Structural and Electronic Properties of (Al2O3)n Clusters with n = 1–10 from First Principles Calculations

The atomic structures, growth behavior, and electronic properties of (Al2O3)n, n = 1–10, clusters have been studied within the framework of density functional pseudopotential theory and generalized gradient approximation for the exchange–correlation energy. The lowest energy isomers of these clusters show preference for 4-membered Al2O2 and 6-membered Al3O3 rings. There are 3-, 4-, and 5-fold coordinated Al atoms and 2-, 3-, and 4-fold coordinated oxygen atoms. The atomic structures have similarity with that of the γ-Al2O3 phase and the average coordinations of Al and O atoms in clusters are much lower from the values in the ground state of α-Al2O3 (corundum structure). In general, isomers with cage structures lie significantly higher in energy compared with the lowest energy structures we have obtained. The bonding characteristics for clusters of different sizes is studied using Bader charge analysis. It is found that with increasing size, the charge transfer from Al atoms to oxygen increases toward the value in bulk. Further, the infrared (IR) and Raman spectra have been calculated. For n = 4, a comparison of the calculated IR spectra for a few isomers with the available experimental results on cation shows the possibility of the occurrence of a mixture of isomers in experiments. The Raman spectra of these isomers are, however, quite different. Therefore, it is suggested that measurements on Raman spectra could give a clear indication of the isomer present in experiments.