Dynamics of Displacive and Order-Disorder Phase Transitions

In the preceding chapters we have dealt with the thermodynamic characteristics of crystals, i.e., we have considered the various equations of state relating the equilibrium values of the generalized thermodynamic forces and coordinates. An example of such an equation is (3.3) or the relation derived from it: ??? (A = 1/2α (T–T_c)), which characterizes the variation of the order parameter for a proper ferroelectric under the action of a low electric field with T → T_c. The quantity aE may here be regarded as a “force” coupled to the order parameter and the coefficient A has the meaning of “rigidity” (corresponding to η) or of inverse susceptibility. It can be seen that with T→T_c the quantity A also tends to zero, i.e., upon approach to T_c the rigidity of the system falls off.