Room temperature chemically oxidized La₂CuO_4+y: Phase separation induced by thermal treatment

The structure of roam temperature chemically oxidized La2CuO4+y [y = 0.103(4)] has been refined from powder neutron diffraction data using the 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 measured for this material. The thermogravimetric analysis(TGA) of La2CuO4.103(4) evidences an unexplained two-step mass loss process. Based on this observation, three samples obtained by different thermal treatments of the fully oxidized material were studied by TGA, X-ray diffraction 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 estimated y(2) = 0.12(1) and T-c2 = 17 K. By increasing the annealing times, phase 2 transforms to phase I and finally disappears. Therefore, it seems likely that the plateau observed in the TGA curve of La2CuO4.103(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 this phase could be related-to the presence of one-dimensional interstitial oxygen ordering along the c-axis (stage 3), although no experimental evidence for this has been found. (C) 1997 Elsevier Science B.V.