SUPERCONDUCTING AND MAGNETIC-PROPERTIES OF MATERIALS WITH VARYING CLUSTERING IN YBA2 (CU1-XMX)3O] WITH M = FE, CO, NI

We present structural, magnetic and superconducting properties of conv entionally, and reductively prepared equilibrium states of YBa2(Cu1-xM x)3Oy with M = Fe, Co, Ni. Depending on preparation conditions, varyin g clustering tendencies for M on Cu(I) are found. A highly ordered clu ster state (U) manifests itself in relatively high T(c) and strong ort horhombicities amongst others. This has to do with the partial condens ation of M into cluster domain walls. Mode and conditions for this clu stering are different for different M. As an example, maximum values o f T(c) at a given x are found at lower annealing temperatures for M = Co compared to Fe both, experimentally and on a local thermodynamic mo del. Comparisons of this model with data from Mossbauer spectroscopy a re drawn. The clustered M = Fe materials show an increase in critical currents through flux pinning on the Fe clusters, indicating technolog ical potential.