Incorporation in Layered Polar α-In2Se3: Crystal-Chemical Control of Localized Spin States and Ferroic Responses

Site-selective transition-metal incorporation provides a crystal-chemical route for tuning local bonding, spin states, and ferroic responses in layered inorganic chalcogenides. Here, Fe-incorporated α-In2Se3 is investigated to clarify how Fe site preference and spatial distribution regulate the structure–property relationship of a polar van der Waals semiconductor. Experimentally, two incorporation regimes are distinguished: lattice-incorporated Fe–In2Se3 (LI-Fe–In2Se3) and cluster-rich Fe–In2Se3 (CR-Fe–In2Se3). Structural, spectroscopic, and microscopic analyses reveal that LI-Fe–In2Se3 preserves the layered α-In2Se3 framework with relatively dispersed Fe-related centers, whereas CR-Fe–In2Se3 exhibits local Fe enrichment, structural inhomogeneity, and red-shifted optical absorption. Piezoresponse force microscopy and magnetic measurements further show pathway-dependent polar switching and spin-related magnetic responses. First-principles calculations further reveal that Fe preferentially substitutes In sites, especially the six-coordinated W4 site, while retaining Fe-centered localized spin-polarized states coupled to the surrounding Se-derived host states. A vertical graphene/LI-Fe–In2Se3/graphene junction demonstrates polarization-associated resistance modulation. These results establish site-selective Fe incorporation as a key crystal-chemical parameter for regulating localized spin states and ferroic responses in layered polar α-In2Se3.