New K2NiF4-type superconductors obtained by Cl2 treatment: synthesis, neutron diffraction and X-ray absorption spectroscopy characterization

Chlorine gas treatment of La 2 CuO 4 leads to an oxygen-doped material which is superconducting below T c = 32 K. The structural properties of this oxygen-doped La 2 CuO 4+8 have been investigated by Rietveld refinement of the powder neutron diffraction data and analysis of the X-ray 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 the formula La 2 CuO 4.08 . These atoms are located between two successive (LaO) planes of a rock salt layer and are surrounded by four La and four O1 atoms, both groups of atoms forming tetrahedra. The four O1 atoms are located at 2.035 and 2.176 A from the interstitial oxygen. Such short distances have also been found in other oxygen-excess phases, e.g. La 2 CuO 4.048 and La 2 NiO 4.25 . The X-ray absorption spectroscopy results show that the local environment of La atoms is disturbed by the presence of anion excess. The thermal stability of this compound has been compared with that of La 2 Cu(O,F) 4+ y oxyfluoride. Cl 2 − gas treatment has also been used to obtain superconducting phases in the La 2− x Ba x CuO 4 system with x ⩽0.05.