Raman Spectroscopic Investigation of Ferroelectric Phase Transitions in Rochelle Salt

Raman scattering spectra of single crystals of Rochelle salt are recorded at 2 K and at several temperatures between 248 and 300 K in the spectral region from 0 to 4000 cm−1. The frequencies of some phonon modes soften or harden in the intermediate ferroelectric phase between the two Curie points which are known to exist in this material. In the paraelectric phases the frequencies remain constant. This behaviour can be explained by applying a simple nonlinear pseudospinphonon coupling theory originally developed for the TGS family and the incommensurate phase transition in thiourea. The anomalous specific heat due to temperature‐dependent optical phonon modes is also discussed.