PHASE-DIAGRAM OF A 2-DIMENSIONAL LATTICE-GAS MODEL OF OXYGEN ORDERINGIN YBA2CU3OZ WITH REALISTIC INTERACTIONS

We have determined a first-principles temperature-composition phase di agram describing the ordering of oxygen atoms in the ''chain'' layers of YBa2Cu3Oz. The calculations are based on a transfer-matrix finite-s ize-scaling study of a two-dimensional lattice-gas Hamiltonian (the as ymmetric next-nearest-neighbor Ising [ASYNNNI] model). The interaction parameters are obtained from ab initio electronic structure calculati ons. The resulting phase diagram compares favorably with experiment. W e analyze its sensitivity to the relative size of the interactions. We also show that the addition of a third-neighbor interaction has a min imal effect on the phase boundaries. Based on these results, we assess the applicability of the ASYNNNI model to YBa2Cu3Oz.