Valence compounds at the border to intermetallics: Alkali and alkaline earth metal stannides and plumbides

The element Sn has the outstanding property that it appears in a metallic and a semi-metallic modification, which possess almost the same stability. The associated interplay between delocalized and localized bonding governs the structures of alkali and alkaline earth metal stannides and leads to a large variety of different structure types. Even though the element Pb crystallizes in the fcc lattice, which is typical for metals, directed bonds between Pb atoms occur in compounds with electropositive metals. During the last decade several new structures in the phase systems of alkali or alkaline-earth metals with tin or lead were characterized. Many examples show the gradual transition from valence compounds to intermetallic phases and present new possibilities for structural and bonding motives for Sn and Pb. These structures reinforce interesting links between the various structure types. Structural relationships of Zintl phases containing discrete and linked polyhedra, as well as the gradual transition to typical intermetallic compounds are highlighted.