SECONDARY AND TERTIARY ORDERING IN NONSTOICHIOMETRIC YBA2CU3O6+DELTA

The kinetics of the secondary and tertiary oxygen ordering in non-stoichiometric YBa2Cu3O6+delta are studied by the computer simulation method. The effect of long-range interactions between O atoms, the Coulomb repulsion and the strain-induced interaction, on the kinetics is investigated in terms of the crystal lattice site diffusion theory. A previous computer simulation by Semenovskaya and Khachaturyan in 1991 predicted the formation of (110) twins of the primary O(I) ordered phase. It is shown in the present work that the nanoscale O(I) twins strongly effect the path of the secondary and tertiary ordering reactions. Secondary ordering within the twins results in the formation of the metastable 2a0 x 2a0 phase prior to the occurrence of the stable 2a0 x a0 (or O(II)) ordered phase. Twin boundaries also promote the appearance of the tertiary ordered O' phase with 2 square-root 2 a0 x 2 square-root 2 a0 x 3a0 unit cell.