Self-diffusion of lithium in molten LiBeF3 and Li2BeF4

Abstract

The self-diffusion coefficients of lithium in molten Li₂BeF₄ and LiBeF₃ have been measured by the capillary reservoir technique using ⁶Li as a tracer. The concentration profiles of ⁶Li in a capillary were measured by an ion micro-mass-analyser and the self-diffusion coefficients D were calculated. The results can be described using the Arrhenius equation, D= 9.27 × 10^–7 exp [(–32.5 ± 8.4)× 10³/RT] standard error: ± 1.1 × 10^–9(Li₂BeF₄) and D= 1.12 × 10^–6 exp [(–38.9 ± 12.5)× 10³/RT] standard error: ± 4.3 × 10^–10(LiBeF₃), where D is expressed in m² s^–1, R in J mol^–1 K^–1 and T in K. The activation energy for diffusion of lithium is much smaller than that of fluorine and slightly larger than that for electrical conductivity. These results suggest that the movement of fluorine does not involve electric current transfer and the mobility of lithium ions is much larger than that of the fluoroberyllate ion.

The self-diffusion coefficients of lithium in molten LiBeF₃ are smaller than those in molten Li₂BeF₄. However, the activation energies are similar in molten Li₂BeF₄ and LiBeF₃.