THERMAL HISTORY DEPENDENCE OF SUPERCONDUCTING PROPERTIES IN LA2CUO4+DELTA

We studied the thermal history dependence of the superconducting prope rties below/above room temperature (RT) in the ceramic La2CuO4+delta. With excess oxygen. The phase separation (O-rich phase: superconductin g and O-poor phase: antiferromagnetic) was concluded to occur above 37 3 K, in contrast with the usual report of the phase separation around 320 K. As for the superconducting phases, the well-known T-c onset of 32 or 36 K, dependent on thermal history around 200 K, in the samples annealed in high-pressure oxygen gas, was not changed by thermal histo ry between RT and 373 K. The samples electrochemically oxidized at RT included the phase with the high T-c of 45 K, which was not changed by thermal history below RT, and the phase with the low T-c of 32 or 36 K. The 45 It phase was changed into the low-T-c phase by annealing at 373 K. The samples electrochemically oxidized at 333 K, which was acco mpanied with the diffusion of excess oxygen, showed gradual change of superconducting behavior: the single low-T-c (32 or 36 K) phase (oxida tion time = 24 h), coexistence of the low-T-c phase and the high-T-c ( 45 K) phase (36 h), and the single high T-c phase (48 and 72 h). Thus, the single superconducting phase with the high T-c of 45 K has been o btained, which showed a metallic behavior in normal resistivity and ap parent changes of lattice constant in comparison with that of stoichio metric La2CuO4. (C) 1998 Elsevier Science B.V. All rights reserved.