Phase diagram of a two-dimensional lattice-gas model of oxygen ordering in YBa2Cu3Oz, with realistic interactions

Abstract

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