THERMODYNAMICS OF O ORDERING IN YBA2CU3O6+X - EFFECT OF COULOMB REPULSION

We have calculated several thermodynamic properties of a lattice gas mode with screened Coulomb repulsions V(i) for ith nearest neighbors up to i = 5, but for second-nearest neighbors with a Cu in between the repulsion V2cu is reduced by charge transfer effects (V2cu =fV2, f < 1). We used the 3 x 3 point approximation of the cluster-variation method (CVM). The composition (x)-temperature (T) phase diagram for V3 = V4 = V5 = 0, V1 = + infinity and f = - 1, is similar to the one published by other authors. Taking instead f approximately 0 and all V(i) not-equal 0, the low-T, low-x phase disappears from the phase diagram and structures that explain the experimentally observed diffraction patterns appear; the tetragonal-orthorhombic II transition becomes continuous and displaces to lower temperatures. We also explain the structure in the derivative of x with respect to the chemical potential (mu) observed experimentally in the tetragonal phase.