  We study the complex ferroelastic/ferroelectric domain structure in the
prototypical ferroelectric PbTiO3 epitaxially strained on (110)o-oriented
DyScO3 substrates, using a combination of atomic force microscopy, laboratory
and synchrotron x-ray diffraction and high resolution scanning transmission
electron microscopy. We observe that the anisotropic strain imposed by the
orthorhombic substrate creates a large asymmetry in the domain configuration,
with domain walls macroscopically aligned along one of the two in-plane
directions. We show that the periodicity as a function of film thickness
deviates from the Kittel law. As the ferroelectric film thickness increases, we
find that the domain configuration evolves from flux-closure to a/c-phase, with
a larger scale arrangement of domains into superdomains.
