Phase diagram of oxygen ordering in YBa2Cu3O6+2c. The three-atom interactions
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A cluster variation method (CVM) has been used to calculate the phase diagram of oxygen ordering in YBa2-Cu3O6+2c in terms of two dimensional Ising model that, besides of repulsive nearest-neighbor (NN) interaction V-1 GT 0 and two types of next-to-nearest neighbor (NNN) pair interactions V-2 LT 0 and V-3 GT 0, includes additional three-atom effective interaction V-th coupling two NNN oxygen atoms already bounded by V-2 (through Cu(l) ion) with the third O atom that is connected with one of these two by the repulsive NNN Coulomb interaction V-3 GT 0. Phase diagrams were calculated for several different sets of interaction constants V-1, V-2, V-3, and V-th together with the corresponding phase diagrams obtained for V-th = 0. The arrangement of main orthorhombic structures in (c, T) space and the nature of the phase transition curves between them is found to be in a very good agreement with those of the corresponding phase diagrams of the well-known ASYNNNI model, which had already prove...n itself as accurately reproducing the number of experimental findings on the structural phase diagram of YBa2Cu3O6+2c. On the other hand, unlike to the predictions of the ASYNNNI model (V-th = 0), it is shown that inclusion of three-atom interaction V-th results in significant increase of the number of isolated basal plane oxygen ions which are generally considered as not giving rise to the charge transfer process and the onset of superconductivity. The importance of obtained oxygen superstructures of low temperature oxygen ordering and its possible impact on existence of plateaus in T-c(c) dependence has been discussed. (c) 2005 Elsevier B.V. All rights reserved.