3.3 In-depth study of AlBe3 and AlBe8
Based on the preceding analysis of energetic and electronic properties, it is interesting to find that the AlBe3 and AlBe8 clusters possess special stability among the AlBen series. Hence, the electronic structure and density of states (DOS) of these two clusters are further investigated. To facilitate comparison, the total and partial density of states (TDOS and PDOS) of AlBe3 and AlBe8 as well as their corresponding Be4 and Be9 clusters are plotted as a function of energy in Figure 5, in which their valence molecular orbitals (MOs) obtained at the ROMP2//B3LYP level are also illustrated.
From Figure 5b, the singly-occupied HOMO orbital of AlBe3 exhibits dominant s state, the HOMO-1 to HOMO-3 orbitals present typical p shape, and the HOMO-4 orbital shows spherically symmetric s character. So the valence electron configuration of AlBe3 is1S 21P 62S 1. As for the AlBe8 cluster, its nineteen valence electrons are arranged in the1S 21P 61D 102S 1pattern, which accords well with the prediction of the spherical jellium model.[15] Compared with AlBe8, the triplet Be9 cluster is one D electron less and has a electronic shell structure of1S 21P 61D 92S 1(see Figure 5c). As the studied AlBen clusters are all open-shell species, there are not any shell-closed magic clusters in this series. However, the AlBe3 and AlBe8 clusters do exhibit enhanced stability relative to their congeners. A possible explanation is that both of them are one electron short of shell closure, and their desire for an additional electron makes the component atoms bind so tightly with each other that the clusters are stable against dissociation of any kind.
From Figure 5, the introduction of Al impurity has more effect on the DOS of p-type molecular orbitals of beryllium clusters. The comparison between TDOS of Be4 and AlBe3 shows that the peak near -6 eV for the former splits into two peaks in the latter. Similarly, the peak around -9.5 eV in the TDOS plot of Be9 also splits into two peaks at -9.5 eV and -10 eV in that of AlBe8. In contrast, the Al doping effect is minor on the DOS of other states.