Exploring Molecular Motion through an Order–Disorder Transition within the Erythrosiderite Mineral Compound, (NH₄)₂ [FeCl₅(H₂O)]

The erythrosiderite mineral class is fascinating, in particular, the compound (NH4)2 [FeCl5(H2O)], a well-known multiferroic. At 7.3 K, the sample enters an antiferromagnetic state changing to a collinear magnetic structure at 6.8 K with magneto-electric coupling leading to a multiferroic state. At higher temperatures, approximately 79 K, the sample is believed to undergo an order–disorder transition related to the ammonium cations. The behavior at this transition has been speculative, and more is needed to confirm the exact nature of the induced disorder at higher temperatures. Our work utilizes two techniques: quasielastic neutron scattering (QENS) and elastic incoherent neutron scattering (EINS). QENS is able to study the time scales of any molecular motion with spatial resolution, and in this case, we have taken slices at different energies to analyze the data as inelastic fixed window scans, whereas EINS focuses on the spatial nature of any incoherent scattering purely on the elastic line. The results presented provide an energy and time scale for the order–disorder transition from fits to Arrhenius behavior, and we have also been able to pick apart the spatial nature of the molecular motion, showing that at 79 K, the ammonium cations are able to diffuse around a sphere with a circumradius equal to that of a N–H bond length.