Density-functional investigation of the size dependence of the electronic structure of mixed aluminum-sodium clusters

Abstract

Equilibrium geometries and electronic-structure properties have been obtained for cationic, anionic, and neutral ${\mathrm{Al}}_{n}\mathrm{Na}$ and ${\mathrm{Al}}_{n}{\mathrm{Na}}_{2}$ $(n=1--12)$ clusters within the density-functional theory using the generalized gradient approximation for the exchange-correlation potential. The resulting geometries show that the sodium atom prefers to be on the periphery and does not get trapped. The stability has been investigated by analyzing the binding energy, the dissociation energy, and the second difference in energy. The results indicate that for neutral clusters, ${\mathrm{Al}}_{4}\mathrm{Na}$ and ${\mathrm{Al}}_{7}\mathrm{Na}$ are stable. Considerable enhanced stability is also seen for ${\mathrm{Al}}_{4}{\mathrm{Na}}_{2},$ ${\mathrm{Al}}_{6}{\mathrm{Na}}_{2},$ ${\mathrm{Al}}_{4}{\mathrm{Na}}^{\ensuremath{-}},$ and ${\mathrm{Al}}_{6}{\mathrm{Na}}^{\ensuremath{-}}$ clusters. The stability of these clusters cannot be explained on the basis of a simple jellium model. The calculated vertical ionization potentials are in good agreement with their experimental counterparts. No consistent signature of a monovalent nature for Al is observed in these systems. The addition of sodium atoms is found to quench the weak magnetic moment of pure Al clusters and all ${\mathrm{Al}}_{n}{\mathrm{Na}}_{2}$ clusters are found to be nonmagnetic.