Self-diffusion Coefficients of Fluorine in Molten LiF-BeF₂ Mixture System

Abstract

The self-diffusion coefficients and their temperature dependences of fluorine in molten Li₂BeF₄ and LiBeF₃ have been investigated by the capillary reservior technique using a tracer Fluorine-18. The observed diffusion coefficients of fluorine D_F (cm² sec⁻¹) are written in the form of D_F=D₀exp[−E_D(F)⁄RT], where E_D(F) and D₀ are calculated by the least squares method from all experimental data. Those of molten Li₂BeF₄ and LiBeF₃ are as follows:
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\ oindentThese results for fluorine diffusion are considerably difficult to explain by a simple model because of the unusual combination of the relatively large diffusion coefficients and a high activation energy and cannot be explained solely by mass transfer due to migration of large fluoroberyllate anions. One of the possible explanations for these results could be the exchange of fluorine atom between neighboring beryllate units including the rotation of beryllate anions. Another possible explanation could be the diffusion mechanism of a neutral pair of, for example, LiF. Both diffusion mechanisms give high activation energy and are consistent with the observed results. The close analogy between the diffusion behavior of fluorine in molten LiF-BeF₂ mixture and that of oxygen in molten CaO-SiO₂-Al₂O₃ slag was also discussed.