Structure of Deuterated Ammonia IV

The structure of the high-pressure phase IV of ammonia has been solved from neutron powder diffraction data from N${\mathrm{D}}_3$, and preliminary studies have been made of the structural pressure dependence to 9 GPa. The structure is orthorhombic, not hexagonal close packed (hcp) as previously thought. The nitrogen atoms have a pseudo-hcp arrangement, but the deuterium atoms do not and are ordered at all pressures studied. The internal molecular geometry is not significantly distorted from that found in phase I at ambient pressure, but the hydrogen bond geometry is more distorted and is unexpectedly complex.