Proper Ferroelectrics: Anomalies of Physical Properties in Phase Transitions

Before we turn to the analysis of the main consequences of the theory of second-order phase transitions for ferroelectric crystals, we are going to discuss the conditions, the fulfilment of which leads us to believe that the inferences of the theory will be quantitative. The most important question is evidently the following: Is the representation of the nonequilibrium thermodynamic potential in the form of a power series of the order parameter at the point T = T_c permissible? The answer to this question is known and it can be shown that this point is a singular point of the thermodynamic potential and the expansion coefficients vanish at this point or go to infinity. A special temperature dependence is also exhibited by the quantity Φ₀(p,T), which is the part of the thermodynamic potential containing the contributions of all degrees of freedom of a crystal, except η.