Raman spectrum of ammonium bromide and its variation with temperature

Abstract

Using the λ2536.5 mercury resonance radiation as exciter, the Raman spectrum of crystalline ammonium bromide has been photographed with the crystal maintained successively at four different temperatures, namely, 398°, 308°, 173° and 85° T. The following facts emerge from this study:(1) In the region of temperatures investigated, the Raman spectrum of ammonium bromide exhibits a series of low frequency or lattice lines a fact which has been noticed for the first time. The frequency shifts, intensities and widths of the lines are found to vary with temperature. The lines observed in the spectrum taken above the transition temperature (235° T.) have been identified as the fundamental modes of vibration of the cresium chloride structure in which the NH₄ ions oscillate as units as demanded by the new crystal dynamics. (2) The lattice spectrum exhibits discrete frequency shifts even at 398° T., i.e., 163° above the transition temperature. This does not support Pauling's hypothesis that there is onset of free rotation of the NH₄ ions above the transition temperature. (3) The spectrum characteristic of the NH₄ ions exhibits 5 distinct Raman lines at room temperature and 6 lines at liquid-air temperature. From this it is concluded that the oscillating NH₄ ions do not possess the full tetrahedral symmetry. A comparison of the spectrum of NH₄Br with that of NH₄Cl reveals that the osdllating NH₄ ions in NH₄Br possess a higher order of symmetry than those in NH₄Cl. (4) The spectra recorded at all temperatures reveal the presence of a large number of bands of which those lying in the region ≈ 2800 and 3200-3500 are the most prominent.