Oxygen stoichiometry, structure and superconductivity in the superconducting series Pb₂Sr₂Y_1-xCa_xCu₃O_8+∂

The unit cell of Pb₂Sr₂YCu₃O_8+∂ with ∂ = 0 contains the sequence of layers: ...(Y)(CuO₂)(SrO)(PbO)(Cu)(PbO)(SrO)(CuO₂)... in which the (Y) and (Cu) planes are oxygen deficient. Because of the low formal valences of the Cu cations, (Cu²⁺in the (CuO₂) planes and Cu¹⁺ in the (Cu) planes) this compound is not a superconductor. Neutron, x-ray, and electron diffraction measurements show that the Cu layer inserted between two (PbO) layers can easily incorporate extra oxygen. A theoretical composition ∂ = 2 is possible, although in practice only a stoichiometry corresponding to ∂ = 1.9 has been achieved so far. This may be explained by the presence of twin boundaries. In the range of composition 0≤∂≤1, mixtures of two phases are obtained, one with ∂ = 0 and the other with ∂ = 1, whose relative quantities depend on the total amount of oxygen incorporated by the sample. The positive charges induced in Pb₂Sr₂YCu₃O_8+∂ by oxidation, are localized on the Pb sublattice. This localization hinders the charge transfer to the conducting (CuO₂) planes and, for this reason, no superconductivity is present in oxidized samples. Pb₂Sr₂YCu₃O_8+∂ becomes superconducting at ≈ 80K when some of the trivalent Y cations are replaced by divalent Ca. In this case the extra positive charges oxidize the Cu²⁺ cations of the (CuO₂) planes to 3+ instead of the Pb²⁺ to 4+ and the Cu¹⁺ to 2+, as does the incorporation of oxygen. When heat-treated at 450 °C in O₂, Pb₂Sr₂Y_1-xCa_xCu₃O_8+∂ behaves similarly to the undoped compound. The oxygen uptake suppresses the superconducting transition which is re-established by heattreating the sample at same temperature in N₂.