Raman Spectroscopic Investigation of Ferroelectric Phase Transitions in Rochelle Salt

Abstract

Raman scattering spectra of single crystals of Rochelle salt are recorded a t 2 K and a t several temperatures between 248 and 300 K in the spectral region from 0 to 4000 cm-l. 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. I n 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. Es werden die Ramanspektren von Rochellesalz-Einkristallen

Chemical Dynamic Behavior at Solid Surfaces

“…These are the so-called hard-core modes. This type of behaviour has been encountered in other systems previously before (tri-glycine sulphate (TGS), TGSe, thiourea, Rochelle salt) and has been attributed to a non-linear pseudo-spin-phonon coupling (Winterfeldt and Schaack 1977, Winterfeldt 1980. The linewidths in both the infrared and Raman spectra increase considerably with T. As a consequence, weak lines that are well resolved at 2 K disappear rapidly with increasing temperature, while other lines merge with neighbouring lines, producing the appearance of broad bands in the high-temperature spectra.…”

“…This shift can be fitted to the relation ( Winterfeldt andSchaack 1977,1980) and it has been shown that the fitted values of the exponent in (3) agree, within reasonable limits, with the critical exponent / 3 of the order parameter of the SPT. This can be interpreted by an additional term in the force field that is proportional to the order parameter.…”

Structural Disorder and Molecular Dynamics in Ferroelectric Crystals [C(NH2)3]4Br2SO4 and [C(NH2)3]4Cl2SO4

“…The anomalous temperature shifts of the frequencies of Raman lines in the ferroelectric phase of G 4 Br 2 SO 4 can be described in the frame of a nonlinear pseudospin coupling model. − According to this model, the change in the mode frequency is coupled with the ferroelectric order and it fulfills the following relation: where ν 0 is a constant frequency of the mode in the paraelectric phase, η is an order parameter, and m is an integer power. For proper ferroelectrics, the spontaneous polarization P s is the order parameter.…”

Anharmonic Effects in the Electron Spin-Lattice Relaxation of Radiation Defects in Potassium Selenate and Rochelle Salt Single Crystals

“…Experimental optical investigations in Rochelle salt by means of the Raman spectroscopy [9] indicated the presence of optical lattice vibrations at the wave numbers 128 cm -1 and 135 cm-1 . Therefore, the lattice excitation with the wave number v2 = Δ2k/hc = (130 + 5) cm-1 determined from the temperature dependence of T1 may be attributed to this optical lattice vibration.…”