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 Å and 12 Å in diameter, respectively. Initial atomic configurations before relaxation were created by considering two different space groups corresponding to the cubic Fd3̄m 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 lowtemperature 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.