A neutron diffraction study of the crystal structure of (NH 4 ) 2 TeCl 6 is presented. The data support a previous contention that this crystal undergoes a ferrorotative structural phase transition at 85 K, from a cubic structure with space group O h 5 to a trigonal structure with space group C 3i 2 . The primary-order parameter is the rotation angle, Φ, of the TeCl 6 octahedra about a [111] axis of the cubic phase. The data indicate that the temperature dependence of Φ is non-mean-field like over an extended reduced temperature range.
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Armstrong et al. (1991) studied this question.