Neutron diffraction study of the structure of the antifluorite crystal (NH4)2PbCl6

A neutron diffraction study of the crystal structure of (NH4)2PbCl6 is presented. Data taken at 100 and 4.2 K support a previous contention that (NH4)2PbCl6 undergoes a ferrorotative structural phase transition at 80 K, from a cubic structure with space group to a trigonal structure with space group . The primary-order parameter is the rotation angle of the PbCl6 octahedron about a [111] axis of the cubic phase; is 3.1° at 4.2 K. A small lattice distortion and an (NH4)+ ion displacement along [111] also occur and represent secondary-order parameters.