Since the structure of the paraelectric phase has been previously established by powder neutron diffraction, the single crystal neutron diffraction studies presented here focus on the low temperature structure, in which the NH4 + cations are ordered. The symmetry of the M = Mn 2+ and Zn 2+ phases below the transition temperature, 254 K and 192 K respectively, is confirmed as P63, with three crystallographically unique cation environments that sit within the hexagonal channels of the M(HCO2)3 framework. The elusive proton positions of the NH4 + cations were previously determined geometrically by X-ray diffraction using laboratory sources, with low certainty in their positions both because X-rays are relatively insensitive to light atoms and because hydrogen's single electron is located in the covalent bond. The positions of the molecular cations in the polar phase determined by neutron single crystal diffraction are shown in Figure and S3, labelled according to the nitrogen environments (see Table for crystallographic details); we note this technique is particularly sensitive to the N position due to the large neutron scattering length of this element. The N1 cations, which occupy the channels at the vertices of the unit cell, are well ordered along the polar c-axis, which corresponds with the sixfold screw axis. The two central channels of the cell contain alternating N2 and N3 ammonium environments centred on a threefold rotation axis. The two resulting chains of ammonium cations are oriented antiparallel to each other along c. This confirms the arrangement modelled by the single crystal X-ray diffraction (SCXRD) experiment. Although the cations appear well ordered in both Zn 2+ and Mn 2+ structures, there are weak regions of negative scattering density around the N2 ammonium environment. The position of the negative nucleic scattering for the Mn 2+ compound suggests rotational disorder of the NH4 + protons, primarily affecting the basal sites (see Figure ). On the other hand, for the Zn 2+ compound the disorder appears to be associated with tilting of the apical proton off the rotation axis. The N2 cations account for 1/3 of the NH4 + groups in the structure and further details of the modelling of this disorder are discussed in the ESI. The N2 environment in the Zn 2+ analogue has been previously been suggested to be likely responsible for the second N NMR environment, due to differences in its hydrogen bonding network compared to the other two sites which may be linked to the disorder we observe here. The hydrogen-bonding interactions in both compounds occur primarily with the framework, suggesting these host-guest interactions are likely responsible for cation ordering (see Tables for bond distances and angles). In contrast, the neighbouring cations within each channel are well separated, with the nitrogen atoms in neighbouring cations usually over 4 Å apart. This suggests they may not significantly interact with each other, as is also highlighted from Hirshfeld analysis (see ESI, including Figure ). This lack of interactions between NH4 + cations likely plays a role in water molecules being able to insert between ammonium sites in the channels at high humidity. We have previously reported similar shielding of guest disorder by the metal formate framework in the related framework material dimethylammonium manganese(II) formate. The cell dipole was previously established to arise from an asymmetry of charge distribution in the unit cell caused by the displacement of the NH4 + cations in the channels. The ammonium displacements calculated from single-crystal neutron diffraction, which has greater sensitivity to the N positions, are shown in Table . These were determined by measuring displacement relative to the metal cation, which are at equivalent positions in c in the paraelectric phases, as previously used by Xu et al. The overall arrangement has two of the three distinct ammonium cations displaced in one direction. The unit cell polarisation is therefore ferrielectric, with oppositely oriented polarisations that do not cancel completely and thus still give an overall cell polarisation. The N atoms displaced in the same direction are N1 and N2 in the Mn 2+ material and N2 and N3 in the Zn 2+ material. Interestingly, and consistently with the previous work on this system, this indicates that in the Mn 2+ material, one of the two distinct NH4 + cations sharing a channel moves in the same direction as the cations in the other channel; but in the Zn 2+ material, the NH4 + cations in the same channel all move in the same direction, opposite to those in the other channels. This is consistent with the NH4 + cations in each channel having limited interactions with each other, with the positions they adopt at low temperature arising from optimising their interactions with the framework. The absolute values of the displacements of the N1 and N2 nitrogen atoms in the Mn 2+ material are also more varied from those determined previously by X-ray studies at 110 K, suggesting these may evolve further as the material is cooled to 22 K, as used here. The static bandshapes across all temperatures, Figure , are very narrow, indicating significant motion, consistent with random re-orientation of NH4 + cations across the full range. The bandshapes become narrower at higher temperatures (i.e exhibit a smaller averaged CQ), which implies that motion is slowly approaching quasi-isotropic re-orientation. Interestingly, this increased symmetry is independent of the phase transitions, with a smooth evolution of CQ observed with increasing temperature. The motion at low temperature is clearly not a single-axis threefold rotation (C3) jump, which would lead to two distinct spectra: a broad spectrum ~200 kHz wide and a narrow spectrum ~67 = 200/3 kHz wide. Instead, even at low temperature, all the N-H bond vectors must be in exchange at a rate faster than 61 MHz (based on the T1 data). The residual lineshape after averaging corresponds to an axially symmetric coupling tensor, consistent with the different interactions of the basal versus apical positions with the framework, leading to a non-zero CQ after motional averaging. Note that NMR cannot determine the exact motional mechanism for the "scrambling" of N-H orientations. 2 H T1 relaxation measurements showed two regions with a distinct change of gradient either side of ~200 K, corresponding to the crystallographic phase transition at 192 K (see Figure ). The activation energy for the quasi-isotropic exchange is 20.6 ± 0.7 kJ mol-1 from the low temperature region. The lower activation energy of 7.9 ± 0.1 kJ mol -1 at higher temperature is consistent with motion in the high-temperature paraelectric phase being more energetically favourable.