PHASE-SEPARATION AND DEVILS STAIRCASE IN MODELS FOR O ORDERING IN RBA2CU3O6+X

When the Gibbs free energy G of a solid solution depends on a free var iable eta, which is determined minimizing G, a negative contribution t o the second derivative of G with respect to the compositional paramet er arises. Under certain circumstances, this term is the dominant one and phase separation is favored. Here we consider the specific case of different two-dimensional lattice-gas models of O ordering in RBa2Cu3 O6+x (R=Y, rare earths) in the regime of compositions and parameters f or which the O atoms are ordered in infinite Cu-O chains at temperatur e T=0. When a phenomenological term that takes into account the depend ence of the lattice parameter c on the O content x is added to G, phas e separation with two broad two-phase fields takes place at T less tha n or similar to 400 K in the asymmetric next-nearest neighbor Ising (A SYNNNI) model. If instead any two 0 ions interact via a screened Coulo mb repulsion, and c is kept fixed, we obtain further evidence of the e xistence of a complete devil's staircase at T=0. However, when c is re laxed, for any finite screening length only a finite number of phases are stable at T=0 and phase separation also occurs at low enough tempe ratures.