Before discussing the results from this study, we briefly summarise what is known thus far about the [NH4]M(HCO2)3 family (where M = Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , and Zn 2+ ). These materials adopt a helical anionic M(HCO2)3 framework, which has a 4 •6 6 network topology, with hexagonal channels occupied by the NH4 + cations. In the high temperature phase, the NH4 + cations are disordered with P6322 hexagonal symmetry adopted. With the exception of Ni 2+ , these chiral hexagonal materials show a conventional paraelectric to ferroelectric transition at low temperature, via the ordering of the NH4 + cations, with a change in symmetry to the polar hexagonal P63 space group at low temperatures with the unit cell volume tripling; the a-and b-axis are √3 times their values in the high temperature P6322 phase. This manifests structurally as the single disordered ammonium environment in the paraelectric phase ordering to three crystallographically distinct ammonium environments in the low temperature ferroelectric phase. Transition temperatures decrease as the metal cation across the series gets smaller; for the smallest cation, Ni 2+ , the phase transition is supressed, with the structure remaining P6322 down to low temperature. Electron paramagnetic resonance measurements on Mn 2+ -doped [NH4]Zn(HCO2)3 indicate ferroelectric transition is continuous, with an order parameter changing in the manner expected for the transition to belong to a 2D universality class. The dielectric spectra for the chiral hexagonal systems show a peak at the transition with frequency dependence indicative of dielectric relaxation similar to the related [NH3NH2]M(HCO2)3 series; the change in the temperature at which the dielectric response peaks with the electrical field frequency is, however, much smaller in the NH4 + -containing phases. The frameworks with paramagnetic cations (Mn 2+ , Fe 2+ , Co 2+ and Ni 2+ ) adopt antiferromagnetic arrangements, with magnetic structures indicating an alternation of spin orientation across the series. All members of the chiral hexagonal [NH4]M(HCO2)3 series show anisotropic negative thermal expansion (NTE) with contraction along the c-axis, which is parallel to the channels in the structure, on heating paired with expansion within the ab plane. This behaviour is the opposite of the NTE exhibited by [NH3NH2]Mg(HCO2)3. The M = Mn 2+ and Co 2+ analogues have been shown to be proton conductors under humid conditions via a mechanism which involves the uptake of water molecules into their pores, suggesting that the pore space is not fully occupied by the NH4 + cations. Like the [NH3NH2]M(HCO2)3 series, the [NH4]M(HCO2)3 systems lack cation doping and have an explicit symmetry change at the temperature associated with their dielectric anomaly, which are not features usually associated with relaxors. Understanding the origins of this behaviour requires probing the structures and dynamics of the low and high-temperature phases beyond what can be gained from initial X-ray crystal structure determination. The simpler paraelectric phase could be well modelled by the previous powder neutron diffraction studies of the [NH4]M(HCO2)3 series, with the tetrahedral NH4 + cations disordered between "up" and "down" orientations, where the nitrogen atoms in the two different sites are displaced from each other. The details of the more complex ferroelectric phase, with its relatively large unit cell, could not, however, be reliably determined. The ferroelectric transition of [NH4]Zn(HCO2)3 has also been examined through Zn and N SSNMR, which highlighted the subtle distortions in the ZnO6 octahedral environments at low temperatures. The 14 N experiments distinguished two nitrogen environments in a 2:1 ratio identified as different hydrogen bonding environments, suggesting that two of the three crystallographic distinct sites must be chemically quite similar. Insights into the dynamics of the A-site cations, whose ordering is key to the transition, have however been limited by the lack of 1 H or 2 H SSNMR experiments.