LOCAL-STRUCTURE ABOUT NI ATOMS IN NI-SUBSTITUTED YBA2CU3O7-DELTA

We have performed x-ray-absorption fine-structure measurements on a series of Ni-substituted YBa2Cu3O7- (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, Tc decreases linearly with Ni concentration, with Tc approaching zero at a Ni concentration of 8.5 at. %. © 1990 The American Physical Society.