Self-diffusion in solid lithium probed by spin-lattice relaxation of⁸Li nuclei

Abstract

Self-diffusion in solid Li was studied via the spin-lattice relaxation of polarised radioactive 8Li nuclei using their asymmetric beta -decay radiation. The diffusion-induced spin-lattice relaxation rate 1/T1diff was measured as a function of temperature from the melting point Tm=454K down to about 1/2Tm and as a function of the magnetic field from 8 to 700 mT. 1/T1diff can be explained assuming the nuclear dipole-dipole interaction alone. The data are analysed in the frame of the encounter model of correlated self-diffusion. The macroscopic self-diffusion coefficient DSD is determined over almost seven decades. A deviation from Arrhenius behaviour at elevated temperatures is interpreted in terms of a combined monovacancy/divacancy (1V/2V) mechanism. The correlation factor obtained from DSD and tracer self-diffusion coefficients DT, measured by mass spectroscopy, has a temperature dependence which is consistent with the 1V/2V model. The question of the possible existence of a non-classical isotope effect cannot be settled by a comparison of the present DSD values with those known from conventional nuclear magnetic resonance on 7Li.