SUPERCONDUCTING CRITICAL-TEMPERATURE, OXYGEN ORDERING AND DEVILS STAIRCASE IN YBA2CU3O6+X

Using the appropriate extension of the three-band Hubbard model to des cribe the basal CuO2+x subsystem of YBa2Cu3O6+x for a certain region o f parameters we obtain: a) the number of holes n(H) in the superconduc ting planes and T-c as a function of x have a plateau, b) using a stru ctural model to include the effect of interatomic O-O repulsions, ther e is a transition from structures without first or second nearest-neig hbor O atoms (RS) to order in chains (CS) at x similar to 0.45 for inc reasing x. We also study the structural model for values of x for whic h the CS are stable.