Effect of ordering on the infra-red spectrum of ammonium chloride

Abstract

The infra-red spectra of sublimed films of ammonium chloride have been recorded between 250 and 4000 cm −1as functions of temperature from 85 to 300°K. Special emphasis was given to data near the lambda temperature at 242°K. Two new features are reported in detail. The binary overtone of the v 6 librational mode was directly observed in the low-temperature spectra as a band at 749 cm −1. At low temperatures, the v 4 NH 4 + bending vibration gives rise to a strong, sharp band at 1403 cm −1; as the temperature is increased from 100 to 300°K, a new component appears at 1444 cm −1 and grows rapidly in intensity. This behavior is associated with the breakdown of translational symmetry as the crystal becomes disordered, and the intensity variation can be quantitatively correlated with the short-range order parameter.