The Vibrational Spectra of Molecules and Complex Ions in Crystals IV. Ammonium Bromide and Deutero-Ammonium Bromide

The infra-red spectra of thin non-scattering films of NH4Br and ND4Br were obtained at 28°C, −78°C and −190°C. The spectra of both compounds in Phase III are in agreement with the x-ray symmetry, D4h7. The torsional oscillation frequencies occur at 319 cm−1 and 230 cm−1 in NH4Br and ND4Br, respectively. The spectra of Phase II are similar to those of the chloride and indicate a structure in which the NH4+ ions are disordered in their equilibrium orientations. There is no evidence of free rotation at any temperature studied. The spectra support x-ray evidence for a further transition at −100°C in ND4Br to Phase IV which has symmetry Td1 as in NH4Cl.