Two-Sublattice Model of Ferroelectric Phase Transitions

Dielectric properties of two nonequivalent sublattice polarizations are studied. Assuming a strong coupling favoring an antiparallel alignment of polarizations, the model reveals some new features of a ferroelectric transition with no multiple change of the unit cell volume. The Curie-Weiss constant is unusually small and this fact directly accounts for a possible sign change of the spontaneous polarization by only a slight variation of model parameters with temperature. The model also can describe a para-ferroelectric phase transition followed by an isomorphous phase transition between two ferroelectric phase with no change of symmetry. Temperature dependence of susceptibility is derived with the emphasis on the critical region in which the applied pressure or electric field smears out the difference between two isomorphous phases. The model can explain some properties of ammonium sulfate, dicalcium strontium and lead propionate.

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