Non-Equilibrium Thermodynamics of Ferroelectric Phase Transitions

It is well known that the Landau theory of continuous phase transitions is a milestone in the process of the development of phase transition theories. Though it does not tally with the nature of phase transitions in the critical regions, the Landau theory as a phenomenological one has been very successful in many kinds of phase transitions such as the ferroelectric phase transitions, i.e. the vast studies centering on it have been carried out. In the ferroelectric case, we should pay attention to the Landau theory extended by A.F. Devonshire to the first-order phase transitions (Devonshire, 1949; Devonshire, 1951; Devonshire, 1954). This daring act was said to be successful. However, the Landau theory is based on the equilibrium (reversible) thermodynamics in essence. Can it deal with the outstanding irreversible phenomenon of first-order ferroelectric phase transitions, which is the “thermal hysteresis”? The Landau-Devonshire theory attributes the phenomenon to a series of metastable states existing around the Curie temperature TC. In principle, the metastable states are not the equilibrium ones and can not be processed by using the equilibrium thermodynamics. Therefore, we believe that the extension of Devonshire is problematic though it is successful in mathematics. The real processes of phase transition were distorted. In Section 2, we will show the unpleasant consequence caused by the metastable states hypothesis, and the evidence for the non-existence of metastable states, i.e. the logical conflict. Then in Section 3, we will show the evidence (experimental and theoretical) for the existence of stationary states to a ferroelectric phase transition. In Section 4 and 5, we will give the non-equilibrium (irreversible) thermodynamic description of phase transitions, which eliminates the unpleasant consequence caused by the metastable states hypothesis. At last, in Section 6 we will give the non-equilibrium thermodynamic explanation of the irreversibility of ferroelectric phase transitions, i.e. the thermal hysteresis and the domain occurrences in ferroelectrics.