Superconductivity in sulfur-containing R-Ba-Cu-O compounds

We have prepared samples of RBa2Cu3OγSy (R=Y,Er,Nd) and studied them by x-ray powder diffraction, dc resistivity, and thermoelectric power measurements. All the compounds present a large proportion of secondary phases which are dominated by BaSO4, as confirmed by the x-ray-diffraction patterns. Samples with R=Y or Er are still high-temperature superconductors confirming the fact that the sulfur atoms are not inserted, or with a very low proportion, in the 1:2:3 phase. In contrast, for the R=Nd samples, a semiconducting behavior is observed. A gradual transition from an orthorhombic to a tetragonal structure is obtained as the sulfur content is increased in NdBa2Cu3OγSy (0≤y≤1). This is not due to a sulfur insertion in the 1:2:3 phase. In fact, this phase is Ba deficient because of the formation of BaSO4 and some of the Nd substitutes for Ba in the 1:2:3 phase. That is, the resulting perovskitelike composition is Nd(Ba2-xNdx)Cu3O7+δ. This interpretation is consistent with a previous study of the solid solubility in the R(Ba2-xRx)Cu3O7+δ systems.