Abstract
Neutron-diffraction measurements of the local structure in deuterated ammonia have been conducted up to pressures of 2.1 GPa at ambient temperature. Total pair-distribution functions, determined by Fourier analysis of the static structure factor, are used to examine the structural changes from the first neighbors to extended ranges of similar to 30 angstrom in both the liquid and solid state. In the proton-disordered crystalline phase III, the first coordination shell is almost identical to that of the higher-pressure, ordered phase IV. The H-bond correlation is observed as a distinct shoulder at 2.5 angstrom. A similar local structure is seen in the liquid at a pressure just below freezing, and, in particular, a pronounced H-bond correlation is observed in the liquid across the pressure range studied. A substantial increase in the ordering length scale of the liquid is observed at high pressure with correlations extending to at least 25 angstrom compared to similar to 12 angstrom at ambient. The decay of the primary oscillations in the extended range is exponential and well described by a simple-liquid model, implying that, despite persistent H bonding, packing considerations become the dominant structural driver as density
