High symmetry Nbn and Tan (n = 12, 15, and 17) clusters: High magnetic moments and the finding of superatoms with doping

Abstract

Using ab initio calculations, we study Nbn and Tan clusters with n=12, 15, and 17 and find superatoms made of transition metals. Nb12 and Ta12 have an empty cage icosahedral structure. Neutral Ta12 has icosahedral symmetry and 2μB magnetic moments while the cage for Nb12 is slightly distorted and has no magnetic moment. These clusters behave like a divalent superatom. Accordingly an oxygen atom interacts exohedrally on a 3-fold site of the icosahedral cage like an MgO molecule and leaves the cage intact while Fe, Ru, and Os atoms can be endohedrally doped in Ta12 to produce electronically closed shell clusters with large highest occupied-lowest unoccupied molecular orbital (HOMO–LUMO) gap. Also isoelectronic Co@X12 and Rh@X12 (X=Nb and Ta) cations have a large HOMO–LUMO gap of up to about 1eV. In these cases the magnetic moment is quenched. On the other hand doping with Ni leaves the magnetic behavior of Ta12 the same. Further anions of Nb15 and Ta15 have a closed electronic shell structure and a singlet ground state with a large HOMO–LUMO gap. We show that isoelectronic Nb14Mo, Nb14W, Ta14Mo, Ta14W, Nb14Mn+, Nb14Re+, Ta14Mn+, and Ta14Re+ all have a large HOMO–LUMO gap of up to about 1.0eV and are magic clusters. Most interestingly Ta17 has a tetrahedrally symmetric Z16 Frank–Kasper polyhedral structure with a large magnetic moment of 5μB. On the other hand the magnetic moment on Nb17 cluster is 1μB. The magnetic moment on Ta17 is surprisingly large and it is counter intuitive as normally the magnetic moments decrease as one goes down in a column in the periodic table but some tantalum clusters behave differently. Doping of the n=17 clusters with Zr at the center leaves Zr@Ta16 magnetic with 2μB and 6μB magnetic moments nearly degenerate while Zr@Nb16 has zero magnetic moment.