Properties of Rochelle Salt

Abstract

In a qualitative way the anomalous properties of Rochelle salt can be explained on the basis of four different theories: ($a$) on the dipole theory, ($b$) by assuming polymorphic transitions at the Curie points, ($c$) by postulating an anomalous piezoelectric effect, and ($d$) with an inter-action theory which assumes that the structure and the fundamental properties of the crystal have no unusual characteristics and which ascribes the observed anomalies to an accidental degeneration of the piezoelectric interaction between the elastic deformation and the electric polarization. The existence of a polymorphic transition at the upper Curie point is demonstrated in a new experiment and it is shown that this transition is a spontaneous elastic deformation. It can be explained on the basis of either one of the four theories. A quantitative analysis of the experimental data, which leads to a new interpretation of the elastic and piezoelectric measurements, shows that the theories ($b$) and ($c$) are not valid for Rochelle salt. The experimental data verify the conclusions of the dipole theory insofar as they lead to the result that Rochelle salt would show an abnormal temperature dependence of the dielectric constant even if the crystal were not piezo-electric or if the piezoelectric deformations could be suppressed. However, it is doubtful whether the clamped crystal has a Curie point. The observed Curie points of the free crystal result from the lattice-dipole coupling as described by the interaction theory.