ELECTROCHEMICAL EVIDENCE FOR CU3+-CU2+-CU+ TRANSITIONS IN THE ORTHORHOMBIC YBA2CU3O7-X PHASE

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 SHE and (ii) with an inability of Cu+ to be reduced to Cu0. The occurrence of the two one-electron steps Cu3+ + e- --> Cu2+ and Cu2+ + e- --> 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- --> Cu+.