The structure of saturated lithium– and potassium–ammonia solutions as studied by using neutron diffraction

The technique of isotopic substitution in neutron diffraction has been used to measure the structure of saturated solutions of lithium and potassium in ammonia. Isotopic substitution of ^*N by ${}^{15}\mathrm{N},$ combined with difference analysis, shows that ${\mathrm{K}}^{+}$ acts as a “structure breaking” ion while ${\mathrm{Li}}^{+}$ acts as a “structure making” ion. As a consequence, it is only for the latter that we observe intermediate range order, in the form of a pre-peak at ∼1 Å⁻¹ in the k-space data. From our analysis, we also find that lithium is tetrahedrally coordinated by ammonia molecules, at both 230 and 100 K. Potassium, on the other hand, is octahedrally coordinated at 160 K.