One-phonon density-of-states and spatial correlation from the Raman spectrum of molecular crystals: NH4I

Abstract

The missing translational invariance of a disordered crystal is introduced into the second order perturbation theory of the Raman effect. The disorder of the crystal lifts the wave vector conservation of the scattering process so that phonons from the whole Brillouin zone contribute to the light scattering spectrum. Measurements of the temperature dependence of the Raman spectrum allow the study of spatial correlations in the disordered crystal. We measured the low energy Raman spectrum of NH₄I in its NaCl‐type phase. The random orientations of the ammonium ions of NH₄I in the octahedral lattice cell lift the translational invariance of the crystal but do not affect the local point symmetry of the matrix elements. We obtain the one‐phonon density of states from the low energy Raman spectrum of NH₄I and compare our results with ir and incoherent neutron scattering spectra.