ИСТИНА

Polyoxopalladates (POPs) are a new class of nanoscale materials with a well-defined structure and ions coordination which is very attractive either for fundamental research or hydrogen-uptake and catalysis applications. Cuboid-shaped POPs are very flexible and can adopt a variety of d-ions and lanthanides in their centres while the whole nanocluster can be stabilized by different heterogroups like PO4, AsO4, PhAsO3, etc [1,2]. Electronic and magnetic properties of central metal ions and Pd(2+) ions in the shell are very sensitive to their coordination, local distortions and external influence. X-ray absorption near edge structure (XANES) and its associated X-ray magnetic circular and linear dichroisms (XMCD and XMLD) can provide an information about the valence state, magnetic moments and local symmetry of probed ions in an element-specific way, and in some cases can be even site-specific. The shape of XANES spectra reflects the symmetry of the crystal field and the energy levels splitting; by a sum-rule based analysis of the XMCD signal the spin and orbital magnetic moments of chosen ions can be determined; XMLD signal relates to the charge asymmetry and can be used for a better spectra refinement. XANES, XMCD and XMLD spectra for POPs with the central Fe or Pd ions were recorded at the Fe L2,3 and the Pd M2,3 absorption edges at LT (4.5K) under the external magnetic fields up to 6T. Particular features of obtained spectra for 3d central ions and palladium ions in different local environments were described through comparison with results of multiplet calculations performed by the CTM4XAS program [3]. The crystal field parameters, valence states and magnetic moments of probed ions were quantified. In addition, an influence of a soft hydrogen plasma treatment and re-oxidation process on electronic and magnetic properties of Fe and Pd ions was studied. It was found that POPs with Fe(3+) central ions in the as-prepared states show different XMCD and XMLD signals depending on the capping group (PO4 or PhAsO3) and exhibit distinct reduction behaviour under the hydrogen treatment with visible decrease of magnetic moments; the partial re-oxidation with oxygen gas was going easier for POPs stabilized with PhAsO3 heterogroup. The Pd(2+) ions in the central and shell positions bear different magnetic moments due to the diverse local coordinations; a possible influence of heterogroup on Pd(2+) magnetism and magnetic polarisation associated with hybridized Oxygen states is discussed. In general, it was shown that individual properties of particular ions in POPs can be tailored (e.g., by heterogroups), and can be further adapted for needs of technology. The work was funded by Helmholtz Association (Young Investigator’s group “Borderline Magnetism”, VH-NG-1031). [1] M. Barsukova-Stuckart, N.V. Izarova et al., Inorg. Chem., 18 (2012) 6167-6171. [2] M. Barsukova, N.V. Izarova et al., Chem. Eur. J., 16 (2010) 9076 – 9085. [3] E. Stavitski, F.M.F. de Groot, Micron, 41 (2010) 687-694.