The diffusion of ⁶Li and ⁷Li and the isotope effect in liquid lithium

Absolute values of the diffusivities of ⁶Li in isotopically enriched ⁶Li melts and of ⁷Li in natural liquid lithium have been measured by means of the pulsed-field-gradient nuclear magnetic resonance technique (PFG NMR) between 454 K and 580 K using sample sizes and geometries that did not require any corrections for systematic errors. The diffusivities of ⁶Li and ⁷Li in ⁶Li/⁷Li isotopic alloys decreased significantly with increasing ⁷Li concentration. This result (which is thought to be of general significance for self-diffusion in liquid metals) constitutes clear evidence for the collective nature of self-diffusion in liquid metals. The melting-point ratio of the self-diffusivities of ⁶Li and of ⁷Li, D⁶/D⁷=1.25, is significantly greater than the square root of the inverse mass ratio, (m⁷/m⁶)¹2 / equivalent to 1.08. This result is incompatible with any classical theory but agrees with a quantum mechanical calculation by Omini.