Orientational Changes in the ab plane of YBA₂Cu₃O_7−δ 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 diffractometry with Co Kα and synchrotron radiation (λ = 0.1540 nm) using the q-scan technique. Small local film rotations about [001] have been introduced by two different mechanisms: (1) the orthorhombic/tetragonal phase transition driven by iron substitution or changes of the oxygen content; (2) heavy-ion irradiation (1.4 GeV ²³⁸U 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° to those of the substrate. With decreasing orthorhombicity, this angle decreases to zero for tetragonal symmetry. Irradiation of the orthorhombic film by heavy ions produces detwinning and leads to a macroscopic preferential parallel alignment of the 〈100〉 axes of film and substrate. This reorientation is completed for a dose between 10¹¹ and 10¹² particles cm⁻². High-resolution transmission electron microscopy investigations reveal a rotation of crystallites in the ab plane of irradiated samples of up to 3°. For irradiation with 200 (456) MeV ⁴⁰Ca, no detwinning is observed.