The role of oxygen in YBa2Cu3O7-δ

Five observations concerning the role of oxygen in the high-Tc superconductors YBa2Cu3O7-δ are emphasized; (1) An internal electric field parallel to the c-axis constrains the redox reactions associated with the intercalation/disintercalation of oxygen primarily to the [CuO3-δ]3- layers located between Ba2+ layers. (2) Intercalation of O2- into the tetragonal YBa2Cu3O6 phase results in an ordering onto one of the basal-plane axes of the Cu layer, the b-axis of the orthorhombic phase. (3) Ordering between and within b-axis chains gives rise to discrete phases predicted to occur at O6.875, O6.75, O6.5, O6.25, and O6.125. (4) An equilibrium oxidation state for the intercalation layer depends not only on the oxygen partial pressure and temperature, but also on the cations of the structure. (5) At higher oxidation states of the intercalation layer, any oxygen atom on the a-axis interacts with a near-neighbor oxygen on the b-axis to trap out holes in a peroxide ion (O2)2-