Electronic structure and properties of high-T_c substituted YBCO superconductors: III. Charged cluster models relating to high-T_c Cl-substituted YBCO superconductors

Using the extended Hückel molecular orbital (EHMO) and the EH band calculation in connection with the tight-binding model, we have studied electronic structure and related properties of the charged cluster models relating to superconducting YBa₂Cu₃O_7-x crystals in which O-atoms in regular sites were selectively replaced with Cl atoms. In analogy to the isomorphism problem with molecules, we discuss all possible combinations of Cl-substitutions in O-sites with one, two, and four Cl atoms in our cluster calculation. It shows that the electronic structure of the symmetrically Cl-substituted YBCO is closer to that of the superconducting YBCO than that obtained from the unsymmetrical substitution. This applies in particular if O is replaced with Cl around the Cu(1) site. In both EHMO and EH band calculations for Cl-substitution around Cu(1) site, the decreasing order of charge density for each Cl site is Cl(4) > Cl(5) > Cl(1) which agrees with the result of NMR study.