TRANSPORT AND MAGNETIC-PROPERTIES OF THE SUPERCONDUCTING LA2CUO4+DELTA PHASES (0-LESS-THAN-DELTA-LESS-THAN-0.09) PREPARED BY ELECTROCHEMICAL OXIDATION

A preliminary phase diagram is given for the system La2CuO4+delta, o less-than-or-equal-to delta less-than-or-equal-to 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O(I) for delta<0.055 and O(M) for delta>0.055. The orthorhombicity of O(I) decreases with increasing delta, that of O(M) increases with delta. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K < T less-than-or-equal-to T(sp); below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220<T<T(sp). A similar transport behavior occurs in the range T(c)<T<T(rho) less-than-or-equal-to 120 K in samples 0.02 less-than-or-equal-to delta less-than-or-equal-to 0.04; from comparisons with the La2-xSrxCuO4 system, we suggest that al lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 less-than-or-equal-to delta less-than-or-equal-to 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO2 planes. We conclude that in the O(M) phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes DELTAp = 2delta-0.12 from the CuO2 planes. An H(c1) = 64 mT was obtained for delta = 0.09. A step in the resistivity drop at T(c) signals a temperature interval T(c)<T<T(on) within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing delta or should be considered a pair-fluctuation phenomenon is not resolved.