Structure and bonding in crystalline and molten Li-Sn alloys: A first-principles density-functional study

Ab-initio local-density-functional investigations of crystalline and molten Li-Sn alloys at equiatomic compositions and a stoichiometry close to Li₄Sn (allowing to complete the octet-shell of the Sn-ions if complete charge-transfer is assumed) have been performed. The aim of these studies is to investigate the influence of the formation of direct covalent Sn-Sn bonds on the structure and stability of the alloys. At the equiatomic composition we find that the monoclinic LiSn structure with the Sn-atoms forming rumpled layers (like those in crystalline As) is more stable than a simple ionic CsCl lattice; in the molten phase, this is reflected by the formation of a dense disordered network of Sn-Sn bonds. At the octet composition, an ionic structure with no direct Sn-Sn neighbors and a partially covalent structure with Sn₂ dimers are of comparable stability. In the molten phase, the importance of covalent Sn-Sn bonding is reduced and only a small fraction of the dimers present in the crystal structure survive. Detailed ab-initio molecular dynamics studies for the structural, dynamic, and electronic properties of the liquid alloys are presented. The results are discussed in relation to a generalized Zintl principle for structure and bonding in such strongly interacting alloy systems.