Structures and Magnetism of Cationic Chromium-Manganese Bimetallic Oxide Clusters

The chromium-manganese bimetallic oxide clusters CrMnO4+, CrMn2O5,6+, Cr2MnO6,7+, CrMn3O6+, Cr2Mn2O7,8+, and Cr3MnO8,9+ were synthesized in a laser ablation cluster source and characterized by infrared multiple photon dissociation spectroscopy on the corresponding weakly bound cluster-O-2 complexes. The geometric and electronic structures of CrMnO2-6+, CrxMnyO3-8+ (x,y = 2,1 and 1,2), and CrxMnyO5-10+ (x,y = 1,3, 2,2, and 3,1) were studied using density functional theory, and for certain sizes, the spectroscopic data allowed to assign the predicted structures. It is found computationally that, for a given number of metal atoms, neither the Cr:Mn ratio nor the presence of additional oxygen atoms significantly affects the geometric structures, but their magnetic properties are strongly dependent on the chemical composition. While total magnetic moments of the small CrMnOz+ (z = 2-6) clusters monotonically decrease upon addition of oxygen atoms, those of CrxMnyOz+ (x + y = 3, 4) are lower because of antiferromagnetic interactions and are less dependent on the number of oxygen atoms. The local magnetic moments of metallic sites significantly decrease with additional oxygen atoms, leading to almost quenched magnetism for the saturated oxide clusters. In general, the local magnetic moments on the Mn atoms are found to be higher than on the Cr atoms, and upon addition of oxygen, the local magnetic moments on Cr sites decrease first.