GROUND-STATE OF THE ATOMIC-STRUCTURE OF YBA2CU3O6+X

We study the ordering of O atoms in YBa2Cu3)6+x using a two-dimensional Ising model. We assume a screened Coulomb repulsion between any two O ions of the CuO(x) planes, but the interaction between second-neighbor O ions with a Cu ion in between is reduced by a factor f due to charge-transfer effects. We construct the symmetry-independent atomic configurations, which enable us to compare the energy of a large number of structures. We obtain a rich x-f phase diagram that for screening lengths larger than approximately 1 angstrom and low values of f contains structures compatible with all the diffraction patterns so far observed. The ground state consists of structures composed of linear CuO chains for f < 0, or x near 0.5 and f < 1, while for f = 1 simple lattices of nearly hexagonal symmetry of O ions or vacancies added to the x = 1 structure dominate.