OXYGEN ORDERING AND SUPERCONDUCTIVITY IN CDBASRCU3O7-X

A special preparation process has been developed to stabilise GdBaSrCu 3O(7-x) compounds in both tetragonal and orthorhombic structures with oxygen deficiency x close to zero. Powder X-ray diffraction analysis ( XRD) gives lattice parameters of a=3.839 Angstrom and c=11.572 Angstro m for a tetragonal structure, and a=3.804 Angstrom, b= 3.863 Angstrom and c=11.588 Angstrom for a typical orthorhombic structure. Gd-155-Mos sbauer spectroscopy reveals via the electric quadrupole interaction an asymmetry parameter eta=0.14(6) for the tetragonal phase (T-phase) an d eta=0.31(5) for the orthorhombic phase (O-phase). The small asymmetr y parameter for the T-phase indicates an oxygen disordering in the bas al plane. Superconducting transition temperatures T-c above 80 K were verified for both tetragonal and orthorhombic phases by resistivity an d susceptibility measurements. Despite of a similar T-c-value the norm al-state transport properties studied by resistivity and Hall effect m easurements revealed higher resistivity in the tetragonal phase, which may be caused by both the destruction of the CuO chains and an enhanc ed scattering introduced by disorder of oxygen atoms in the basal plan e.