Crystal structure, atomic ordering and charge localization in Pb₂Sr₂Y_1-xCa_xCu₃O_8+δ (x=0, δ=1.47)

Neutron, X-ray and electron diffraction measurements have been carried out on Pb₂Sr₂Y_1-xCa_xCu₃O_8+δ samples. The oxygen incorporated in the structure during the oxidation is located on the (Cu) planes sandwiched between the two (PbO) layers. A theoretical composition of δ=2 is possible, although in practice only a stoichiometry corresponding to δ=1.9 has been achieved so far. The extra oxygen present for δ620 forms ordered structures in which the Cu cations of the (CuO_δ) planes have square planar coordination in Pb₂Sr₂YCu₃O₉, either square planar, pyramidal, and octahedral, or only pyramidal coordination in Pb₂Sr₂YCu₃O_9.5, and exclusively octahedral coordination in Pb₂Sr₂YCu₃O₁₀. In the range of composition 0≤δ≤1, mixtures of two phases are obtained, one with δ=0 stoichiometry and the other with δ=1, and whose relative quantities depend on the total amount of oxygen incorporated by the sample. The positive charges induced in Pb₂Sr₂YCu₃O_8+δ by oxygen incorporation oxidize the Cu¹⁺ cations to 2+ and some of the Pb²⁺ cations to 4+. An order between Pb²⁺ and Pb⁴⁺ is established and this localization hinders the charge transfer to the conducting (CuO₂) planes and, for this reason, no superconductivity is present in oxidized samples. The cation valences are estimated from the co-ordination numbers and from the bond length-bond strength relationship. Pb₂Sr₂YCu₃O₈ becomes superconducting at ≈ 80 K when some of the trivalent Y cations are replaced by divalent Ca. In this case the extra positive charges oxidize the Cu²⁺ cations in the CuO₂ planes instead of Pb²⁺ to 4+ and Cu¹⁺ to 2+, as does the incorporation of oxygen. This different behavior can be explained as a concentration effect which changes the oxidation/reduction potentials. When heat treated at 500°C in O₂, Pb₂Sr₂Y_0.5Ca_0.5Cu₃O₈ behaves similarly to the undoped compound. The oxygen uptake suppresses the superconducting transition which is re-established by heat treating the sample at the same temperature in N₂.