“…3. With decreasing film thickness, the mean polarization decreases, while the mean susceptibility increases, then goes up rapidly accompanying with a dielectric divergence, which is in agreement with the experimental data of KDP [34], BaTiO 3 [35] and PbTiO 3 fine particles [36,37]. For ferroelectric thin films, when the thickness is smaller than the critical value, the polarization cannot line up under thermoequilibrium because of the weakened long-range interaction by the sample size, but when a weak electric field is applied, they line up easily and show a large dielectric susceptibility.…”