Lone pair interactions in Zintl phases and intermetallic compounds:: Influence on electronic properties in stannides and plumbides

The phases K6Sn23Bi2, K6Sn25, NaSn5, BaSn3, BaSn5, and K5Pb24 depict the structural transition from Zintl phases with localized chemical bonds to typical intermetallic compounds which may even have superconducting properties. The question of the nature of the chemical bond in these compounds is studied with the help of quantum mechanical calculations. Tight binding band structure calculations and real space representations using the Electron Localization Function (ELF) show that free electron pairs play a crucial role for the description of the chemical bond in polar intermetallic compounds. Interactions between lone pairs have a dominant influence on the electronic structures. The coincident appearance of quasi-molecular localized states in form of lone pairs and disperse delocalized bands at the Fermi level E-F, is discussed with respect to a 'chemical view' of the superconductivity observed for BaSn3, BaSn5, and K5P24.