RAMAN-SCATTERING STUDY OF THE SOFT PHONON MODE IN THE HEXAGONAL FERROELECTRIC CRYSTAL KNICL3

Raman spectra of some phonon modes of the hexagonal ferroelectric crys tal KNiCl3 are obtained in the temperature range between 290 K and 590 K, which includes the structural phase transition point T-2(=561 K) a t which previous measurements of dielectric constant and spontaneous p olarization as a function of temperature had shown that KNiCl3 undergo es a transition between polar phases II and III. An optical birefringe nce measurement carried out as a complement to the present Raman scatt ering revealed that this transition is of second order. Towards this t ransition point, the totally symmetric phonon mode with the lowest fre quency observed in the room-temperature phase was found to soften with increasing temperature. The present results provide new information o n the phase-transition mechanism and the space groups of the higher (I I)- and lower (III)-symmetric phases around T-2.