Polymorphism of Rochelle Salt

A careful analysis of the properties of Rochelle salt leads to the conclusion that this crystal belongs to the monoclinic hemimorphic symmetry class between the so-called Curie points (+23.7^∘{}C and -18^∘{}C), whereas it is orthorhombic hemihedric at other temperatures. The Curie points are to be regarded as crystallographic inversion points like the high-low inversion points in quartz and other crystals. Dielectric anomalies as observed for Rochelle salt must be expected near inversions from a pyroelectric to a nonpyroelectric crystal glass. Other properties of Rochelle salt discussed in the light of the new classification are the electrooptic effect, the converse and the direct piezoelectric effect, and the specific heat. A comparison with symmetry conditions in ferromagnetic crystals is made.