Practical Electron Microscopy and Database

Curie temperature (also called Curie point) is the temperature above which a ferroelectric material loses its ferroelectricity and becomes paramagnetic. It is important to note that all ferroelectric materials have their own Curie temperature. When the temperature decreases through the Curie temperature, the ferroelectric crystal presents a phase transition from a non-ferroelectric to a ferroelectric phase. If there are more than one ferroelectric phases, the temperature at which the crystal transforms from one ferroelectric state to another is called the transition temperature. This corresponds to the structural change and especially to symmetry. Note that many physical properties such as dielectric, thermal, optical and elastic properties, birefringence, thermal expansion, dielectric susceptibility and photoluminescence at the Curie temperature are often interesting and anomalous.

Table 2382 lists the Curie temperatures of some typical materials.

Table 2382. Curie temperatures of some typical materials.

Figure 2382a shows an example of T_c as a function of the grain size.

The first-order ferroelectric phase transition induces thermal hysteresis. Figure 2382b shows elastic Gibbs free energy for a first order ferroelectric phase transition at different temperatures. The polarization here is also called remanent polarization, or zero field electric displacement; the Curie-Weiss temperature represents the lowest temperature at which the paraelectric phase may exist; the Curie temperature represents the phase transition temperature; the ferroelectric limit temperature represents the highest temperature at which the ferroelectric phase may exist; and the limit temperature of field induced phase transition represents the highest temperature at which the ferroelectric phase may be induced by the external electric field.

T_c also depends on impurities in materials. For instance, Figure 2382c shows the dependence of T_c on Mn concentration in cubic zirconia, obtained by KKR (Korringa-Kohn-Rostoker) theory.

In some ferroelectric crystals, the temperature dependence of the dielectric constant can be reasonably accurately represented by the Curie-Weiss law,
         ε - ε₀ = C/(T-T₀) ------------------------- [2382a]
where,
         C -- The Curie constant.
         ε -- The permittivity of the material.
         ε₀ -- The permittivity of vacuum.
         T₀ -- The Curie-Weiss temperature, which in general differs from the Curie temperature (T_c).

Equation 2382a indicates that when the temperature of the crystal is close to T₀, the dielectric constant becomes very large.

In some ferroelectrics, T_c is aproximately equal to T₀, while in others T_c is far from T₀.
         
         
         

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