Local Symmetry Broken Induced by the Surface and Inside Cr-vacancy layers of Two-Dimensional Cr2Se3 System

in two-dimensional (2D) Cr₂Se₃ magnetic films, surface terminations (Cr–Cr, Cr–Se, Se–Se) and thickness (1L–6L) critically influence structural stability and local symmetry. Our first-principles study reveals that Cr–Cr termination causes a strong competition between surface CrSe layer compression (~23%) from dangling bonds and inner Cr-vacancy layer expansion (~14%), producing a net vertical compression that weakens with thickness. For Se–Se termination, both surface (~4%) and inner (~13%) layers compress, yielding a strong vertical strain that increases with thickness. In Cr–Se termination, the interplay of surface compression (~23%, ~4%) and inner expansion (~14%) induces compression in thin films and slight expansion in thicker ones. These strain-driven distortions break local symmetries in bond lengths and angles, suggesting significant modifications in the electronic and magnetic properties compared with bulk Cr₂Se₃.