Curie Temperature/Curie Point
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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.

Materials

Curie temperature
(Tc, °C)

Material formation/growth temperature (°C)

Reference

Ba(Co,Zr)0.4 Fe11.6O19
426

[1]
Ba(Co,Zr)1.2Fe10.8O19
330

[1]
Ba(Co,Zr)2.0 Fe10.0O19
295

[1]
BaFe12O19
453

[1]
Ba(Ni,Zr)0.4 Fe11.6O19
434

[1]
Ba(Ni,Zr)1.2 Fe10.8O19
374

[1]
Ba(Ni,Zr)2.0 Fe10.0O19
315

[1]
BaxSr1-xTiO3 (BST)
Linear from 0 to ~120 (~ 0 for x=0.5)

[1]
BaTiO3
see Figure 2382a
α-Fe
770

ε'-Fe2.2C (hcp)
450

χ-Fe5C2 (monoclinic)
257

θ-Fe3C (orthorhombic)
212

Ga(Mn)As
-100

KxNa1-xNbO3 (KNN) (perovskite)
410

Li2Ge7O15 (Lithium heptagermanate, LGO)
20   Weak ferroelectric
25% Mn–HfO2
300

[3]
Mn–ZrO2
390

[3]
NaNbO3 (Sodium niobate)
+63

NH4H2PO4
-125

PbZrO3 (Lead zirconate, PZ)
+233

PbTiO3
+490
570

ZnTiO3
~5.0

[2]
Zn(Zr0.01Ti0.99)O3
11.5
900

[2]
Zn(Zr0.05Ti0.95)O3
13.0
900

[2]
Zn(Zr0.10Ti0.90)O3
13.5
900

[2]
Zr1-xMnxO2-y
See Figure 2382c

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

Figure 2382a. Theoretical Tc in BaTiO3 as a function of the grain size. Adapted from [4]

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.

 Figure 2382b. Free energy for a first order ferroelectric phase transition at different temperatures. Four characteristic temperatures in the phase transition process: (1) T0: Curie-Weiss temperature, (2) Tc: Curie temperature, (3) T1: Ferroelectric limit temperature, and (4) T2: Limit temperature of field induced phase transition.

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

Figure 2382c. Dependence of Tc on Mn concentration in cubic zirconia, obtained by KKR (Korringa-Kohn-Rostoker) theory. The inset presents the variation of Tc of Zr0.75Mn0.25O2-y as a function of the oxygen vacancies. Adapted from [5]

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 (Tc).

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

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

[1] Synthesis and determination of Curie temperature of ferrites from the systems BaO-CoO-ZrO2-Fe2O3 and BaO-NiO-ZrO2-Fe2O3, B. Boyanov, Journal of the University of Chemical Technology and Metallurgy, 41, 1, 2006.
[2] Influence of ZrO2 addition on the structure, thermal stability, and dielectric properties of ZnTiO3 ceramics, Yin-Lai Chai, Yee-Shin Chang, Lay-Gaik Teoh, Yi-Jing Lin, and Yu-Jen Hsiao, J Mater Sci (2008) 43:6771–6776.
[3] First-principles study of manganese-stabilized hafnia, I.V. Maznichenko, S. Ostanin, A. Ernst, and I. Mertig, Journal of Magnetism and Magnetic Materials 321 (2009) 913–916.
[4] P. Perriat, J.C. Niepce, G. Caboche: J. Thermal Anal. 41, 635–649 (1994).
[5] Mn-Stabilized Zirconia: From Imitation Diamonds to a New Potential High-TC Ferromagnetic Spintronics Material S. Ostanin, A. Ernst, L. M. Sandratskii, P. Bruno, M. Däne, I. D. Hughes, J. B. Staunton, W. Hergert, I. Mertig, and J. Kudrnovský, Physical Review Letters, 98, 016101 (2007).

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