Oxygen order-disorder transition in the superconductorYBa2Cu3O6+δ

The order-disorder transition involving oxygen atoms and vacancies in the Cu-O basal plane of the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$ superconductor is studied using the cluster-variation method in the square approximation. We are able to reproduce most of the features observed in recent neutron-diffraction experiments by assuming that the chemical potential in the solid and gas phases varies linearly with temperature, and that the dominant interaction for the order-disorder transition is the oxygen-oxygen nearest-neighbor repulsion.