Room-temperature electrochemical reduction of YBa₂Cu₃O_{7 – x}: Solid-state and solution chemical results

The oxygen content of polycrystalline samples of YBa₂Cu₃O_{7 – x} can be reduced quantitatively, in a controlled fashion, by electrochemical techniques at room temperature in propylene carbonate. Upon reduction, the propylene carbonate undergoes an unusual reaction at the YBa₂Cu₃O_{7 – x} cathode to produce propanal, apparently because of the production of an active oxygen species on the surface. The reduced materials have been characterized by X-ray powder diffraction, electrical resistivity and magnetic susceptibility. The large-grained, reduced pellets are found to be inhomogeneous with respect to x. The reduced materials exhibit a broadened transition to the superconducting state. This effect is ascribed to the formation of metastable phases formed during reduction. After a low-temperature anneal, 80, 60 and 20 K transition temperatures are observed. These results indicate that T_c is a continuous function of oxygen content, but a discontinuous function of oxygen ordering.