Phase diagram of oxygen ordering in YBa2Cu3O6+2c.

A cluster variation method (CVM) has been used to calculate the phase diagram of oxygen ordering in YBa2Cu3O6+2c in terms of two dimensional Ising model that, besides of repulsive nearest-neighbor (NN) interaction V1 > 0 and two types of next-to-nearest neighbor (NNN) pair interactions V2 < 0 and V3 > 0, includes additional three-atom effective interaction Vth coupling two NNN oxygen atoms already bounded by V2 (through Cu(1) ion) with the third O atom that is connected with one of these two by the repulsive NNN Coulomb interaction V3 > 0. Phase diagrams were calculated for several different sets of interaction constants V1, V2, V3, and Vth together with the corresponding phase diagrams obtained for Vth = 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 proven 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 (Vth = 0), it is shown that inclusion of three-atom interaction Vth 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 Tc(c) dependence has been discussed.