Analysis of the Raman Spectrum of the Higher Frequency Modes of Rb₂ZnCl₄

Abstract

The stretching-region (250–330 cm -1 ) internal modes of Rb 2 ZnCl 4 were studied by Raman-scattering from 190 to 330 K. Four lines, peaking at about 275, 285, 295 and 304 cm -1 are clearly resolved below the normal-to-incommensurate phase transition (307 K in these measurements); above 307 K, the four merge into three and the 285 cm -1 peak is no longer detectable. The derivatives of these spectra were fitted to the derivative of an overlapped, spectrometer-corrected, four-Lorentzian lineshape. The resulting temperature dependence of the integrated intensity of the 285 cm -1 peak decreases by at most a factor of 3 up to 305 K whereas the square of the order parameter in this range decreases by a factor of about 50. Thus, its disappearance above 307 K is as yet unexplained.