Structural properties of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The structural properties of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ have been determined by neutron powder diffraction for 0.07<δ<0.91. The samples were produced by quenching into liquid nitrogen from controlled oxygen partial pressures at 520 °C, and they exhibit a clearly defined ‘‘plateau’’ behavior of $T_c$ versus δ. Superconductivity disappears at the orthorhombic-to-tetragonal transition that occurs near δ=0.65. Structural parameters, including the copper-oxygen bond lengths, vary smoothly with δ within each phase but exhibit different behavior in the superconducting and nonsuperconducting phases. These observations are consistent with a model in which superconducting behavior is controlled by charge transfer between the conducting two-dimensional ${\mathrm{CuO}}_2$ planes and the ${\mathrm{CuO}}_{\mathrm{x}}$ chains, which act as reservoirs of charge.