ИСТИНА

Polyoxometallates (POMs) represent a large class of nanosized, polynuclear metal-oxo anions with a wide compositional and structural variety. Cuboid shaped polyoxopalladates (POPs) shells can incorporate various transitions metal ions placed in the cubic crystal field for studying their magnetic properties. Two types of POPs, Fe-POP (with Fe ion in the center) and Pd-POP (with Pd ion in the center), have been investigated by x-ray absorption spectroscopy (XAS) at the L2,3 absorption edges of Fe and at the M2,3 absorption edges of Pd. Spin and orbital magnetic moments were quantified by an integral sum-rules analysis of the x-ray magnetic circular dichroism (XMCD). For Fe-POP, the derived magnetic moments of Fe correspond with the values expected from a simple atomic model. A small XMCD at Pd was found only for the Pd-POP sample. For Fe-POP it is considerably smaller or even absent. This assumes that the magnetism of Pd arises only from the central Pd ion and it is very sensitive to the coordination environment. In addition, the influence of a hydrogen plasma treatment was investigated. The Fe ions were reduced from the initial 3d5 to 3d6 state, the crystal field was changed, and no XMCD for Pd was found in both samples.