The stability of solid and liquid compounds with strong anion-anion interactions

The equiatomic alkali-group-IV (Si,Ge,Sn,Pb) compounds usually crystallize in the NaPb structure. This structure contains negatively charged tetrahedra of group-IV ions, which are separated by alkali ions. The Li compounds, however, have either a CsCl (LiSn,LiPb) structure or a structure with a three-dimensional network of group-IV atoms, e.g., LiGe. A similar difference between Li compounds on the one hand, and Na, K, Rb, Cs compounds on the other hand, has been found in the corresponding liquid alloys. The author explains this difference in structures in terms of two factors: the existence of a gap in the density of states at the Fermi level in the phase with tetrahedra, and the radius of the alkali atom. When the alkali ion is too small, like Li+, to separate the tetrahedra, then either the CsCl structure is the most stable, or a threefold-coordinated anion network will be the most stable structure. The latter structure occurs when the group-IV anions have a strong covalent interaction like for Ge and Si.