Oxygen ordering in YBa2Cu3O6+x: low-temperature expansions

Low-temperature series expansions are used to investigate the phase transition due to oxygen ordering in the high-temperature superconductor YBa2Cu3O6+x. The formulation is based on the lattice-gas model, which has been studied by many authors, and excellent agreement with previous results is obtained. The authors also investigate the effect of a sublattice-dependent chemical potential which plays a role similar to a uniaxial stress in breaking the degeneracy between twinned structures.