The high-pressure behaviour of ammonium metal formates has been investigated using highpressure single-crystal X-ray diffraction on ammonium iron and nickel formates, and neutron powder diffraction on ammonium zinc formate in the pressure range of 0–2.3 GPa. A structural phase transition in the pressure range of 0.4–1.4 GPa, depending on the metal cation, is observed for all three ammonium metal formates. The hexagonal to monoclinic high-pressure transition gives rise to a characteristic sixfold twinning for the single-crystal diffraction data. Structure solution of the single-crystal data and refinement of the neutron powder diffraction characterise the pressureinduced distortions of the metal formate frameworks. The pressure dependence of the principal axes show significantly larger anisotropic compressibilities in the high-pressure monoclinic phase (K1 = 48 TPa^-1, K3 = -7 TPa^-1) compared to the ambient hexagonal phase (K1 = 16 TPa^-1, K3= -2 TPa^-1), and can be related to the symmetry-breaking distortions that cause deformation of the honeycomb motifs in the metal formate framework. While high-pressure Raman spectroscopy suggest that the ammonium cations remain dynamically disordered upon the phase transition, the pressure-induced distortion in the metal formate framework cause polar displacements in the ammonium cations. The magnitude of polarisation in the high-pressure phase of ammonium zinc formate was calculated based upon the offset of the ammonium cation relative to the anionic zinc formate framework and showed an enhanced polarisation of Ps ~ 4 μ C cm^-2 at the transition, which then decreases with increasing pressure.
