NMR Measurements of Self-Diffusion in Lithium-Ammonia and Sodium-Ammonia Solutions

Abstract

Self-diffusion coefficients of $^{7}\mathrm{Li}$, $^{23}\mathrm{Na}$, and $^{1}\mathrm{H}$ have been measured in 1-20-MPM (100\ifmmode\times\else\texttimes\fi{}moles metal/total of moles metal and moles ammonia) solutions of lithium ammonia at 223 and 233 K and in 2-15.5-MPM solutions of sodium ammonia at 233 K by the NMR spin-echo pulsed-magnetic-field-gradient technique. In addition, the temperature dependences of the self-diffusion coefficients were measured for seven representative concentrations of lithium ammonia below 240 K. The data indicate that the lithium and sodium ions are solvated by four ammonia molecules over the time scale of molecular diffusion. The concentration dependence of the self-diffusion coefficients of the metal-ion complexes and of the free-ammonia molecules is found to be consistent, evaluated by the Stokes-Einstein relation, with the available viscosity data. The melting-point self-diffusion coefficient and the temperature dependence of 20-MPM lithium ammonia are fitted to the Ascarelli-Paskin and Cohen-Turnbull diffusion models for a reasonable choice of packing fraction.