NEW K2NIF4-TYPE SUPERCONDUCTORS OBTAINED BY CL2 TREATMENT - SYNTHESIS, NEUTRON-DIFFRACTION AND X-RAY-ABSORPTION SPECTROSCOPY CHARACTERIZATION

Chlorine ps treatment of La2CuO4 leads to an oxygen-doped material whi ch is superconducting below T(c) = 32 K. The structural properties of this oxygen-doped La2CuO4+delta have been investigated by Rietveld ref inement of the powder neutron diffraction data and analysis of the X-r ay absorption spectra obtained in the La L(III) edge (LaO) region. The extra oxygen atoms occupy the (1/4, 1/4, y) positions (O4) with y = 0 .2629 at room temperature. Their occupancy factor n = 0.042 leads to t he formula La2CuO4.08. These atoms are located between two successive (LaO) planes of a rock salt layer and are surrounded by four La and fo ur O1 atoms, both groups of atoms forming tetrahedra. The four O1 atom s are located at 2.035 and 2.176 angstrom from the interstitial oxygen . Such short distances have also been found in other oxygen-excess pha ses, eg. La2CuO4.048 and LaNiO4.25. The X-ray absorption spectroscopy results show that the local environment of La atoms is disturbed by th e presence of anion excess. The thermal stability of this compound has been compared with that of La2Cu(O,F)4+y oxyfluoride. Cl2- gas treatm ent has also been used to obtain superconducting phases in the La2-xBa CuO4 system with x less-than-or-equal-to 0.05.