Electrochemical evidence for Cu³⁺-Cu²⁺-Cu⁺ transitions in the orthorhombic YBa₂Cu₃O_7-x phase

Comparative studies of the electrochemical behaviour of solid YBa₂Cu₃O_7-x, CuO and Cu₂O were performed, using 1M NaOH as electrolyte and applying the method of abrasive stripping voltammetry. The results show that high-temperature superconductivity is connected (i) with the observation of two one-electron steps between Cu³⁺, Cu²⁺ and Cu⁺ at +0.658 V and +0.052 V versus SRE and (ii) with an inability of Cu⁺ to be reduced to Cu⁰. The occurrence of the two one-electron steps Cu³⁺+e^- to Cu²⁺ and Cu²⁺+e^- to Cu⁺ supports the disproportionation theory of high-temperature superconductivity, which assumes the formation of Cooper pairs of electrons due to disproportionation of Cu²⁺. A decrease of the oxygen content of YBa₂Cu₃O_7-x from x=0 to x=0.7 is accompanied by the disappearance of the one-electron reduction step Cu²⁺+e^- to Cu⁺.