CHEMICAL-BONDS AND ELECTRONIC CONFIGURATIONS OF CU-O CLUSTERS IN THE Y-BA-CU-O SYSTEM

Discrete variational DV-Xalpha calculations were carried out on the Cu 3O1015-, Cu3O1217- and Cu4O1417- clusters as models of Y-Ba-Cu-O super conductors. The calculated results reveal that the valence levels cons ist mainly of Cu d and O p orbitals, with strong covalent hybridizatio n. The unpaired spins are mostly localized on the planar Cu2 sites, wh ile the spin densities of the other atoms are very small compared with the experimental results. The charge distributions obtained support t he absence of Cu3+ and the presence of Cu+, Cu2+, O- and O2- electroni c configurations due to the covalent interaction.