Electronic Structures of KNa₃In₉ and Na₂In, Two Metallic Phases with Classical Closed-Shell Electronic Configurations

The cluster compounds KNa₃In₉ [K₂Na₆(In₁₂)(In)₆] and Na₂In [(Na)₈(In₄)], which contain In₁₂ icosahedra interbridged by 4-bonded In atoms and isolated In₄ tetrahedra, respectively, both have classical closed-shell electronic configurations but show metallic transport properties. These contrasts have been studied by means of first-principles density functional methods (LMTO-ASA). Several bands cross the Fermi level in both compounds, consistent with their metallic properties. In KNa₃In₉, the metal atom framework alone is sufficient to generate a metallic characteristic. The alkali-metal s and indium p orbitals mix considerably in both phases, providing for substantial covalent contributions to their stabilities as well as bands crossing E_f. The participation of Na atoms in the 3D bonding networks is more striking in cation-richer Na₂In than in KNa₃In₉.