VII. A thermodynamic treatment of ferroelectricity and antiferroelectricity in pseudo-cubic dielectrics

Abstract

The Kittel-Devonshire phenomenological treatment of antiferroelectricity is extended to the case of polarizations in three dimensions in a pseudo-cubic material. There are ten possible solutions of the proposed free energy equation, corresponding to non-polar, ferroelectric, antiferroelectric and ferrielectric states, and expressions are derived for the polarization and dielectric stiffness coefficients in these forms. Using the experimental data for NaNbO₃, values of the constants in the free energy expression are derived which give satisfactory agreement with the observed phase changes, optical anisotropies and dielectric constants in this material. It is also shown that the major features of the phase diagram for (NaK)NbO₃ solid solutions are reproduced from the NaNbO₃ free energy expression if T _a, the antiferroelectric ‘curie’ temperature decreases linearly with increasing KNbO₃ concentration. These results are discussed in terms of the spatial relationships in the NaNbO₃ and KNbO₃ structures, and compared to the properties of the corresponding alkali tantalates.