NEUTRON-DIFFRACTION STUDY OF THE STRUCTURE OF THE ANTIFLUORITE CRYSTAL (NH4)2TECL6

A neutron diffraction study of the crystal structure of (NH4)2TeCl6 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 C3i2. The primary-order parameter is the rotation angle, PHI, of the TeCl6 octahedra about a [111] axis of the cubic phase. The data indicate that the temperature dependence of PHI is non-mean-field like over an extended reduced temperature range.