Some aspects of the existence of trivalent copper in YBa 2Cu 3O 7− x superconductors and related corrosion phenomena

Abstract

Direct evidence was obtained for the presence of trivalent copper (3d 8) in the YBa 2Cu 3O 7− x (0⪕ x⪕0.5) lattice through extraction of the copper components of the superconductor in aqueous KOH solutions in the presence of (IO 6) 5- anions, with formation of the stable bis-periodato cuprate anion, [Cu III(IO 6) 2] 7-. Solutions of this substance have a characteristic intense yellow colour (with an absorbance maximum at a wavelength of 415 nm and a molar decadic extinction coefficient of ϵ 415=1.3×10 4), permitting sensitive, specific detection of Cu III in solution. The analogous Cu II complex has a light blue- green colour (λ max=621 nm, ϵ 621=45) and is poorly soluble (5̃×10 -4M). Bis-tellurato cuprate, [Cu III(TeO 6) 2] 9-, behaves similarly. The complexes of Cu III and Cu II also yield characteristic polarographic reduction waves in alkaline media. Both the spectrophotometric and the polarographic method can be used for the quantitative determination of both forms of copper in solution. The superconductor was also studied in the solid phase by the electrochemical method of chronopotentiometric curves. The results obtained also indicated the presence of trivalent copper in the superconductor. Some aspects of corrosion are discussed in connection with the high oxidation potential of Cu III.