Influence of the Anionic Composition on Molar Volumes of the Beryllium-Containing Molten Salt Mixtures

Abstract—Along with technological aspects, an analysis of the interaction of components of molten salt mixtures is significant for the development of models of the structures of molten salts. From this point of view, it seems interesting to study the influence of the anionic composition of electrolytes on a possible structure of the melts. The construction of the composition–property diagrams at various temperatures of the processes is an efficient way of physicochemical analysis of the interaction of components of salt compositions. The use of molar values for properties is the most correct in this respect. The data on the concentration dependence of molar volumes of molten salt mixtures of alkaline metal and beryllium chlorides and fluorides are presented. The molar volumes are calculated from the data on the density of the corresponding melts, which have been obtained by the authors and other researchers and published previously. Mixture compacting (decreasing molar volume) in the concentration range close to 30 mol % beryllium halide is observed in the systems containing alkaline metal and beryllium chlorides and in analogous fluoride systems. A decrease in the molar volume increases on going from lithium halide to cesium halides. Such a change in the properties is characteristic of the beryllium-containing electrolytes and is caused by the charge and small radius of the beryllium ion, which favors the intensification of its interaction with anions. The properties of the anions also affect the ionic interaction: the compacting in the chloride systems is somewhat higher than that in the corresponding fluoride systems. The concentration range of the maximum compacting of the molten salt mixtures corresponds to the stoichiometry of complex M₂BeHal₄, where M is alkaline metal, and Hal is Cl or F. The further increase in the beryllium halide concentration in the mixture results in melt structure loosening, which is reflected as a flat maximum on the molar volume isotherms. The concentration coordinate of the maximum is close to that corresponding to the stoichiometry of compound MBe₂Hal₅. Such congruently melted compounds are also present in the MCl–BeCl₂ and MF–BeF₂ systems. No similar points are observed in the MCl–BeF₂ systems on both the fusibility curves and molar volume isotherms.