Ordering of O vacancies inYBa2Cu3O7−δ

Using a repulsive interaction of electrostatic origin between O ions lying at the plane which contains O vacancies, we determine the minimum energy structure for different values of the composition parameter $\delta$. For $\delta =\frac{1}{2}$ or larger, the Cu-O chains disappear, and though the structure has no nearest-neighbor O ions, the main diffraction peaks show the same intensity as if the O ions were totally disordered. For small $\delta$ a long-distance ordering of the vacancies is obtained.