Motion of the Ammonium Ion in Lattices of the NaCl Type

The infrared spectrum of NH4I in the NaCl-type phase gives insufficient information about the motion of the ammonium ions in that phase. Dilute solutions of NH4+ in NaCl-type alkali halides provide considerably more detail in the form of a fine structure in most of the absorption bands which becomes visible on sufficient cooling. This fine structure is most certainly correlated with a rotational motion of the NH4+ ion. However, both models suggested in the literature—free rotation and rotation of the ion about a N–H···X— axis—deviate in many important respects from the experimental results.