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L. Ginzburg et al. Ψ Fig. 5. The thermodynamic potential as a function of the order parameter at a temperature somewhat higher than that of the first-order transition. which η changes from zero to ± ηχ are connected with a small change of thermodynamic potential and therefore are quite probable and should be considered on the same footing with small " p h o n o n - l i k e " fluctuations near the point η = 0. Note, however, that fig. 5 is for space-homogeneous order parameter variations. At the same time, the appearance of a localized heterophase fluctuation (a change of η from zero to a finite value η * in some region, say, inside a sphere of radius R) implies the appearance of an extra ("surface") energy.

F o r example, in the case of K H 2 P 0 4 , where one can set η = Pz (z component of the polarization vector) and ξ = uxy (component of the strain deformation tensor), alone that changes it turns out that it is the quantity φξηη = (ο2φ/δΡ2)^ essentially with temperature near the phase transition while the coefficient φηξξ = (δ2φ/du2xy)Pz remains practically constant. F o r this reason it is just the polarization vector component Pz that is usually taken for the order parameter in K H 2 P 0 4 (Jona and Shirane 1962).

F o r a second-order phase transition far from the tricritical point the quantities φρηη and Y\\ depend on temperature in the same way as below Tc. Thus, as is seen from eq. (57), the presence of shear deformations does not affect the temperature dependence of ( h ( t f ) | 2) as compared with the spatially homogeneous case but influences only the amplitude of this dependence. Another situation takes place for a tricritical point. Here φρηη goes to zero (for |τ|-*0) as 7/ 2|τ| 1 / ,2 i- e> steeper than η\ (see eqs.