Low frequency Raman spectra of ammonium halides in the disordered phases I and II

Abstract

The infrared and Raman spectra of matrix isolated binary and mixed mercury halides. II. Extension to low frequenciesThe low frequency Raman spectrum of single crystals of NH4 G, NH4 Br, and NH4 I has been measured in disordered phases I and II. Particular attention has been paid to the polarization selection rules.· The temperature dependence of the intensity of the disorder induced spectrum on entering ordered phase III or IV has also been measured. The data for disordered phase II in NH4 G were interpreted by performing a calculation of the Raman spectrum, based on the theory of Loveluck and Sokoloff of Raman scattering from phonons in a disordered system, using the lattice dynamical calculations performed by Teh. Excellent agreement is obtained with the major features of the observed polarization selection rules. The two polarization configurations are shown to pick out either NHt vibrations or Cl-vibrations alone. The implications for phase II in NH4Br are also discussed in the light of these calculations. A brief discussion of NH41 in phase I is also given.

Mise en évidence d'un ordre a courte distance dans la phase II des cristaux de NH4Cl, rappelant la phase III des cristaux de NH4Br

“…La seconde phase (phase II), observée entre 243 K et 458 K, est une phase « désordonnée » dans laquelle les groupements ammonium peuvent avoir, avec la même probabilité, les deux orientations équivalentes représentées figure 2a [1]. Les principales caractéristiques du spectre Raman de basse fréquence observé pour NH4Cl, à la température ambiante et à la pression atmosphérique [3][4][5], c'est-à-dire dans la phase II, ont pu être expliquées [5] à l'aide d'un modèle de cristal « désordonné » [6]. Cependant, l'évolution du spectre de la phase II, lorsque la température varie, semble indiquer un désordre non statistique et suggère l'existence de phénomènes d'ordre à courte distance [4,5].…”

Far‐Infrared and Raman Studies on (NH4)Cl1−xBrx Mixed Crystals in the Cubic Phases β and δ

“…For pure NH4C1, x = 0, again, a single symmetric peak a t 92 cm-1 is observed. The group-theoretical Raman selection rules for the disordered @-phase have been given for all points in the Brillouin zone by Couzi et al [8]. From the high symmetry points, I?, R, M, X, only LA(X), and the lowest M-point mode TA,(M) will contribute t o the Al, + 4Eg spectra.…”

Linewidth of Phonons in Order–disorder Phase Transitions With Random Telegraph Noise

“…The Raman linewidths of the zone boundary ν 7 (93 cm −1 ) lattice mode of NH 4 Cl also broaden in the disordered β phase 6 as the ν 7 (56 cm −1 ) lattice mode of NH 4 Br does. As the temperature lowers below the λ-transition temperature (T λ ∼ = 243 K), the observed data 6 shows that the linewidths of the ν 7 (93 cm −1 ) Raman mode decrease in the ferro-ordered δ phase where all the NH + 4 ions are ordered parallel to the c-axis in NH 4 Cl.…”