Neutron diffraction study of the structure of saturated sodium-ammonia solutions

The structure of saturated solutions of sodium in ammonia at 200 K has been measured by using the method of isotope substitution in neutron diffraction. Substitution of *N by N-15, combined with difference analysis, yields detailed structural information about the local environment around the host solvent. We find that the solution's are structured over both short- and intermediate-length scales. The solvent structure is significantly "stretched" due to the presence of excess valence electrons, which reduce the overall density. The data are consistent with five-fold ammoniation of sodium, with a nearest neighbour Na-N distance of 2.45(2) Angstrom. Contacts between solvated sodium ions then give rise to a first diffraction peak at 0.85(3) Angstrom(-1).