  We report observation of 90-degree ferroelectric domain structures in
transmission electron microscopy (TEM) of epitaxially-grown films of PbTiO3.
Using molecular dynamics (MD) simulations based on first-principles effective
Hamiltonian of bulk PbTiO3, we corroborate the occurance of such domains
showing that it arises as metastable states only in cooling simulations (as the
temperature is lowered) and establish characteristic stability of 90-degree
domain structures in PbTiO3. In contrast, such domains do not manifest in
similar simulations of BaTiO3. Through a detailed analysis based on energetics
and comparison between PbTiO3 and BaTiO3, we find that 90-degree domain
structures are energetically favorable only in the former, and the origin of
their stability lies in the polarization-strain coupling. Our analysis suggests
that they may form in BaTiO3 due to special boundary condition and/or
defect-related inhomogeneities.
