Properties of Rochelle Salt. III

The interaction theory of Rochelle salt is extended by supplementing the expression for the free energy of the deformed and polarized crystal by a term proportional to the fourth power of the polarization in the $a$ direction. This second approximation explains the dielectric and elastic saturation phenomena above the upper Curie point and the dielectric, elastic, piezoelectric and pyroelectric properties in the ferroelectric temperature range. All experimental results are compatible with the hypothesis that the clamped crystal has only one transition point at about 5° C, where its dielectric constant has a very high maximum. The two Curie points of the free crystal are due to the interaction between the elastic deformation and the electric polarization. Instead of sudden changes of the heat capacity the new theory predicts slight changes of the slope of the specific heat curve at the Curie points. The dispersion of the dielectric constant is explained without assuming a relaxation time.