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.

中文翻译：

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通过赤黄铁矿矿物化合物 （NH4）2 [FeCl5（H2O）] 内的有序-无序转变探索分子运动


赤铁铁矿矿物类别令人着迷，尤其是化合物 （NH₄）₂ [FeCl₅（H₂O）]，一种众所周知的多铁性化合物。在 7.3 K 时，样品进入反铁磁状态，在 6.8 K 时变为共线磁性结构，磁电耦合导致多铁态。在较高的温度下，大约 79 K，样品被认为会经历与铵阳离子相关的有序-无序转变。这种转变的行为是推测性的，需要更多的方法来确认在较高温度下诱导的无序的确切性质。我们的工作利用了两种技术：准弹性中子散射 （QENS） 和弹性非相干中子散射 （EINS）。QENS 能够以空间分辨率研究任何分子运动的时间尺度，在这种情况下，我们以不同能量的切片来分析数据，作为非弹性固定窗口扫描，而 EINS 则专注于纯粹在弹性线上的任何非相干散射的空间性质。所提出的结果为从拟合到 Arrhenius 行为的有序-无序转变提供了一个能量和时间尺度，我们还能够分离出分子运动的空间性质，表明在 79 K 时，铵阳离子能够在圆周半径等于 N-H 键长的球体周围扩散。