Thermodynamic modeling of critical properties of ferroelectric superlattices in nano-scale

Modeling nano-scale ferroelectric superlattices using the Landau free-energy functional approach requires incorporating contributions from the interfacial and depolarization field effects. The choice of the order parameter then becomes a vital issue. In this paper, we compare the predictions of models using the spontaneous polarization as order parameter (SPOP approach) with models using the total polarization as order parameter (TPOP approach). We have comprehensively calculated the critical properties of nano-scale ferroelectric superlattices, such as the phase-transition temperature, critical thickness and Curie–Weiss-type relation using both approaches. We found that all the SPOP results are in excellent agreement with experimental measurements and first-principle calculations in all cases studied here. The TPOP approach, on the other hand, much overestimates the depolarization by underestimating the effect of the dielectric screening and produces results that deviate significantly from the experimental ones. Our results also traced the dependence of the critical properties on the thicknesses of the constituent layers of the ferroelectric superlattices to the interfacial and depolarization field effects.