The NH4M(HCO2)3 series have channels of NH4 + cations which are likely screened from each other by the anionic framework. Similarly, within the channels the lack of hydrogen bonding between neighbouring NH4 + cations suggest they are relatively well isolated from each other, unlike in the related [NH3NH2]Mg(HCO2)3 where the larger molecular cations interact significantly. Thus the hydrogen-bonding network formed with the anionic framework likely plays a key role in the NH4 + cations ordering at the phase transition. The greater motion of NH4 + cations in the Zn 2+ material suggested by the QENS is likely responsible for its lower temperature phase transition compared to the Mn 2+ compound because less thermal energy is required for the transition to the high temperature phase to occur. Since the channels of the Zn 2+ phase are smaller than those in the Mn 2+ phase (c.f. 8.3946(8) Å to 8.4461 (11) Å based on the structures of Xu et al. ), it is unlikely that mechanical effects are primarily responsible for the greater motion of the Zn 2+ phase; Instead this likely involves subtle differences in the hydrogen bonding network between the NH4 + cations and the framework. The general trend of the structural phase transition decreasing in temperature with decreasing ionic size across the [NH4]M(HCO2)3 series might be linked to the motion of the NH4 + cations increasing with smaller size B-site cations. In this case, the motion of these A-site cations in the Ni 2+ framework may be sufficiently high to supress the phase transition completely. Both the 2 H SSNMR and QENS data suggest that the motion is similar, albeit slower, in the low-temperature and high-temperature phases. To investigate this further, we used nudged elastic band calculations to estimate the energy barrier to reorientation of a single N2 ammonium ion in the low-temperature phase under different motional models. While the diffraction data imply that the N2 site is somewhat more disordered than others, it is clear from the NMR spectra that all the ammonium sites are undergoing essentially the same motion. As shown in Fig. , we find a particularly low energy barrier of 15.2 kJ mol -1 for a tilt-jump process that switches between "up" and "down" states while also converting between axial and equatorial sites, thus avoiding a full 180º rotation. Alternative mechanisms for inverting the ammonium orientation involving breaking more hydrogen bonds, e.g. a C3-like motion about the c axis, and have higher energy barriers (see Videos S2-3 & Figure ). The energy barrier is similar in magnitude to the barrier to rotation in ammonium bromide, estimated at 14-18 kJ mol -1 ; 47 interestingly, this is also approximately constant across the III-II order-disorder transition at 235 K in that material. A pair of "tilt-jumps" will return the cations to their original, low-energy, orientation, but crucially will scramble the N-H orientations. Hence, we propose that this low-energy process is responsible for the dynamics observed in the NMR and QENS experiments. In the low-temperature phase, the "flipped" state is 12.3 kJ mol -1 above the groundstate structure and will have negligible equilibrium population (i.e. will not be observed in experiments).