Pressure Dependence of Self-Diffusion in Lithium and Sodium

The activation volume ΔVₐ 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 (T₂^')d is proportional to the self-diffusion coefficient D. Measurements of (T₂^')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 cm³ and 9.6±{}0.5 cm³ 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 lnD is satisfactorily accounted for in terms of a single variable, the reduced melting temperature TₘT.