Structural properties of oxygen-deficient<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

The structural properties of oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ have been determined by neutron powder diffraction for 0.07&lt;\ensuremath{\delta}&lt;0.91. The samples were produced by quenching into liquid nitrogen from controlled oxygen partial pressures at 520 \ifmmode^\circ\else\textdegree\fi{}C, and they exhibit a clearly defined ``plateau'' behavior of ${T}_{c}$ versus \ensuremath{\delta}. Superconductivity disappears at the orthorhombic-to-tetragonal transition that occurs near \ensuremath{\delta}=0.65. Structural parameters, including the copper-oxygen bond lengths, vary smoothly with \ensuremath{\delta} 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.

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