Long-wavelength lattice vibrations in ammonium sulfate are analysed group-theoretically, and symmetry coordinates of external modes belonging to the irreducible representation B 1 u , which is responsible for the phase transition from D 2 h to C 2 v , are obtained. They are composed of two groups; one is made of the purely translational and polar modes, and the other is made of the purely librational and nonpolar modes. Some peculiar properties of ammonium sulfate, including, especially, the very small value of the Curie-Weiss constant, are explained phenomenologically by a model in which a normal mode with a large component of a librational nonpolar mode admixed with a small contribution of a translational polar mode becomes soft.
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Sawada et al. (1973) studied this question.
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