XAFS DETERMINES MINI-CLUSTERS OF NI-O IN NI DOPED YBA2CU3O7-DELTA

X-ray Absorption Fine Structure (XAFS) measurements probe the very local environment of atoms. A sizeable amount of the Ni atoms in Ni substituted YBCO was shown to form Ni-O clusters with bond lengths characteristic of NiO even when x-ray diffraction indicated single phase samples. Subtracting the amount of Ni in NiO precipitates, T(c) decreases linearly with Ni concentration and extrapolates to zero at 8.5% Ni.