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 (<i>e.g.</i>, Ti₂PbC and Ti₂SnC), dimer/trimers (<i>e.g.</i>, Ti₂PC), tetramers (<i>e.g.</i>, Ti₂SiC and Ti₂GeC), pentagon chains (<i>e.g.</i>, Ti₂SiC), or Kagome lattices (<i>e.g.</i>, 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.