Structure, fragmentation patterns, and magnetic properties of small nickel oxide clusters

We report a comprehensive theoretical study of the structural and electronic properties of neutral and charged nickel oxide clusters, Ni_nO_m^0/± (n = 3-8 and m = 1-10), in the context of recent experiments of photodissociation and Ion Mobility Mass Spectrometry. By means of density functional theory calculations in the generalized gradient approximation for exchange and correlation, we determined the putative ground states as well as the low-energy structural- and spin-isomers which were then used to explore the favorable fragmentation channels of the nickel oxide cationic clusters, and the resulting most abundant products, in good qualitative agreement with photodissociation measurements. Apart from stoichiometries different from those of their nickel oxide macroscopic counterparts, we found a tendency to form compact Ni subclusters, with reentrance of low-coordinated structures close to the equiatomic Ni-O concentration, taking the form of alternating Ni-O rings in the smaller sizes, in good qualitative agreement with Ion Mobility Mass Spectrometry measurements. This structural pattern is manifested in a drop of the total spin magnetic moment close to the equiatomic concentration due to the formation of antiparallel magnetic couplings. Although antiparallel couplings are found to a more or less extent in most clusters, especially in the oxygen rich phase, we identified certain clusters of special interest in the context of magnetic grains because of their large total magnetic moment and abundance. There are even some nickel oxide clusters with a higher total moment than their pure Ni counterparts, due to parallel magnetic couplings and the contribution of the oxygen atoms to the total moment.