Electronic structure of metallic and semiconducting alkali-metal–lead compounds

We present an investigation of the electronic structure and the crystal binding in a number of metallic and semiconducting alkali-metal--lead compounds based on linear muffin-tin orbital calculations. We show that at all compositions the electronic structure is dominated by the strong attractive Pb potential. In compounds close to the ``octet'' composition A₄Pb (Na₁₅{Pb}₄$,${Li}₇Pb₂) we find a narrow gap separating the nearly full occupied anion bands from the lowest cation band. However, the bonding is not truly ionic because of the strong delocalization of the lead orbital. In the equi- atomic compounds containing Pb₄ tetrahedra we find a gap at the Fermi level which originates from the splitting of the bands which are bonding or antibonding within the tetrahedra. However, the bonding within the Pb₄ units is again not a classical covalent bond. In the Pb-rich compounds the electronic structure approaches that of pure lead.