The effects of oxidation and reduction on the X-ray photoelectron spectra of YBa₂Cu₃O_7-x

Abstract

Detailed X-ray photoelectron spectra (XPS) have been obtained from YBa₂Cu₃O_7-x specimens which have a superconducting transition temperature of 93 K. The effects of reduction on heating in uacuo at 400°C and of oxidation at 400°C in air have been considered. It is found that in general the XPS signatures of the Cu 2p peaks are consistent with the presence of formally mixed valence Cu(II) and Cu(III) atoms, although a unique valence cannot be assigned. Reduction in vacuo apparently reduces all copper atoms to divalency as shown by the increasing intensity in the ‘shake-up’ satellites. Re-oxidation in air does not fully restore the sample to its original state, although the superconducting transition and the Meissner effect are recovered. It is also found that the low-binding-energy components of the O 1s and Ba 3d XPS peaks are characteristic of bulk states and cannot be ascribed to surface contamination and/or surface-specific states. Evidence is found for migration of barium to the free surface during heat treatment, probably as a result of thermally activated diffusion along grain boundaries. Ion-beam etching of the material results in a general structural and electronic rearrangement with apparent partial reduction of the copper species to monovalency, the disappearance of substructure in the Ba 3d and Y 3d XPS envelopes and an enhancement of the low-binding-energy component of the O 1s peak.