Pressure Dependence of Self-Diffusion in Lithium and Sodium

The activation volume $\Delta V_a$ for self-diffusion in lithium and sodium has been measured using nuclear magnetic resonance spin-echo techniques. In the region where the resonance line shape is narrowed by diffusion, the nuclear dipole-dipole phase memory time ${\left({T_2}^{\prime }\right)}_d$ is proportional to the self-diffusion coefficient $D$. Measurements of ${\left({T_2}^{\prime }\right)}_d$ for lithium as a function of pressure at constant temperature have been made at five temperatures between 35° and 80°C, and at pressures to 7000 atm. The phase memory time has been measured for sodium at three temperatures near -45°C at pressures to 3500 atm. Molar activation volumes of 3.6±0.3 ${\mathrm{cm}}^3$ and 9.6±0.5 ${\mathrm{cm}}^3$ have been obtained for lithium and sodium, respectively. These values are, respectively, 28 and 41% of the molar volumes of lithium and sodium, indicating that there occurs considerable relaxation inward on a vacancy by the nearest- and perhaps next-nearest-neighbor atoms of the vacancy. In addition, the experimental values are found to be in excellent agreement with a dynamical theory of diffusion which predicts that corresponding activation parameters for diffusion and melting are proportional. In both metals, the behavior of $\ln D$ is satisfactorily accounted for in terms of a single variable, the reduced melting temperature $\frac{T_m}{T}$.