Abstract: XPS studies show that carbonates are formed on the surfaces of Y₂O₃,BaO,CuO and YBa₂Cu₃O_～7 superconductor therefrom stored in air.The valence states of the elements on the surface of the superconductor are as follows: Y³⁺,Ba²⁺and Cu²⁺.Individual raw CuO can be converted mostly into Cu₂O after heating at 400℃ under high vacuum,while in newly prepared superconductor at the same conditions only a small portion of Cu²⁺ions are converted to Cu¹⁺ ions.These results indicate that sintering enhanced the stability of Y-Ba-Cu-Osystem. However,for the superconductor stored long in air this conversion of Cu²⁺ to Cu¹⁺ takes place readily,which reflects that the"ceramic superconductor"does not behave as a true ceramic in stability,it still shows a"metastable"feature.

Key words: X-ray Photoelectron, Spectroscopy(XPS), Superconductor of Y-Ba-Cu-O type, Valency state of elements