The structure of lithium–ammonia and sodium–ammonia solutions by neutron diffraction

Abstract

The microscopic structures of lithium–ammonia and sodium–ammonia solutions have been measured by the technique of isotopic labeling in neutron diffraction, at and above the metal–nonmetal transition that occurs in the range 2–8 mole percent metal (MPM). Substitution of Li* by Li6 has been used to obtain the lithium-centered first-order difference function at 8 MPM and 230 K. This function shows us that the lithium cations are strongly solvated by 4 ammonia molecules. Substitution of N* by N15 has then been used to probe the nitrogen-centered structure in lithium–ammonia solutions at 4, 8, and 12.5 MPM and sodium–ammonia at 12.5 MPM. These functions give us new insight into both the disruption of hydrogen bonding as alkali metal is added to ammonia, and the solvation structure of the sodium cations. The former manifests itself through a progressive loss of the hydrogen-bonded N–D peak at ∼2.4 Å. The latter appears as an N–Na shoulder at ∼2.5 Å, and shows us that sodium is solvated by ∼5.5 ammonia molecules. In contrast to previous data for saturated (∼21 MPM) metal–ammonia solutions, we do not observe intermediate-range ordering of the solvated cations at the concentrations studied here.