Motion of the Ammonium Ion in Lattices of the NaCl Type

Abstract
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.

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