Effect of Heat Treatment on Tin Doping High-<I>Tc</I> Superconductor YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−<I>y</I></SUB>

This study was performed in order to establish the heat treatment for substitution of Sn for Cu in high-Tc superconductor YBa2Cu3O7−y. Two methods of YBa2(Cu, Sn)3O7−y preparation were tired. The crystal structure and the impurity phase in the fired samples were examined by X-ray diffraction, and the superconductivity was estimated by the ac-susceptibility measurement. First the substitution of Sn for Cu in CuO prior to the synthesis of YBa2(Cu, Sn)3O7−y was tried and solubility of Sn in CuO was obtained as a function of firing temperature. Since it was found that Sn is substituted for 0.5 at%Cu at most, this preparation method was abandoned. Second, the ordinary firing of the mixture of 2BaCO3, 1/2Y2O3, 3(1−x)CuO and 3xSnO2 was performed. The Sn solubility was obtained as a function of firing temperature from 900°C to 970°C. Sn of x=0.011 dissolves at 960°C. From the lattice constant, it was shown that Sn is substituted not only for Cu(1) site but also for Ba or Y sites in the case of large x and high firing temperature. In the samples fired at high temperatures, Sn doping did not change Tconset but it decreased the superconductive volume mainly by the orthorhombic-tetragonal phase transition. In the samples fired at 900°C, the Meissner signal decreased by 40%, though Sn of only x=0.001 is soluted, which means that Sn may occupy the Cu(2) site in the samples.