Model E fits well to all temperatures and is similar to F where this is employed at high temperature. Thus, the EISF data is also consistent with a smooth evolution of the motion across the phase transition rather than a sudden change. While neither model directly matches the quasi-isotropic scrambling indicated by the 2 H NMR the trends we draw from these fits will likely remain correct even if the absolute values determined for f and m may be somewhat model dependent. An evaluation of f between the two compounds shows that the NH4 + cations in the Zn 2+ sample are much more dynamic than in the Mn 2+ analogue at comparable temperatures. While f is usually expected to increase linearly with temperature this is not observed here (see Table and Figure ). This is attributed to some fraction of the molecules moving faster than the ~1 ps rate that can be resolved by the dynamic range of the OSIRIS spectrometer. Deviation of the f-values from a linear trend with temperature occurs at about the same temperature that the level of the EISF rises, consistent with some fraction of the hydrogen atoms moving faster than the range of the spectrometer. Interestingly, the observed curving of the f parameter begins below the crystallographic phase transition temperature, suggesting that the fast motion is already occurring for a fraction of molecules in the ferroelectric phase. The variation in the Lorentzian width 𝛤 with Q is minimal (see Figure ), suggesting a lack of long-range diffusion, and consistent with localised rotational motion. The increase in 𝛤 with temperature indicates the motion increases in speed with temperature, while the increase in f indicates that more molecules are reorienting on the OSIRIS timescale. The mean residency times of the protons are calculated as described in the ESI and presented in Table ; again, these suggest the proton motion is faster in the Zn 2+ compound than the Mn 2+ analogue. With increasing temperature, the mean residency times reduce but without a discontinuity through the phase transition, consistent with the gradual increase in motion observed on the 2 H NMR timescale. Activation energies extracted from a plot of the evolution of ln 𝛤 against inverse temperature yield estimated values for the activation energy for the motion observed as ~10 kJ mol -1 for both compounds (see Figure ). This activation energy is averaged across both ferroelectric and paraelectric phases due to the limited data available in the high temperature phase and thus primarily suggests that the preexponent of the Zn 2+ phase is larger than for Mn 2+ , given other aspects of the QENS analysis suggests motion is faster in the former.