LOW-TEMPERATURE ORDERING IN YBA2CU3O6-GREATER-THAN-0.5 - COMPUTER-SIMULATION(X OXIDES AT X)

Recent experimental data are used to formulate the effective pair inte raction model of oxygen ordering in YBa2Cu3O6+x oxides at x>0.5 which is able to predict the main observed features of the low-temperature o rdering. The computer simulation which uses this model characterizes t he low-temperature secondary and tertiary ordering and describes the m icrostructure development resulting in formation of the Magneli-type a nd 2 root 2a(0)x2 root 2a(0) phases (a(0) is the perovskite lattice pa rameter). It is shown that these phases form stable rather than transi ent states. The stability fields of these phases on the phase diagram are estimated. Physical reasons distinguishing specific features of or dering in YBa2Cu3O6+x from the conventional ordering are discussed. Th e computer simulation results suggest certain heat treatment regimes w hich may provide formation of well-developed domains of the triple-per iod O-III and Magneli phases. The latter is important since lack of ma terial with large domains poses a serious difficulty in experimental s tudies of these phases.