Shell model investigation of the Raman scattering in the ordered and disordered phases of NH4Br

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Abstract
We describe a formalism for the Raman scattering from orientationally disordered solids, in particular for the ammonium halides. The electronic polarizability is treated in a shell model. Applying the theory to the ordered phase of NH4Br we analyze the extreme scattering anisotropy of the 56 cm−1 band observed by Wang and Wright. This allows us to determine the temperature dependence of the order parameter and to deduce a potential for the orientation dependent interaction of the Br shell with neighboring H atoms. With the use of this potential predictions can be made for all the polarizability coefficients which enter numerical calculations of the Raman scattering intensity in the disordered phase. These calculations are in good agreement with experiment and show that one does not have to assume short range order among the NH4+ ions to explain the appearance of sharp bands in the Raman spectra of the disordered phase.