Published online by Cambridge University Press: 28 February 2011
We describe the properties of a model for the absorption of oxygen in the basal planes of YBa2 Cu3 O7-y. The model consists of a layered oxygen lattice gas in equilibrium with an external source of O2 wherethe Cu-mediated oxygen-oxygen interactions are coupled to the elastic energy of the crystal. Our predictions for the dependence of oxygen-ordering, and unit cell dimensions with temperature and oxygen partial pressure are in quantitative agreement with experiment.