NATURE OF THE COPPER SPECIES IN SUPERCONDUCTING YBA2CU3O7+

The nominal composition of YBa+ZZ2Cu//3O//7 oxide suggests that 33% of the copper should be in the 3 plus state. In structural studies, Cu**3** plus and Cu**2** plus ions have been assigned specific sites based on their preferred coordinations, and many of the models of superconductivity also require the presence of Cu**3** plus . Our studies owever show that Cu is present only in the Cu**1** plus and Cu**2** plus states, the proportion of the former increasing with decreasing temperatures. It appears that there is no clear experimental evidence of the presence of Cu**3** plus in YBa//2Cu//3O//7.