Superconductivity and microstructure of ${\mathrm{YSr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.875}$

The crystal structure of ${\mathrm{YSr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ synthesized under high pressure has been studied in detail using transmission electron microscopy and powder neutron diffraction. It has been shown that ${\mathrm{YSr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ are not isostructural after all. The much lower $T_c$ of ${\mathrm{YSr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ (YSCO) with respect to that of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ 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 Cu1 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 ${\mathrm{O}}_{7.20}$ and ${\mathrm{O}}_{6.92}$ for the as-prepared and annealed phases, respectively.