Ferroelectricity has been uncovered in Rochelle Salt (RS) by Valasek in 1920 (1). RS is a complicated crystal with four molecules [NaKC4 H4 06 , 4H2 0] in the primitive unit cell (2). Several external lattice vibrations have been observed in the 10 - 100 cm-1 far infrared range at 80 K and 5 K in crystal plates of increasing thickness : 160-550 and 6,500 μm. At 80 K absorption increases with thickness according to Beer's law. At 5 K, the lower frequency absorption line (21 cm-1) seems independent of thickness. This line had been shown to shift to a lower frequency at T = 130 K, and to disappear from the FIR range at a still higher temperature (3). More recently (4) it has been ascribed to the soft mode responsible for the FE transition with a resonant behaviour at 5 K and 80 K. It is softening into a relaxation mode whose frequency goes to zero when temperature increases up to the lower Curie temperature (T1 = 255 K). At 5 K the soft mode is a very weakly damped one, and we have to take into account the instrumental function for the discussion of the effect of plate thickness.
"Far IR Transmission Spectra Of Rochelle Salt At 5 K.", Proc. SPIE 1039, 13th Intl Conf on Infrared and Millimeter Waves, (18 November 1989); doi: 10.1117/12.978480; https://doi.org/10.1117/12.978480