CRYSTAL-STRUCTURE, ATOMIC ORDERING AND CHARGE LOCALIZATION IN PB2SR2Y1-0CA0CU3O8+DELTA

Neutron, X-ray and electron diffraction measurements have been carried out on Pb2Sr2Y1-xCaxCu3O8+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 (CuOδ) planes have square planar coordination in Pb2Sr2YCu3O9, either square planar, pyramidal, and octahedral, or only pyramidal coordination in Pb2Sr2YCu3O9.5, 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 Pb2Sr2YCu3O8+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, Pb2Sr2Y0.5Ca0.5Cu3O8 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. © 1990.