X-ray diffraction studies of the epitaxy of a/b-axes oriented YBa2Cu3O7−δ films grown by liquid phase epitaxy

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Abstract
A detailed x‐ray diffraction study is reported for a/b‐axes oriented YBa2Cu3O7−δ films obtained by the liquid phase epitaxy technique. The films were grown epitaxially in the tetragonal state on (110) NdGaO3 substrates so that [001]film∥ [110]subs and [100]film∥ [001]subs, 90°‐[100]/[010] boundaries were almost absent. Below the tetragonal‐to‐orthorhombic transition temperature, the film undergoes intensive {hh0}‐type twinning. The volume of the a‐axis oriented material is similar to that of the b‐axis oriented fraction and the presence of both orientations is likely to be controlled by {hh0}‐type twinning. High temperature diffraction of the substrate indicates that nucleation of YBa2Cu3O7−δ on (110) NdGaO3 takes place on a square two‐dimensional lattice and therefore the observed strong in‐plane alignment is probably controlled by impurity action and/or surface relaxation rather than simple mismatch effects. The difference between the thermal expansion coefficients of the film and the substrate (αfilm[100], αfilm[010]≳αsubs[001]) is compensated for upon cooling by a slight rotation (ε∼0.12°) of {hh0}‐type domain boundaries around the common [001] direction.