ORIENTATIONAL CHANGES IN THE AB PLANE OF YBA2CU3O7-DELTA FILMS ON DIFFERENT SUBSTRATES

YBa2Cu3O7 (YBCO) thin films with c-axis orientation on (001) SrTiO3, ( 001) MgO and (110) NdGaO3 substrates were examined by four-circle diff ractometry with Co K alpha and synchrotron radiation (lambda = 0.1540 nm) using the q-scan technique. Small local film rotations about [001] have been introduced by two different mechanisms: (1) the orthorhombi c/tetragonal phase transition driven by iron substitution or changes o f the oxygen content; (2) heavy-ion irradiation (1.4 GeV U-238 UNILAC, GSI Darmstadt). In the original orthorhombic state, the [110] axes of film and substrate are parallel so that the [100](film) axes form an angle of 0.45 degrees to those of the substrate. With decreasing ortho rhombicity, this angle decreases to zero for tetragonal symmetry. Irra diation of the orthorhombic film by heavy ions produces detwinning and leads to a macroscopic preferential parallel alignment of the (100) a xes of film and substrate. This reorientation is completed for a dose between 10(11) and 10(12) particles cm(-2). High-resolution transmissi on electron microscopy investigations reveal a rotation of crystallite s in the ab plane of irradiated samples of up to 3 degrees. For irradi ation with 200 (456) MeV Ca-40, no detwinning is observed.