EFFECT OF ZINC AND IRON ON THE (MICRO-)STRUCTURE AND COPPER CHARGE EXCESS OF THE YBACUO SUPERCONDUCTOR

Samples of YBa2(Cu1-yZny)(3)O-7-y and YBa2(Cu1-yFey)(3)O-7+x,O- with y in the range 0.0-0.16 for zinc and 0.0-0.30 for iron were synthesized by solid-state reaction. The solubility limit has been found to be eq ual to 7% and 19 at % for zinc and iron, respectively. Zinc has little effect on the structure which remains orthorhombic throughout all the zinc concentration range, while iron induces an important structural modification (tweed structure formation). Both impurities induce grain -growth inhibition but densification appears to be independent of the impurity content with respect to the undoped material. However, iodome try shows that the oxygen stoichiometry decreases in zinc-doped sample s while it increases in iron-doped samples. Charge balance resulting f rom the dopant charge and the evolution of the copper charge with dopi ng have been invoked. In zinc-doped samples, the copper charge excess (copper charge fraction > 2) decreases sharply for 0.0 less than or eq ual to y < 0.04, then it shows a plateaulike behaviour for 0.04 less t han or equal to y < 0.06, while in iron-doped samples, it decreases al most monotonically. Confirming some of our previous results there is a correlated T-c decrease in the case of zinc-doped samples (occurrence of a T-c plateau) and in the case of iron-doped samples (quasi monoto nical decrease). This difference has been interpreted in terms of stru ctural changes related to the different substitution behaviour of zinc and iron.