Theory of hypothetical ferroelectric superlattices incorporating   head-to-head and tail-to-tail 180$^\circ$ domain walls
  While electrical compatibility constraints normally prevent head-to-head (HH) and tail-to-tail (TT) domain walls from forming in ferroelectric materials, we propose that such domain walls could be stabilized by intentional growth of atomic layers in which the cations are substituted from a neighboring column of the periodic table. In particular, we carry out predictive first-principles calculations of superlattices in which Sc, Nb, or other substitutional layers are inserted periodically into PbTiO$_3$. We confirm that this gives rise to a domain structure with the longitudinal component of the polarization alternating from domain to domain, and with the substitutional layers serving as HH and TT domain walls. We also find that a substantial transverse component of the polarization can also be present. 
Xifan Wu and David Vanderbilt