TWIN AND TWEED MICROSTRUCTURES IN YBA2CU3O7-DELTA DOPED BY TRIVALENT CATIONS

Computer-simulation techniques and TEM analysis were employed to study the dependence of the twin and tweed microstructures in YBa2Cu3-xMxO7 -delta on doping by trivalent atoms (like Fe, Co, or Al). Since the tr ivalent atoms substituting the bivalent Cu(I) atoms have a greater num ber of the nearest-neighbor oxygen atoms, it was suggested that doping can be described by the M-O nearest-neighbor attractive interaction. This interaction generates a local oxygen disorder which plays an impo rtant role in the formation of the tweed structure. The M-O interactio n and the long-range O-O interaction (screened Coulomb and strain indu ced) were taken into account to simulate the oxygen ordering kinetics. The simulation is based on equations describing the microscopic diffu sion of oxygen atoms. The obtained simulated microstructures are in qu alitative and even quantitative agreement with electron microscopic ob servations for YBa2Cu3-xFexO7-delta and with previous electron microsc opic data. Both the computer simulations and the TEM results have show n that the usual twin structure, formed through coarsening and refinin g of the transient tweed structure, is produced at small doping (x les s-than-or-equal-to approximately 0.08). At larger x (approximately 0.0 8 < x < approximately 0.2-0.3), the dopant atoms prevent coarsening an d a metastable (or stable) mesoscopic tweed pattern appears. Although the crystal lattice is locally distorted by ultrafine orthorhombic dom ains forming the tweed pattern, the average crystal lattice determinin g the diffraction spot pattern is tetragonal. At higher x, the ultrafi ne orthorhombic domains producing the tweed structure disappear and a disordered tetragonal phase is formed. It is found that doping does no t affect the microstructure if the long-range model for the M-O intera ctions is assumed.