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 ${\mathrm{cm}}^{\mathrm{-}1}$ were classified according to their types of symmetry. The temperature dependence of the soft-mode frequency (∼20 ${\mathrm{cm}}^{\mathrm{-}1}$ at 30 K and ∼0.15 ${\mathrm{cm}}^{\mathrm{-}1}$ at ${\mathit{T}}_{\mathit{C}1}$=255 K) and the soft-mode oscillator strength (ten times higher at ${\mathit{T}}_{\mathit{C}1}$ than at low temperatures) was explained by coupling of the mode near 75 ${\mathrm{cm}}^{\mathrm{-}1}$ which partially softens with infrared inactive hard modes at 37 and 13 ${\mathrm{cm}}^{\mathrm{-}1}$ 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 ${\mathit{T}}_{\mathit{C}1}$ and ${\mathit{T}}_{\mathit{C}2}$=297 K, but softening of the optical phonon near 75 ${\mathrm{cm}}^{\mathrm{-}1}$. The phase transitions in Rochelle salt can be treated as a mixture of displacive and order-disorder type.

• Received 16 January 1995

DOI:https://doi.org/10.1103/PhysRevB.51.14998