Nature of the copper species in superconducting YBa₂Cu₃O₇

Abstract

The nominal composition of YBa₂Cu₃O₇ oxide suggests that 33% of the copper should be in the 3+ state. In structural studies, Cu³⁺ and Cu²⁺ ions have been assigned specific sites based on their preferred coordinations, and many of the models of superconductivity also require the presence of Cu³⁺. Our studies however show that Cu is present only in the Cu¹⁺ and Cu²⁺ states, the proportion of the former increasing with decreasing temperature. It appears that there is no clear experimental evidence for the presence of Cu³⁺ in YBa₂Cu₃O₇.