90-degree Ferroelectric Domains in PbTiO3: Experimental Observation and   Molecular Dynamics Simulations
  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. 
Kenta Aoyagi, Takeshi Nishimatsu, Takanori Kiguchi, Toyohiko J. Konno,   Yoshiyuki Kawazoe, Hiroshi Funakubo, Anil Kumar, an...
cond-mat.mtrl-sci