Ferroelectricity is temperature dependent phenomena. Spontaneous polarization exists below a temperature defined as Curie temperature TC. Above TC, ferroelectric materials undertake a phase transition to normal dielectrics that are paraelectric. The variation of Pr as a function of temperature in ferroelectric materials is schematically shown in Figure 2.4 a. Moreover, upon heating, the dielectric permittivity of the ferroelectric phase presents an irregular variation; that is it increases abruptly around the Curie temperature to decrease again at temperatures above the transition point. The decrease of the dielectric permittivity above the Curie temperature is conventionally described by the Curie-Weiss’ law:
?_r/?_0 =C/(T-T_C )
Figure 2.4 b schematically depicts the temperature dependence of the dielectric constant around the Curie temperature.
Phase transitions can be generally classified in two broad categories: first order and second order. First order phase transitions involve with a latent heat and macroscopic variables, such as the entropy, volume, polarization and the structure of a crystal present a discontinuous variation with respect to the temperature. Unlike first order transitions, second order phase transitions present a continuous variation of these parameters with respect to the temperature whereas their first order derivatives with respect to the temperature are discontinuous at the transition point.