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

Abstract

In this study we present results of electronic structure calculations for some iron oxide clusters of the form ${\text{Fe}}_{n}{\text{O}}_{m}$ on the basis of the $\text{GGA}+U$ approximation. The cluster size ranged between 33 and 113 atoms corresponding to length scales between around $7\text{ }\text{\AA{}}$ and $12\text{ }\text{\AA{}}$ in diameter, respectively. Initial atomic configurations before relaxation were created by considering two different space groups corresponding to the cubic $Fd\overline{3}m$ 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.