A model consisting of oxygen-occupied and -vacant chains is considered, with repulsive first- and second-nearest-neighbor interactions ${\mathit{V}}_1$ and ${\mathit{V}}_2$, respectively. The statistical mechanics and the diffraction spectrum of the model are solved exactly and analytically with the only assumption ${\mathit{V}}_1$≫${\mathit{V}}_2$. At temperatures T∼${\mathit{V}}_1$ only a broad maximum at (1/2,0,0) is present, while for ‖δ-1/2‖>1/14 at low enough T, the peak splits into two. The simple expression for the diffraction intensity obtained for T≪${\mathit{V}}_1$ represents in a more compact form previous results of Khachaturyan and Morris, extends them to all δ and T/${\mathit{V}}_2$, and leads to a good agreement with experiment.

• Received 29 January 1993

DOI:https://doi.org/10.1103/PhysRevB.47.15308