Structure of Metal Clusters — A High Resolution Electron Microscopy Study of Osmium Clusters on Magnesium Oxide

Abstract

High resolution electron microscpy has been used to obtain direct structure images of small aggregates of osmium dispersed on MgO. Some molecular clusters of Os₁₀C(CO)₂₄ ^2- were deposited directly onto the MgO support and others were synthesised on the support from chloroosmic acid. The electron micrographs show a distribution of aggregate sizes in the latter sample, the larger aggregates having the osmium hexagonal-close-packed structure. Also evident are very small particles ranging in size from one or two atoms up to several atoms, with structures which cannot readily be interpreted as being hep. The Os₁₀ clusters deposited on the support do not have the normal osmium hexagonal-close-packed structure, nor can the images be interpreted as being face-centred-cubic. The images can be interpreted on the basis of the assumption that the clusters have an hexagonal-close-packed structure with a larger c/a ratio. There is a strong interaction between the MgO and the Os aggregates, which are aligned parallel to the MgO {002} planes. The strong interaction could be responsible for the distortion of the cluster away from the normal bulk osmium hexagonal-close-packed structure.