Superconductivity and microstructure of YSr2Cu3O6.875

The crystal structure of YSr2Cu3O6+x synthesized under high pressure has been studied in detail using transmission electron microscopy and powder neutron diffraction. It has been shown that YSr2Cu3O6+x and YBa2Cu3O6+x are not isostructural after all. The much lower Tc of YSr2Cu3O6+x (YSCO) with respect to that of YBa2Cu3O6+x has to be ascribed to the different structural arrangement of the reservoir blocks. The different oxygen ordering and the consequent displacement of the Sr and Cu cations in YSCO yield an orthorhombic modulated structure with a propagation vector of 1/4[012]*. Perfectly ordered YSCO would have an oxygen stoichiometry equal to 6.875. Powder neutron-diffraction data yielded O7.20 and O6.92 for the as-prepared and annealed phases, respectively