  Nonlinear electrostatic interaction between the surface ions of
electrochemical nature and ferroelectric dipoles gives rise to the coupled
ferroionic states in nanoscale ferroelectrics. Here, we investigated the role
of the surface ions formation energy value on the polarization states and
polarization reversal mechanisms, domain structure and corresponding phase
diagrams of ferroelectric thin films. Using 3D finite elements modeling we
analyze the distribution and hysteresis loops of ferroelectric polarization and
ionic charge, and dynamics of the domain states. These calculations performed
over large parameter space delineate the regions of single- and poly- domain
ferroelectric, ferroionic, antiferroionic and non-ferroelectric states as a
function of surface ions formation energy, film thickness, applied voltage and
temperature. We further map the analytical theory for 1D system onto effective
Landau-Ginzburg free energy and establish the correspondence between the 3D
numerical and 1D analytical results. This approach allows performing the
overview of the phase diagrams of ferroionic systems and exploring the specific
of switching and domain evolution phenomena.
