OXYGEN CLUSTERING IN 123 COPPER OXIDES HAVING DISORDERED OR EXCESS (GREATER-THAN 7.0) OXYGEN

It is shown that in the high-Tc copper oxide superconductors, Tc varies for a given structure with the concentration of mobile holes in excess of the formal Cu(II) oxidation state in CuO2 sheets with a bridging oxygen atom making about 180° Cu-O-Cu interactions. It is also demonstrated by XPS spectroscopy as well as by indirect experiments that, in the YBa2Cu3O6+x (123) structure, the presence of a-axis oxygen in the Cu(1) planes at compositions x > 0.5 leads to the trapping of holes at oxygen-cluster states. We conclude that the π* band holes are in states of primarily O 2pπ character as opposed to Cu 3t2 character and suggest that the critical common feature of the high-Tc copper oxides and Ba2-xKxBiO3 is an overlapping of partially occupied Cu 3e σ* or Bi 6s σ* and O 2ppiπ* bands.