RELATION BETWEEN ELECTRONIC AND ATOMIC-STRUCTURE OF YBA2CU3O6+X

Using a model with strong interatomic correlations previously proposed to explain the electronic structure and pairing in CuO2 superconductors, we obtain a staircase behavior of the Fermi level, and a tendency of O(1) ions to order in Cu-O chains. We study the competition between this tendency and long-range Coulomb repulsion, using an Ising model and group-theoretical techniques. Using the cluster variation method, we also study the orthorhombic-tetragonal transition and correlations of the tetragonal phase. All results agree qualitatively with experiments. © 1990 The American Physical Society.