Oxygen stoichiometry, superconductivity and normal-state properties of YBa₂Cu₃O_7-δ

Probably the most unusual crystal-chemical aspect of the new high-temperature oxide superconductors is their significant variability in oxygen content. For the YBa₂Cu₃O_7-δ family of compounds, the oxygen stoichiometry varies in the range 0 < δ < 1 under normal synthetic conditions, with the optimal 91-K superconductive transitions obtained near δ=0 (see, for example, ref. 1). Work presented so far on the dependence of both superconducting and normal-state properties on oxygen stoichiometry has considered only the value of δ in YBa ₂Cu₃O_7-δ. Here we present data that indicate the issue to be considerably more interesting and complex, and propose that the microscopic variables thus far not considered include the distribution of oxygen over the available sets of sites in the structure, with that distribution strongly dependent on synthetic conditions. In addition to the dependence of the bulk transition temperture (T_c), normal-state resistivity and crystallographic symmetry on the microscopic distribution, we find the magnitude of the magnetic susceptibility in the normal state to be strongly affected by the microscopic state of the oxygen subsystem. We also report the existence of a single-phase 60-K bulk superconductor at stoichiometry YBa₂Cu₃O_6.6-6.7, which we postulate to have an ordered array of oxygen vacancies, and two distinct plateau regions in the variation of T_c with x which are either unobserved or only hinted at under other synthetic conditions.