Weakly bound oxygen and the superconductivity of YBa2Cu3O<i>x</i>

The oxygen content and its influence (during subsequent cooling) on the superconducting properties of the metaloxide ceramic YBa2Cu3Ox are measured at temperatures (T) 300−1000 °C under partial pressures (pO2) 10−105 Pa. The position of the oxygen isoconcentrates for 6.2 &amp;lt; x &amp;lt; 7 is determined from the pO2-T diagram by gravimetric method with absolute calibration by reduction to metallic copper. It is shown that the oxygen in the compound exists in the strongly bound form (6 g-atom/mol.) and in a weakly bound form which is responsible for superconductivity. The amount δ of weakly bound oxygen is determined by quenching the samples in liquid hydrogen. The nature of correlation of δ with the critical temperature Tc and the diamagnetic response LYBCO is reconstructed. It is found that the shape of the LYBCO(δ) and Tc (δ) is in accord with the formation of stoichiometric ionic crystals with Cul+ , Cu2+, and Cu3+ ions in the basal plane of the compound for δ = 0, 0.5, and 1, respectively. It is established that for x &amp;lt; 6.5, the metaloxide ceramic YBa2Cu3Ox exists in a tetragonal superconducting phase.

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