Atomic and electronic structures of niobium clusters

First-principles electronic structure calculations have been carried out on Nb_N clusters with N = 2 - 23 atoms using ultrasoft pseudopotentials, a plane wave basis and generalized gradient approximation for the exchange-correlation energy, to elucidate the growth behavior and the evolution of the electronic structure. Our results show that clusters with N>13, prefer high coordination structures with hexagonal antiprisms except for N = 15 which has a distorted body centered cubic structure (bcc). Clusters with N>2 favor lowest spin configurations. Of particular importance is the finding that icosahedral growth is not favored in these clusters. There are large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps for clusters with 4, 6, 8, 10, and 16 atoms. These are in agreement with the observed low reactivities as well as the photoemission data of 6-, 8-, 10-, and 16-atom clusters. Clusters with 4, 8, 10, 15, and 16 atoms are found to be magic, suggesting the importance of the atomically closed shell bcc structure of Nb₁₅ even though the HOMO-LUMO gap is small. However, Nb₆ is not magic. A dimerization behavior is obtained in Nb₁₀ and it is found to be even more prominent in the isoelectronic V₁₀. But, this tendency is absent in Ta₁₀. The calculated binding energies, electronic structures and fragmentation behavior are in good agreement with available experimental data.