Effect of Pressure on the Isotope Effect in Sodium Self-Diffusion

Abstract

The diffusion of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ in sodium has been measured over the temperature range - 78.5 to 97°C and the data do not fit a linear Arrhenius relation. The data indicate that at least two, and possibly three, jump mechanisms could be responsible for the diffusion behavior. The diffusion of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ in sodium has been measured as a function of pressure over the range from ambient pressure to 9500 kg ${\mathrm{cm}}^{-2\mathrm{}}$ at 14.8 and 91.3°C. The $\ln D$-vs-pressure results also show curvature and are in substantial agreement with the ambient-pressure data. Isotope-effect measurements have been made over the temperature-range - 25.0 to 97°C at ambient pressure. The isotope effect decreases as the temperature increases over the entire temperature range, with a more rapid decrease above 70°C. Isotope-effect measurements have been made at 7000 kg ${\mathrm{cm}}^{-2\mathrm{}}$ over a temperature range 14.8-117°C. The effect of pressure on the isotope effect is not large. The variation of isotope effect with temperature appears to be less at 7000 kg ${\mathrm{cm}}^{-2\mathrm{}}$. Collectively, the data are interpreted in terms of one, two, or three mechanisms of diffusion. It is not possible to determine unambiguously the mechanisms of diffusion in sodium. The evidence appears to favor a vacancy mechanism at low temperatures with an increasing contribution from divacancies as the melting temperature is approached.