Electronic structures, bonding natures and defect processes in Sn-based 211 MAX phases

The electronic structure, bonding natures, and defect processes of the new superconducting MAX phase Lu2SnC are investigated by using density functional theory, and are compared to other existing M2SnC phases. The formation of M2SnC MAX phases is exothermic and these compounds are intrinsically stable in agreement with experiment. The finite value of DOS, in addition to the d-resonance at the vicinity of the Fermi level, indicates a metallic nature and conductivity of M2SnC MAX phases. The strength of the covalent M–C bond is higher than that of the covalent M–Sn bond. The calculated effective valence charge also indicates the dominance of covalency in the chemical bonding in the studied compounds. The charge transfer in M2SnC phases indicates the ionic nature of their chemical bonds. The ionic character of their chemical bonds can also be understood from the spherical nature of charge distribution in their contour maps of electron charge density. Therefore, the overall bonding nature in the studied M2SnC MAX phases is a combination of metallic, covalent, and, ionic. The bond length is directly proportional to the crystal radius, while bond covalency is inversely proportional to the crystal radius. Additionally, the Fermi surface topology is also investigated. Considering the intrinsic defect processes it is calculated that Nb2SnC is the material that is predicted to have better radiation tolerance.