Structure and electronic properties of iron oxide clusters: A first-principles study

In this study we present results of electronic structure calculations for some iron oxide clusters of the form FenOm on the basis of the GGA+U approximation. The cluster size ranged between 33 and 113 atoms corresponding to length scales between around 7 A degrees and 12 A degrees in diameter, respectively. Initial atomic configurations before relaxation were created by considering two different space groups corresponding to the cubic Fd3m and monoclinic P2/c symmetries. The charge and the magnetization per atom were computed. In particular, the charge distribution of the cluster relaxed from cubic symmetry and containing 113 atoms reveals a well-defined periodic pattern of Fe pairs consistent with a partial charge-ordering scenario. Results evidence that the ground-state cohesive energy is smaller in the clusters originated from the P2/c symmetry. This fact indicates that at least in the largest cluster, having more tendency to preserve the initial structure, the low-temperature monoclinic phase is energetically more stable. Clusters starting from monoclinic symmetry are characterized by an insulating state, whereas those optimized from cubic symmetry exhibit a very small electronic gap. Finally, radial and angular distribution functions reveal strong modifications of the starting crystalline structures after relaxation with a tendency of forming cagelike structures.