Evolution of the structural and electronic properties of small alkali metal-doped aluminum clusters

Abstract

The atomic arrangement of Aln and AlnM clusters (n=2–14, M=Li, Na o K) was determined by combining both the basin hopping (BH) algorithm using the Gupta potential, and density functional theory (DFT) calculations. The BH yielded hundreds of structures that were refined by DFT using the PBE framework, and Ahlrichs-VDZ basis sets. Anions, neutrals and cations of Aln and AlnM clusters were calculated by DFT resulting in a set of ground state structures. These structures were considered for studying different stability criteria such as binding energy, dissociation energy, second-order difference of energies, and HOMO-LUMO gaps. Furthermore, the calculated ionization potential, and electron affinity of the clusters yielded values comparable to the experimental ones. A further bonding analysis of the clusters was carried out by the quantum theory of atoms in molecules (AIM) and by using the electronic localization function (ELF). Based on the stability criteria, we determinate the following clusters as relative stable against neighbors: Al7+, Al13−, Al2M−, Al4M−, Al6M−, and Al13M. Except by the Al4M− clusters, all clusters are closed shell structures and they follow the jellium model predictions. The high relative stability of the Al4M− clusters was attributable to their aromaticity. Moreover, AIM and ELF results revealed that when the size of the clusters increases then the transition from covalent to metallic AlAl bonding emerges. Otherwise, in the case of AlnM clusters, it was found that the nature of AlM bonds is modified with the size, from metallic for Al2M− to ionic when their size increases.