Thermodynamic Theory of Primary Ferroics

In the vicinity of phase transition one can expand thermodynamic potential 0(r],r,p) into the power series in t]  

Here Oq is the thermodynamic potential of high-symmetry prototype phase (high-temperature phase) of ferroic coefficients a. A, p, 5 are the functions of T andp, the magnitudes of the coefficients before higher powers of r are usually smaller than those before lower powers. 

It is easy to see that a = 0 as only in this case 9 5/9h = 0 in high-temperature phase, which subsequently we call paraphrase. Substituting a = 0 into Eq. (1.1), we find after differentiation 

Equating the expression in the brackets (1.3) to zero, we obtain the order parameter in the low-temperature phase of a ferroic. Before doing so, we analyze the behavior of the coefficients. Keeping in mind that ho = 0 at T = 7), one finds in the transition point 

The supposition about possibility to leave only linear term in Eq. (1.8) is the postulate of Landau theory that gives good agreement with experimental data [10]. Indeed, adding the term r h to thermodynamic potential (1.5), where h is the external field, conjugated to order parameter, one can find that isothermal susceptibility of a system x = r dh equals to 

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