THE MICROSTRUCTURE OF THE TWIN TO TWEED TRANSITION IN ALUMINUM SUBSTITUTED YBA2CU3O7-DELTA SINGLE-CRYSTALS

Single Crystals of the superconductor YBa2Cu3O7-partial derivative hav e been grown with controlled amounts of aluminium doping just below th e twin to tweed transition (1 and 1.5 cationic%) and are examined usin g single crystal x-ray diffraction and TEM. The nature of the twinning changes from well defined twins and colonies in undoped crystals to a n interpenetrating twin morphology as the tweed transition is approach ed.