The hole concentration on oxygen sites in the highT c superconductor Y1?Ba2?Cu3?O7?x

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x is constructed.