Curie-Weiss equation

The function of I2g> (T) in the vicinity of the phase transition to centrosymmetric conditions usually has a linear character. Such behavior corresponds to ferroelectrics that undergo type II phase transitions and for which the SHG signal, l2Curie temperature is described by the Curie - Weiss Equation ... 

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

In an analogous maimer, the complete form of the Curie-Weiss equation is ... 

The relationship, i = 2.83 "VX (T+ A ) shows that changes in susceptibility could result from changes in the moment (i.e.,changes in oxidation state) and/ or from changes in the Weiss interaction constant, A. The susceptiblity, X, is related to the interaction constant,A, by the Curie-Weiss equation ... 

Ideally the graph is linear, with a slope of 1/C and an intercept on the T axis of (Figure 11.18). Frequently, the point of intercept, Tq, is slightly different from the measured value of T, and the Curie-Weiss equation is often written in the form... 

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