Electrochemical evidence for Cu3+-Cu2+-Cu+transitions in the orthorhombic YBa2Cu3O7-xphase

Abstract
Comparative studies of the electrochemical behaviour of solid YBa2Cu3O7-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 YBa2Cu3O7-x from x=0 to x=0.7 is accompanied by the disappearance of the one-electron reduction step Cu2++e- to Cu+.

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