Evaluation of the mean squared displacement (MSD) parameter with temperature, presented in Figure , indicates proton motion becomes sufficiently fast to be detected above 100 K for each system; this is a similar temperature to that at which CH3 rotations often activate, and is attributed here to the onset of NH3 rotations. The motion then increases rapidly for each system as it approaches the crystallographic phase transition. This observation is typical, as locally dynamics will speed up as the transition temperature approaches on heating, and the local proton motion will be detected before the bulk phase change. A phase transition will typically involve a sudden increase in the gradient of this curve, as softer modes become activated and the Debye temperature decreases. The MSD increases more gradually at higher temperatures above the phase transition, the decrease in gradient for the Mn 2+ material occurs immediately following the crystallographically observed phase transition temperature, whilst in the Zn 2+ material this occurs about 20 K above the phase change (see Figure ). The higher MSD values in the high temperature phase suggests the motion involves a lower energy barrier in the lower temperature phase, consistent with the significant decrease in activation energy for motion above the phase transition determined by the 2 H SSNMR. The elastic incoherent structure factor (EISF), which is the proportion of scattering that is elastic as a function of scattering vector Q (see Equation ), can be used to describe the geometry of the rotational diffusion of the NH4 + cations about their given centre of mass. As is commonly done to determine the "best" EISF model, possible models for the [NH4]M(HCO2)3 compounds are presented in Table with representative fits for each model at low temperature and high temperature for M = Zn 2+ (see Figures ).