Effect of zinc and iron on the (micro-)structure and copper charge excess of the YBaCuO superconductor

Samples of YBa₂(Cu_1–y Zn _y )₃O_7–y and YBa₂(Cu_1–y Fe _y )₃O_7+x , withy 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 equal 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, iodometry shows that the oxygen stoichiometry decreases in zinc-doped samples while it increases in iron-doped samples. Charge balance resulting from the dopant charge and the evolution of the copper charge with doping have been invoked. In zinc-doped samples, the copper charge excess (copper charge fraction>2) decreases sharply for 0.0y<0.04, then it shows a plateau-like behaviour for 0.04y<0.06, while in iron-doped samples, it decreases almost monotonically. Confirming some of our previous results there is a correlatedT _c decrease in the case of zinc-doped samples (occurrence of aT _c plateau) and in the case of iron-doped samples (quasi monotonical decrease). This difference has been interpreted in terms of structural changes related to the different substitution behaviour of zinc and iron.