X-ray study of in-plane epitaxy ofYBa2Cu3Oxthin films

Detailed x-ray scattering measurements in three dimensions are reported for thin $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ films on (001) SrTi${\mathrm{O}}_3$ and KTa${\mathrm{O}}_3$ substrates. The films, produced by coevaporation followed by a postdeposition anneal, consist of domains epitaxially oriented with either the $a$ axis or the $c$ axis aligned with the substrate normal. The in-plane epitaxy of grains with the $c$ axis parallel to the normal shows an alignment of the $〈110〉$ directions of the film and the substrate, rather than of the $〈100〉$ directions as is generally presumed. We associate this alignment with a strain relaxation during the high-temperature tetragonal-to-orthorhombic phase transition.