SUPERCONDUCTIVITY AND PHASE-SEPARATION IN LA2CUO4+X SINGLE-CRYSTALS

We report a phase separation study on high-quality single crystals of La2CuO4+x for two concentrations x = 0.03 and x = 0.04. The excess oxygen was loaded into the crystals at high oxygen pressure p = 3 kbar and temperature T = 650-700-degrees-C. The X-ray diffraction study of these crystals shows the presence of a phase separation process for the x = 0.04 sample over the temperature range 230 < T < 320 K and the absence of such a decomposition for the x = 0.03 sample within the same temperature region. The superconducting transition is observed for both crystals, though the detected T(c) almost-equal-to 12 K value for the x = 0.03 sample is much lower than usually reported in the La2CuO4+x system (approximately 30 K), which is additional evidence that here superconductivity is not connected with the new phase formation. Possible reasons for such a behavior and phase diagram of La2CuO4+x are discussed.