Charge state of copper ions in the YBa2Cu3O6.8 single crystal (1.5 at % Ce): X-ray photoelectron and absorption studies

X-ray photoelectron and absorption spectra of YBa 2 Cu 3 O 6.8 single crystals (1.5 at % Ce) annealed in vacuum, in argon, and in air have been studied. It has been established that the heat treatment in air at 200°C for 100 h led to the appearance of Cu 1+ ions in a subsurface layer and to an increase in the degree of covalence of the Cu-O bonds in the “bulk” of the sample.