Neutron, X-ray and electron diffraction measurements have been carried out on Pb2Sr2Yl _xCaxCu3Os+6 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 8 = 2 is possible, although in practice only a stoichiometry corresponding to J = 1.9 has been achieved so far. The extra oxygen present for 8> 0 forms ordered structures in which the Cu cations of the (CuO6) planes have square planar coordination in Pb2Sr2YCu3Og, either square planar, pyramidal, and octahedral, or only pyramidal coordination in Pb2Sr2YCuaOg.5, and exclusively octahedral coordination in Pb2Sr2YCu30~o. In the range of composition 0 < J < 1, mixtures of two phases are obtained, one with J= 0 stoichiometry and the other with 8= 1, and whose relative quantities depend on the total amount of oxygen incorporated by the sample. The positive charges induced in Pb2Sr2YCuaOs+6 by oxygen incorporation oxidize the Cul + cations to 2 + and some of the Pb 2 + cations to 4 +. An order between Pb 2 ÷ and Pb 4 + 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.
Pb2Sr2YCu3Os 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 Cu 2+ cations in the CuO2 planes instead of Pb 2+ to 4 + and Cu ~+ 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 02, Pb2Sr2Y0.sCao.sCu3Os 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.