First-principles studies of the stability ofZintl ions in alkali-tin alloys: I. Crystalline intermetalliccompounds

Abstract

The systematic variations of the crystal structure, phase stability,electronic structure and chemical bonding properties of equiatomicalkali-tin alloys as functions of the size of the alkali atom havebeen studied for the example of equiatomic alkali-tin alloys usingab initio local density calculations. It is demonstrated thatthe formation of the polyanionic phases of KSn and NaSn withtetrahedral Sn4 clusters may be interpreted within the Zintlprinciple: the large electronegativity difference leads to an atleast formally complete electron transfer from the alkali to the tinatoms and to the formation of strong covalent bonds stabilizing theSn44- `Zintl ions' which are isoelectronic and isostructuralto the P4 molecule. Charge transfer is also the dominantmechanism in LiSn; however, due to the smaller size of its alkaliion, the remaining intercluster interactions are too strong, so theSn ions form an extended network (in the form of corrugated planes)rather than isolated polyanions. The LiSn structure is alsodiscussed from the point of view of a simple ionic model such as isrealized in the CsCl structure. It is shown that the simple ionicmodel is destabilized by direct Sn-Sn interactions. Local densityfunctional theory is shown to provide an accurate description of thecomplex crystal structures of these alloys and a rationale for theobserved structural trends in alkali-group-IV alloys.