Vibrational spectroscopy and soft-mode behavior in Rochelle salt

A complete set of the far-infrared transmission and reflection spectra of Rochelle salt together with Raman spectra were obtained in the broad temperature range between 15 and 300 K. All observed phonons below 700 cm^-1 were classified according to their types of symmetry. The temperature dependence of the soft-mode frequency ({~}20 cm^-1 at 30 K and {~}0.15 cm^-1 at TC1=255 K) and the soft-mode oscillator strength (ten times higher at TC1 than at low temperatures) was explained by coupling of the mode near 75 cm^-1 which partially softens with infrared inactive hard modes at 37 and 13 cm^-1 of the same symmetry. The dynamical origin of the phase transitions in Rochelle salt is therefore not the soft relaxation observed in microwave and near-millimeter spectra in a restricted temperature range around TC1 and TC2=297 K, but softening of the optical phonon near 75 cm^-1. The phase transitions in Rochelle salt can be treated as a mixture of displacive and order-disorder type.