ORDER-DISORDER PHASE-TRANSITIONS IN YBA2CU3O6- THE ASYMMETRIC NEXT-NEAREST-NEIGHBOR INTERACTION-MODEL REVISED FROM A 1ST-PRINCIPLES POINT-OF-VIEW(2C )

First-principles renormalized effective pair interactions up to the fi fth shell in the oxygen sublattice of the Cu-O basal plane of the YBa2 Cu3O6+2c (0.15 less-than-or-equal-to c less-than-or-equal-to 0.5) syst em have been calculated using the embedded-cluster method in terms of the real-space scattering cluster coherent-potential approximation. Th e calculated weakly concentration-dependent pair interactions with dis tance rapidly decreasing support fairly well the asymmetric next-neare st-neighbour interaction model and are in excellent agreement with the numerical estimate by Hilton et al. In order to illustrate these pair interactions, the Monte Carlo method was used within the lattice-gas model to obtain the order-disorder (c, T) and (mu, T) phase diagrams.