Exploring Molecular\nMotion through an Order–Disorder\nTransition within the Erythrosiderite Mineral Compound, (NH<sub>4</sub>)<sub>2</sub> [FeCl<sub>5</sub>(H<sub>2</sub>O)]

The erythrosiderite\nmineral class is fascinating, in particular,\nthe compound (NH<sub>4</sub>)<sub>2</sub> [FeCl<sub>5</sub>(H<sub>2</sub>O)], a well-known multiferroic. At 7.3 K, the sample enters\nan antiferromagnetic state changing to a collinear magnetic structure\nat 6.8 K with magneto-electric coupling leading to a multiferroic\nstate. At higher temperatures, approximately 79 K, the sample is believed\nto undergo an order–disorder transition related to the ammonium\ncations. The behavior at this transition has been speculative, and\nmore is needed to confirm the exact nature of the induced disorder\nat higher temperatures. Our work utilizes two techniques: quasielastic\nneutron scattering (QENS) and elastic incoherent neutron scattering\n(EINS). QENS is able to study the time scales of any molecular motion\nwith spatial resolution, and in this case, we have taken slices at\ndifferent energies to analyze the data as inelastic fixed window scans,\nwhereas EINS focuses on the spatial nature of any incoherent scattering\npurely on the elastic line. The results presented provide an energy\nand time scale for the order–disorder transition from fits\nto Arrhenius behavior, and we have also been able to pick apart the\nspatial nature of the molecular motion, showing that at 79 K, the\nammonium cations are able to diffuse around a sphere with a circumradius\nequal to that of a N–H bond length.