STRUCTURE OF DEUTERATED AMMONIA-IV

The structure of the high-pressure phase IV of ammonia has been solved from neutron powder diffraction data from ND3, and preliminary studie s have been made of the structural pressure dependence to 9 GPa. The s tructure is orthorhombic, not hexagonal close packed (hcp) as previous ly thought. The nitrogen atoms have a pseudo-hcp arrangement, but the deuterium atoms do not and are ordered at all pressures studied. The i nternal molecular geometry is not significantly distorted from that fo und in phase I at ambient pressure, but the hydrogen bond geometry is more distorted and is unexpectedly complex.