Diverse surface reconstructions in MAX phases

Surface reconstructions in MAX phases demonstrate a complexity similar to that of semiconductor surfaces, influenced by the intricate interactions between their distinct electronic and structural characteristics. Utilizing first-principles electronic and phonon calculations, we explore various surface reconstructions that may occur on the surfaces of transition metal carbides and nitrides, known as MAX phases, especially when the A-element atoms from group 13 to 16 of the periodic table are positioned in the topmost surface layer. In many MAX phases, such as Ti2AlC, Ti2AlN, Ti2GaC, and Ti2InC, the surface A overlayer exhibits dynamic stability and maintains a bulk-like hexagonal configuration. Conversely, certain phases possess dynamic instability, as evidenced by soft phonon modes, leading to reconstructions of the A overlayer that include buckling (e.g., Ti2PbC and Ti2SnC), dimer/trimers (e.g., Ti2PC), tetramers (e.g., Ti2SiC and Ti2GeC), pentagon chains (e.g., Ti2SiC), or Kagome lattices (e.g., Ti2ZnC). Following these surface reconstructions, the surfaces achieve dynamic stability as all soft modes disappear. These reconstructions are associated with energy gains from band splitting due to A–A orbital interactions at low energies and/or the rehybridization of A dangling-bond-like electronic states at the Fermi level. The diversity of surface reconstructions and their connection with electronic properties underscore the intricate nature of surface phenomena in MAX phases.