SYNCHROTRON X-RAY STUDY OF INTERSTITIAL OXYGEN ORDERING IN THE SUPERCONDUCTING PHASE OF LA2CUO4+DELTA

A synchrotron x-ray powder-diffraction study has been carried out on t wo samples with well-defined excess oxygen concentrations, delta = 0.0 45 and 0.055. Evidence for one-dimensional interstitial oxygen orderin g along the c-axis was observed from the appearance of a pair of super lattice satellite peaks at (0, 1, 2 +/- Delta) in both samples. In La2 CuO4.045, a similar to 5% oxygen-poor Bmab phase coexists with a stage -6 phase and with a small amount of the high-temperature-tetragonal (H TT) phase below the phase separation temperature (similar to 280 K), L a2CuO4.055 shows a single stage-5 phase below 170 K, but with two unin dexed peaks between (0, 1, 2 +/- 0.2) satellites which may well result from a further domain modulation, The critical temperature T-c is inc reased by 1.5 K by reducing the stage number from 6 to 5.