ELECTRONIC-STRUCTURE AND PROPERTIES OF HIGH-T(C) SUBSTITUTED YBCO SUPERCONDUCTORS .2. MO CALCULATIONS ON CHARGED CLUSTER-MODELS RELATING TO HIGH-T(C) SE-SUBSTITUTED YBCO SUPERCONDUCTORS

Using the extended Huckel molecular orbital method in connection with the tight binding model, we have studied electronic structure and related properties of the charged cluster models relating to superconducting YBa2Cu3O7-x crystals in which O-atoms in regular sites were selectively replaced with Se atoms. In analogy to the isomorphism problem with molecules, we discuss all possible combinations of Se-substitutions in O-sites with one, two, and four Se atoms. The calculations are carried out within charged cluster models for analogues of YBa-copper oxide. Our results suggest that the electronic structure of the symmetrically Se-substituted or Se-added compound is closer to that of the YBCO superconducting compound than that obtained from the unsymmetrical substitution. This applies in particular if O is replaced with Se around the Cu(1) site. Symmetrical substitutions in the CuO2 layers give rise to large variations in the electronic structure of YBa2Cu3O7. This is consistent with the fact that superconductivity is very sensitive to the electronic population of the CuO2 layers.