Self-diffusion mechanism in solid sodium by NMR

The self-diffusion mechanism in solid sodium has been determined by means of NMR. For that purpose, the spin-lattice relaxation times $T_1$ and $T_{1\rho }$ and the Knight shift $K$ of ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ in ultrapure sodium have been measured as a function of temperature in the range of $10<~T<~371$ K (melting point). At all temperatures, the Zeeman relaxation time $T_1$ is determined by conduction electrons leading to a volume-corrected Korringa relation of $T_{1}T=4.68\mathrm{}\pm 0.13$ K s. In the temperature range 150-280 K, an additional contribution to the rotating-frame relaxation rate, $T_{1\rho }^{-1\mathrm{}}$, arising from fluctuations in the nuclear dipole interaction due to atomic self-diffusion is observed. By comparing the motion-induced part of the relaxation rate with the tracer measurements of Mundy, the correlation factor and thus the self-diffusion mechanism in sodium is determined. The following three diffusion mechanisms have been assumed to interpret the observed curvature in the Arrhenius plot: (1) a combination of mono- and divacancies; (2) monovacancies alone with a temperature-dependent pre-exponential factor and activation enthalpy; and (3) monovacancies with the possibility of vacancy double jumps. It is found that the temperature dependence of the measured correlation factor is consistent with the simultaneous migration of mono- and divacancies while the other two mechanisms can be ruled out as solely responsible for the observed effects.