Twinning behaviour in YBCO and PBCO thin films and in PBCO-YBCO superlattices

It is well established that YBa2Cu3O7−x bulk crystals twin when crossing the tetragonal to orthorhombic transition. Much fewer investigations have been performed on thin films, but one expects a strong influence of the strain induced by the underlying substrate. An analysis of this influence has been undertaken by X-ray diffraction and transmission electron microscopy, from a set of specific samples grown by metal-organic chemical vapor deposition: YBa2Cu3O7 − x PrBa2Cu3O7 − x and YBa2Cu3O7−x-PrBa2Cu3O7−x thin films and superlattices. Results indicate that thick (> 100 nm) YBCO films on cubic SrTiO3 (001) substrates exhibit the same twinning behaviour as bulk crystals. For YBCO films grown on orthorhombic YAIO3, substrates only one twinning direction is observed. YBCO-PBCO superlattices on SrTiO3 exhibit an orthorhombic symmetry down to superperiods of 25 A. The superlattices twinning behaviour is also highly dependent on the total thickness of the stacking.