THEORETICAL-STUDY OF GA4AS4, AL4P4, AND MG4S4 CLUSTERS

Ab initio molecular orbital investigations of the electronic structures, bonding, and stabilities of Ga4AS4, Al4P4, and Mg4S4 are reported. The effects of polarization functions and electron correlation are included in these calculations. Our results indicate that the electronegativity difference between the constituents of a mixed cluster plays a very important role in determining its ground state structure. In A4B4 mixed clusters, a distorted cubic structure with alternating atomic charges leads to a particularly stable ionic form. This T(d) structure consisting of two interpenetrating tetrahedra of the two constituents is the ground state structure for both Al4P4 and Mg4S4. For Ga4AS4, there also exists a more covalent Si8-like C(i) structure that is 3 kcal/mole more stable than the T(d) form. The structures and relative stabilities of these mixed clusters are compared with the results for valence-isoelectronic Si8 and Na4Cl4 clusters.