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

Abstract

The low frequency Raman spectrum of single crystals of NH₄ Cl, 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 Cl 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 NH₄⁺ 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 NH₄I in phase I is also given.