Nickel impurities in the Y-Ba-Cu-O 90-K superconductor

The orthorhombic superconductor ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with nominal values of x=0.02 and 0.05 and $T_c$=86.5 and 79.5 K, respectively, was investigated by x-ray-absorption fine-structure (XAFS) measurements about the Ni and Cu K edges. The sample was carefully characterized by various means including optical and electron microscopy. In the sample with x=0.02, the Ni was predominantly dissolved in the 1:2:3 phase. The maximum solubility found in the nominal 5% phase was x=0.04, with the excess precipitating out as NiO. The Ni atoms in the 1:2:3 phase were found to have 4.75±0.4 oxygen neighbors at 1.95±0.03 Å. The best fit to the next neighbor was 5.1±1.2 Y atoms at 2.96±0.02 Å, though the data are also consistent with a mixed shell of Y and Ba atoms. Although the Ni atoms have a first-neighbor environment of oxygen atoms somewhat similar to the Cu site, the more distant neighbor distances are quite different. It is concluded from both the extended and near edge XAFS that most, if not all, of the Ni atoms do not substitute in either Cu site of the 1:2:3 phase, but are in some defect site.