Infrared Investigation of Structural and Ordering Changes in Ammonium Chloride and Bromide

Abstract

Infrared spectra of NH4Cl, ND4Cl, NH4Br, and ND4Br have been recorded at numerous temperatures between 21 and 300°K using thin sublimed films, single-crystal sections, and pressed disks. At 21°K each spectrum contains a large number of sharp intense bands which can be assigned to combinations involving the internal vibrational modes of the ammonium ion, the librational (torsional oscillation) mode, and various lattice modes. Observation of the first and second librational overtone makes it possible to discuss the potential barrier hindering the rotation of the ammonium ion. Barrier heights of 1860 cm−1 (5.32 kcal mole−1) for NH4Cl and ND4Cl and 1520 cm−1 (4.35 kcal mole−1) for NH4Br and ND4Br were calculated from the Gutowsky–Pake–Bersohn model, but the experimental frequencies show that the libration is more anharmonic than this model would predict. The temperature dependence of an anomalous component of the ν4 bending fundamental was studied in detail. The intensity of this component can be directly correlated with the breakdown of translation symmetry due to disorder in the ammonium ion orientations. In addition, this feature of the spectrum indicates that there is appreciable distortion in the ordered cubic phase of NH4Br and ND4Br prior to the first-order transition into the ordered tetragonal phase.