THE ELECTRONIC-STRUCTURE OF THE [CUO5]8- UNIT - A PHOTOEMISSION-STUDY OF Y2BACUO5

A photoemission study of the 'green phase' Y2BaCuO5, including core level spectra of the elements, Cu L3 VV Auger, and XPS and UPS valence band spectra has been performed. The crystal structure of Y2BaCuO5 contains discrete [CuO5]8- units with divalent copper. The surface status of the Y2BaCuO5 samples is discussed with respect to the problems encountered in photoemission studies of the YBaCuO superconductors. The photoemission data are analysed within a simple cluster configuration interaction model by considering in addition previously reported optical spectra. Y2BaCuO5 is a charge transfer insulator with the charge transfer energy-DELTA almost-equal-to 0.5 eV being much smaller than the d-d Coulomb interaction energy U almost-equal-to 6 eV. Cluster model parameters and valence band shapes of Y2BaCuO5 are compared with those of Nd2CuO4 the structure of which contains divalent copper within an extended copper-oxygen network. More detailed cluster and impurity models for cuprates described in literature are discussed in view of the present results.