LOW-TEMPERATURE SUPERSTRUCTURES AND INCOMMENSURATE PHASES OF YBA2CU3O6+X

The structure factors for the cell doubled (O-II) and the cell tripled (O-III) phases of YBa2Cu3O6+x, are calculated by the cluster variatio n method for the asymmetric-next-nearest-neighbor Ising model with an additional longer-range interaction potential between oxygen atoms. Ve ry good agreement with experimental data is obtained for the phase tra nsition temperatures, the diffraction peak intensities vs temperature, and the ratio of the structural correlation lengths along the a and b axes. Further, incommensurate phases are found in the phase diagram f or x > 0.5, close to room temperature.