OXYGEN, DIFFUSE-SCATTERING, AND TWEED STRUCTURE IN THE YBA2CU3O7-DELTA SYSTEM

Microstructure and diffuse scattering of electron diffraction in YBa2(Cu1-xMx)3O7-delta (M = Fe, Co, Al, Ga, Zn and Ni, 0 less-than-or-equal-to x less-than-to-equal-to 0.33) were studied. It was found that the substitution for Cu by 3-valent Fe, Co and Al at a level x greater-than-or-equal-to 0.03 causes a structural transition from twin to tweed. For 0.03 less-than-or-equal-to x less-than-or-equal-to 0.06, the geometry of the reciprocal spot associated with the tweed structure has a cross-platelet shape extended in 2 equivalent <hh0> directions, which coffesponds to the displacive modulation in real space. For x greater-than-or-equal-to 0.10, the observed diffuse satelhtes and short-square-pipe intensity suggest the existence of a three-dimensional short-range ordering in these materials. Oxygen-reduction and reoxidization experiments indicate that development of tweed depends on the oxygen distribution. We believe that the configuration of oxygen in the Cu(1) layer is responsible for the formation of tweed structure in YBa2Cu3O7-delta system.