ELECTRON-DIFFRACTION AND POWDER NEUTRON-DIFFRACTION STUDIES OF YBA2(CU1-YCOY)(3)O-6+X WITH 0.05-LESS-THAN-Y-LESS-THAN-0.33

Powder samples of YBa2(Cu-1-y)COy)(3)O-6+x compounds with y > 0.05, in the oxidized and reduced state, have been characterized by thermogravimetry, and X-ray, neutron-, and electron-diffraction methods. Neutron experiments showed that (1) the Co atoms substitute exclusively for the Cull) atoms up to y similar to 0.2; (2) extra oxygen atoms are needed to give the Co atoms the appropriate coordination number which is in average equal to 5; (3) the Cu-O distances were found to be a function of the Co concentration and the oxygen stoichiometry. Co-doping yields a lengthening of the distance between the plane Cu and the apical oxygen and a corresponding shortening of the distance between the chain Cu and the same oxygen. For 0.025 less than or equal to y less than or equal to 0.10 the electron-diffraction experiments showed the presence of diffuse scattering indicating the existence of two superposed fields of transverse displacements modulated with long periodicities (4-5) (a+b)) along the [110] directions. For higher Co concentration diffuse scattering still exists, but indicates shorter modulation periodicities along the same directions. This diffuse scattering is due to local orthorhombic-like distortions induced by the Co insertion and distribution. The Co atoms tend to form dimers or small dusters which favor micro-twinning. The bond-distance analysis showed that the formal valence of the plane Cu atoms is the important factor in determining the value of T-c, provided that only the chain Cu atoms are substituted. Our results are in agreement with those obtained by other authors for the Fe-doped 123 compounds.