Some Remarks on the Phenomenological Theory of Ferroelectricity

The free energy of a ferroelectric was expanded as an even power-series of polarization in the customary phenomenological theory. For the case of the transition of the second order, the condition was examined under which the well-known relation holds that the temperature-gradients of reciprocal susceptibility immediately below and above the transition point stand in the ratio -2:1, and it was shown that this relation is valid regardless of the number of terms taken in the expansion and the temperature-dependence of their coefficients. When the transition is of the first order, on the contrary, this ratio was found to be very sensitive to the them, though a simple ratio -8:1 is obtained by postulating the temperature-independent coefficients of fourth- and sixth-powers and neglecting terms of higher powers. A similar result was also obtained concerning the antiferroelectricity.