The Cu valence in the highT c superconductors and in monovalent,divalent and trivalent copper oxides determined from XPS core level spectroscopy

A number of theoretical models have been developed to explain the unexpected high temperature superconductivity in La-Me-Cu-oxides (Me=Ba, Sr) and Y-Ba-Cu-oxides. Some of these models invoke charge fluctuations on the copper ions in the superconductors between a 2⁺ and a 3⁺ state. In order to test these possibilities we have measured the Cu–2p _3/2-core level spectra of NaCuO₂ in which the copper ion is in a 3⁺ state and compared it with the core line position in pure CuO and to superconducting oxides. The data strongly suggest, that there is, if any very little Cu³⁺ in the superconducting compounds present. However, we notice, that in comparison to trivalent and monovalent copper oxides the Cu–2p _3/2 line in CuO and the superconducting oxides is unexpectedly broad. The cause of this large linewidths remains so far unexplained.