OXYGEN STOICHIOMETRY IN HIGH-TC SUPERCONDUCTORS

The oxygen content is one of the most important crystal chemical parameters governing the physical properties of high-T(c) superconducting compounds. Studies of the oxidation process of YBa2Cu3O6+x and Pb2Sr2Y1-xCaxO8+delta reveal that oxygen uptake does not necessarily lead to better superconducting properties. In the former series oxygen incorporation results in a higher concentration of positive charges which are transferred from the square Cu chains to the pyramidal Cu layers. During the oxidation the most significant change consists in the movement of the apical oxygen bridging the chains to the pyramids towards the latter. The interatomic distances and, more specifically, the calculated cation valencies show that the 90 and 60 K plateaus in the T(c) vs x curve are associated with the plateaus observed in the valency vs x curve for the Cu pyramidal layers. Pb2Sr2YCu3O8 does not become superconducting. The oxygen-depleted Cu layer inserted between two PbO layers can easily incorporate extra O atoms just like the Cu layer inserted between two BaO layers in YBa2Cu3O6. The additional positive charges are localized on the Pb sublattice and an order between Pb2+ and Pb4+ is established. This localization hinders the charge transfer to the conducting pyramidal Cu layers and no trace of superconductivity has been detected in any oxidized sample. Pb2Sr2YCu3O8 becomes superconducting at approximately 80 K when some of the trivalent Y cations are replaced by divalent Ca. In this case the extra positive charges are transferred from the (Y, Ca) layers to the conducting pyramidal Cu layers and the compound becomes superconducting. The Cu+ and Pb2+ cations remain in the reduced state because their oxidation can occur only if accompanied by a coordination increase. When heat treated at 770 K 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. In this case, the extra O atoms allow the Cu+ and Pb2+ cations to change coordination and the extra positive charges are trapped in the (PbO)(Cu)(PbO) blocks. Moreover, the oxidation of the Cu+ and Pb2+ cations induces the displacement of the apical oxygen of the pyramidal layers. A charge transfer towards the (PbO)(Cu)(PbO) blocks takes place and consequently the superconducting state is suppressed.