  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.
