ONE-DIMENSIONAL OXYGEN ORDERING IN YBA2CU3O7-DELTA

A model consisting of oxygen-occupied and -vacant chains is considered, with repulsive first- and second-nearest-neighbor interactions V1 and V2, respectively. The statistical mechanics and the diffraction spectrum of the model are solved exactly and analytically with the only assumption V1 much greater than V2. At temperatures T approximately V1 only a broad maximum at (1/2,0,0) is present, while for \delta-1/2\ > 1/14 at low enough T, the peak splits into two. The simple expression for the diffraction intensity obtained for T much less than V1 represents in a more compact form previous results of Khachaturyan and Morris, extends them to all delta and T/V2, and leads to a good agreement with experiment.