Structural growth in iron oxide clusters: Rings, towers, and hollow drums

It is shown that the transition from an elementary FeO molecule to the bulk rock-salt FeO proceeds via hollow rings, towers, and drums. Our first-principles electronic structure calculations carried out within a gradient-corrected density functional framework show that small FeₙOₙ (n=2,0.3em0ex3,0.3em0ex4,0.3em0ex5) clusters form single, highly stable rings. Starting at Fe₆O₆, these elementary rings begin to assemble into nano columnar structures to form stable Fe₆O₆, Fe₇O₇, Fe₈O₈, Fe₉O₉, Fe₁₀O₁₀, and Fe₁₂O₁₂ towers. The rings and the empty towers can be further stabilized by capping O atoms at the ends, leading to FeₙOₙ₊₁ and FeₙOₙ₊₂ sequences. The theoretical results provide insight into the progression of mass intensities in the experimental mass spectra and account for the observed peaks in the negative ion photodetachment spectra of iron oxide clusters.