Oxygen clustering in 123 copper oxides having disordered or excess (>7.0) oxygen

Abstract

It is shown that in the high-T_c copper oxide superconductors, T_c 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 degrees Cu-O-Cu interactions. It is also demonstrated by XPS spectroscopy as well as by indirect experiments that, in the YBa₂Cu₃O_6+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. The authors conclude that the pi * band holes are in states of primarily O 2p_pi character as opposed to Cu 3t₂ character and suggest that the critical common feature of the high-T_c copper oxides and Ba_2-xK_xBiO₃ is an overlapping of partially occupied Cu 3e sigma * or Bi 6s sigma * and O 2p_pi pi * bands. A 'magnetic-bag' mechanism cannot be applicable to Ba_2-xK_xBiO₃; it is therefore questionable whether it is applicable in the copper oxides. Polarisation fluctuations offer an alternative mechanism applicable to any system satisfying the condition of overlapping cationic sigma * and anionic pi * bands.