ROOM-TEMPERATURE CHEMICALLY OXIDIZED LA2CUO4- PHASE-SEPARATION INDUCED BY THERMAL-TREATMENT(Y )

The structure of roam temperature chemically oxidized La2CuO4+y [y = 0 .103(4)] has been refined from powder neutron diffraction data using t he space group Bmab. The modifications induced in the CuO2 and the LaO planes by the insertion of oxygen are consistent with the high T-c me asured for this material. The thermogravimetric analysis(TGA) of La2Cu O4.103(4) evidences an unexplained two-step mass loss process. Based o n this observation, three samples obtained by different thermal treatm ents of the fully oxidized material were studied by TGA, X-ray diffrac tion and AC magnetic susceptibility, After a short treatment at 433 K, La2CuO4.103(4) undergoes a phase separation into two phases: phase 1, with estimated y(1) = 0.086(4) and T-cl = 30 K, and phase 2, with est imated y(2) = 0.12(1) and T-c2 = 17 K. By increasing the annealing tim es, phase 2 transforms to phase I and finally disappears. Therefore, i t seems likely that the plateau observed in the TGA curve of La2CuO4.1 03(4) might be due to the formation on heating of a stable phase with a fixed oxygen stoichiometry, i.e. La2CuO4.086(4). The stability of th is phase could be related-to the presence of one-dimensional interstit ial oxygen ordering along the c-axis (stage 3), although no experiment al evidence for this has been found. (C) 1997 Elsevier Science B.V.