Local structure about Ni atoms in Ni-substituted ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have performed x-ray-absorption fine-structure measurements on a series of Ni-substituted ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (Y-Ba-Cu-O) samples. We find that, for low Ni concentrations, the Ni substitutes nearly uniformly on both Cu sites. The Ni(1) site is essentially undistorted, while the Ni(2) site is slightly distorted with a longer Ni-Y bond length and a shorter Ni(2)-Ba length. At higher Ni concentrations, significant amounts of NiO were present as an impurity phase. The amount of NiO was determined, and the Ni concentration in Y-Ba-Cu-O corrected. Using these corrected values, ${\mathit{T}}_{\mathit{c}}$ decreases linearly with Ni concentration, with ${\mathit{T}}_{\mathit{c}}$ approaching zero at a Ni concentration of 8.5 at. %.