Dielectric Anomalies in Structural Nonferroelectric and Improper Ferroelectric Phase Transitions

As was shown in the preceding chapter, the Landau theory allows one to successfully account for anomalies of the basic thermodynamic quantities for ferroelectric crystals in second-order transitions and also in first-order transitions close to second-order ones. While applying this theory to ferroelectrics, we presumed that the order parameter has the same transformation properties as the component of the electric field vector. As a result, it turns out that the equilibrium value of order parameter is proportional to the electric polarization of the crystal. However, there may exist ferroelectric phase transitions in which such a proportionality is not observed.