First-principles studies of the stability of Zintl ions in alkali-tin alloys: I. Crystalline intermetallic compounds

The systematic variations of the crystal structure, phase stability, electronic structure and chemical bonding properties of equiatomic alkali-tin alloys as functions of the size of the alkali atom have been studied for the example of equiatomic alkali-tin alloys using ab initio local density calculations. It is demonstrated that the formation of the polyanionic phases of KSn and NaSn with tetrahedral Sn4 clusters may be interpreted within the Zintl principle: the large electronegativity difference leads to an at least formally complete electron transfer from the alkali to the tin atoms and to the formation of strong covalent bonds stabilizing the Sn44- 'Zintl ions' which are isoelectronic and isostructural to the P4 molecule. Charge transfer is also the dominant mechanism in LiSn; however, due to the smaller size of its alkali ion, the remaining intercluster interactions are too strong, so the Sn ions form an extended network (in the form of corrugated planes) rather than isolated polyanions. The LiSn structure is also discussed from the point of view of a simple ionic model such as is realized in the CsCl structure. It is shown that the simple ionic model is destabilized by direct Sn-Sn interactions. Local density functional theory is shown to provide an accurate description of the complex crystal structures of these alloys and a rationale for the observed structural trends in alkali-group-IV alloys.