CHARGE-TRANSFER AND OXYGEN ORDERING IN YBA2CU3O6+X

The electronic structure of the CuO(x) planes is studied using a gener alized Hubbard model including Cu-O repulsion U(pd), for each value of x and two different assumptions on the oxygen (O) ordering. The resul t explains qualitatively the experimentally observed hole count in the CuO2 planes, the amount of Cu+ and the metal-insulator transition nea r x=0.5. For large enough U(pd) the energy DELTAE favoring ordering in chains is positive. A simple explanation of this and the relation bet ween charge transfer and O ordering is given. The screening length lam bda is calculated using Thomas-Fermi theory, an effective one-band mod el for the CuO2 planes and experimental data. This information is used to construct an effective lattice-gas model for the O ordering, based on O-O screened repulsions in which DELTAE is the only parameter. The superstructures predicted by this model by this model provide an expl anation of almost all observed diffraction patterns,and of recently, o bserved photoinduced changes in the transport properties. The electron ic and structural results are consistent with the observed dependence of the superconducting T(c) vs x.