Nuclear magnetic relaxation by self-diffusion in solid lithium:T ₁-frequency dependence

Abstract

We report on measurements of the⁷Li nuclear spin relaxation timeT ₁ in solid lithium as a function of temperature (−170°C≦T≦+180°C) and Larmor frequency (450kHz≦v _Li≦31.5 MHz). Using a relaxation model developed by Wolf and Cavelius and combining it with Seeger's diffusion formalism, the diffusion parameters for mono-and divacancy migration were evaluated by a least squares fit to the newly obtainedT ₁ data as well as to previousT _1ϱ measurements. The result for the self-diffusion coefficientD ^SD is given byD ^SD=D ₁₀·exp(−Q ₁/RT)·[1+D ₂₁·exp(−Q ₂₁/RT)], withD ₁₀=0.038 cm²s⁻¹,Q ₁=12.0 kcal mol⁻¹,D ₂₁=250,Q ₂₁=4 kcal mol⁻¹ andR=1.98₅·10⁻³ kcal mol⁻¹ degree⁻¹. Due to the flexibility of Seeger's formula, which contrasts with the standard Arrhenius interpretation of diffusion, discrepancies between earlier high- and low-frequency NMR investigations were eliminated. Furthermore, an excellent agreement with available results from tracer experiments was achieved by taking into account the theoretical predictions of the isotope effect and the vacancy correlation factor.