Magic behavior and bonding nature in hydrogenated aluminum clusters

Abstract

Ab initio electronic structure calculations have been performed using plane-wave ultrasoft pseudopotentials as well as Gaussian atomic-orbital methods to understand the interaction of hydrogen with aluminum clusters. Our results show large binding energies of a single hydrogen atom on small Al clusters and large highest occupied and lowest unoccupied molecular-orbital gaps for AlH, ${\mathrm{Al}}_{7}\mathrm{H},$ and ${\mathrm{Al}}_{13}\mathrm{H}$ making these species behave like magic clusters. In general the binding energy of H is found to decrease with an increase in the cluster size. Calculations on two hydrogen atoms on Al clusters show large binding energies for ${\mathrm{Al}}_{n}{\mathrm{H}}_{2}$ with $n=2,$ 4, 6, and 8, but a significant decrease for ${\mathrm{Al}}_{7}{\mathrm{H}}_{2}$ as compared to ${\mathrm{Al}}_{7}\mathrm{H}.$ These results confirm the magic behavior of ${\mathrm{Al}}_{7}\mathrm{H}$ and suggest that hydrogen should be dissociated on ${\mathrm{Al}}_{6},$ in excellent agreement with the available experimental data.