Dopant effects on the superconductivity of YBa2Cu3O7 ceramics

We investigated the effects of SrTiO3 and 17 different oxide dopants, namely Ca, Sc, La, and Zr with possible substitution in the yttrium sites, K, Sr, and Pb in the barium sites, Li, Mg, Fe, Co, Ni, Zn, Al, Ti, Nb, and Si in the copper sites on the superconductivity of YBa2Cu3O7. These dopants can be classified into four categories depending on their solubilities and substitution in the different cation sublattices of YBa2Cu3O7 perovskite. Dopants in the first category dissolve and substitute in the copper sublattice and they can significantly reduce the transition temperature of the superconductivity of YBa2Cu3O7. For example, three dopants in this category, namely Zn, Mg, and Li, at 2 mole % dopant level, reduce the transition temperatures to 66, 65, and 82 K, respectively. The second category dopants substitute in the barium and yttrium sites, and these dopants, e.g., Sr at 2 mole % concentration, do not have any significant effect on the transition temperature of the superconductivity of YBa2Cu3O7. While dopants, e.g., AlO/sub 1.5/ and SiO2, in the third category have a limited solubility in YBa2Cu3O7, they have a strong tendency to decompose the superconducting phase by leaching out some components of YBa2Cu3O7. The transition temperature of the undecomposed superconducting phase remains unaffected at ∼91 K even though the susceptibility signal is reduced substantially. Since the dopants in the fourth category have a limited solubility and very little reactivity with YBa2Cu3O7, they are present simply as an inert second phase among the superconducting matrix