Determination of the thermodynamic activities of LiF and ThF4 in the LixTh1−xF4−3x liquid solution by Knudsen effusion mass spectrometry

Abstract

Knudsen effusion mass spectrometry (KEMS) has been used to investigate the vapour pressure over the molten LiF–ThF₄ salt and determine the thermodynamic activity of LiF and ThF₄ in the liquid solution. As part of the study, the vaporization of pure LiF and pure ThF₄ was examined and the results were compared with the literature values finding a good agreement. Next, the vapour pressure of the Li_xTh_1−xF_4−3x liquid solution was investigated by measuring four samples having different compositions (X_LiF ∼ 0.2, 0.4, 0.6, 0.8 mol%). In order to determine the thermodynamic activities, the vapour pressure of LiF and ThF₄ species over the liquid solution, as calculated from our results, were compared with the vapour pressure over the pure LiF(l) and pure ThF₄(l) systems. A strong deviation from the Raoult's law was observed, more evident in case of LiF species, in agreement with the predictions by our thermodynamic model.