Ferroelectric displacement of atoms in Rochelle salt

The crystal structure of Rochelle salt in the ferroelectric phase is studied by means of x-ray diffraction. The structures of the two paraelectric phases are also redetermined to obtain the displacements of the atoms owing to the phase transitions. Cooperative displacements of the atoms responsible for the appearance of the spontaneous polarization are obtained. The displacements consist of the movements of the tartrate molecules and the water molecules in a frame composed of K and Na ions. During the successive phase transitions there are several interatomic distances maintained constant. The direction of planes made of carbons of the tartrate molecules is practically unchanged throughout the phase transitions.