Neutron-Diffraction Study of the Structures of Ferroelectric and Paraelectric Ammonium Sulfate

Three-dimensional neutron-diffraction data have been collected for both the room-temperature (paraelectric) and the low-temperature (ferroelectric) phases of ammonium sulfate. Both phases are ordered. The transition, occurring at 223°K and involving a change from Space Group Pnam at room temperature to Space Group Pna21 in the ferroelectric phase, results in somewhat stronger hydrogen bonds in the ferroelectric phase, although there is no dramatic change in the barrier to rotation of the ammonium ions as a result of the transition. In both phases the two crystallographically independent ammonium ions have quite similar rotational barriers and hydrogen-bond strengths. The transition also results in less distorted ammonium ions in the ferroelectric phase. At room temperature, there are only two hydrogen bonds with O···H distances less than 2.0 Å. In the ferroelectric phase there are six. The mean distance is decreased by 0.1 Å in the ferroelectric phase, although the range of distances remains the same. The principal effect is thus a strengthening of the hydrogen bonds of intermediate length; this is accomplished by a reorientation of the ammonium groups. The shortest bonds at room temperature are not necessarily the shortest at low temperature.