X-ray photoelectron spectroscopy study of fluorine-treated YBa2Cu3O7−δ crystals

Abstract

An x-ray photoelectron-spectroscopy study of flux-grown YBa2Cu3O7−δ crystals left in moist atmospheric conditions has shown some aspects of the degradation process. Besides peaks which correspond to YO and BaO bonding in the superconducting phase, higher binding-energy components can be attributed to BaCO3, YOH, and/or YCO3 bonding. An x-ray photoreduction of the copper species is observed in the ultrahigh vacuum. Fluorine-gas treatments carried out at low temperature drastically modify the surface properties of the crystals. The peaks attributed to MO bonding (M=Y, Ba, or Cu) disappear and are substituted by MF bonding with a subsequent increase in Eb. In the F 1s region, the peak corresponding to those bondings is observed around Eb ≂684.5 eV, a value which is in agreement with those claimed for the corresponding inorganic fluorides. The signature of those surface fluorides remains, even when the samples are left for long periods in moist air. Within the F 1s envelope, chemisorbed fluoro species with Eb ≂686.5 eV are observed, especially when the fluorination process is performed at room temperature. These species are eliminated by vacuum annealing at 350 °C. In specific conditions, a high-binding-energy contribution arises (Eb ≂689 eV) which can be ascribed to the formation of surface ‘‘carbon fluoride.’’