We propose a high-pressure neutron powder diffraction study of the crystal structure of deuterated ammonium iodate, ND4IO3, which is known to undergo a ferroelectric phase transition at 368 K at ambient pressure. In addition to the covalent bonds within the IO3- and ND4+ ions, the secondary bonds consists of short O...I halogen bonds and N-H¿O hydrogen bonds. The proposed two-part experiment will collect high-quality neutron powder diffraction data at room-temperature up to 8 GPa and will also investigate the ferroelectric phase transition behaviour at 395 K for pressures up to 5 GPa. The main aims of this neutron study will be to: i) determine the structure pressure dependence of ammonium iodate at room temperature, ii) structurally characterise the ferroelectric transition region of p-T space, and iii) determine the structures of any previously unknown phases which we may encounter.