Evolution of the electronic structure and properties of neutral and charged aluminum clusters: A comprehensive analysis

Density-functional theory with generalized gradient approximation for the e xchange-correlation potential has been used to calculate the global equilib rium geometries and electronic structure of neutral, cationic, and anionic aluminum clusters containing up to 15 atoms. The total energies of these cl usters are then used to study the evolution of their binding energy, relati ve stability, fragmentation channels, ionization potential, and vertical an d adiabatic electron affinities as a function of size. The geometries are f ound to undergo a structural change from two dimensional to three dimension al when the cluster contains 6 atoms. An interior atom emerges only when cl usters contain 11 or more atoms. The geometrical changes are accompanied by corresponding changes in the coordination number and the electronic struct ure. The latter is reflected in the relative concentration of the s and p e lectrons of the highest occupied molecular orbital. Aluminum behaves as a m onovalent atom in clusters containing less than seven atoms and as a trival ent atom in clusters containing seven or more atoms. The binding energy evo lves monotonically with size, but Al-7, Al-7(+), Al-7(-), Al-11(-), and Al- 13(-) exhibit greater stability than their neighbors. Although the neutral clusters do not conform to the jellium model, the enhanced stability of the se charged clusters is demonstrated to be due to the electronic shell closu re. The fragmentation proceeds preferably by the ejection of a single atom irrespective of the charge state of the parent clusters. While odd-atom clu sters carry a magnetic moment of 1 mu(B) as expected, clusters containing e ven number of atoms carry 2 mu(B) for n less than or equal to 10 and 0 mu(B ) for n > 10. The calculated results agree very well with all available exp erimental data on magnetic properties, ionization potentials, electron affi nities, and fragmentation channels. The existence of isomers of Al-13 clust er provides a unique perspective on the anomaly in the intensity distributi on of the mass spectra. The unusual stability of Al-7 in neutral, cationic, and anionic form compared to its neighboring clusters is argued to be due to its likely existence in a mixed-valence state. (C) 1999 American Institu te of Physics. [S0021-9606(99)30629-2].