X-RAY-DIFFRACTION STUDIES OF THE EPITAXY OF A/B-AXES ORIENTED YBA2CU3O7-DELTA FILMS GROWN BY LIQUID-PHASE EPITAXY

A detailed x-ray diffraction study is reported for a/b-axes oriented YBa2Cu3O7-delta 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-degrees-[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-delta 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 (alpha(film)[100], alpha(film)[010]>alpha(subs)[001]) is compensated for upon cooling by a slight rotation (epsilon approximately 0.12-degrees) of {hh0}-type domain boundaries around the common [001] direction.