Pressure Dependence of Self-Diffusion ofNa22in NaCl

The effect of hydrostatic pressure of 6.2 kbar on the rate of diffusion of ${\mathrm{Na}}^{22}$ in NaCl has been studied at temperatures of 663 and 731°C using a tracer-sectioning technique. The cation-diffusion process is characterized by a pressure-dependent activation volume varying between about 37 ${\mathrm{cm}}^3$/mole at low pressures to about 25 ${\mathrm{cm}}^3$/mole at high pressures. If possible divacancy contributions can be neglected, this result implies a volume of formation of Schottky defects of about 2.2 molar volumes. Diffusion profiles at high pressure show a marked deviation from the expected Gaussian dependence. This effect is shown to be due to the presence of trace amounts of polyvalent impurity co-diffusing with the "carrier-free" ${\mathrm{Na}}^{22}$ tracer.