Chemical Interaction in the Li⁺,Na⁺,K⁺||F^–,Br^– System and Identification of Low-Melting-Point Phase Fields in a 3D Model of the Stable Composition Triangle LiF–NaF–KBr

Alkali halides have found wide application as thermal energy storage materials, electrolytes for electrochemical cells, and solvents of inorganic substances. Modeling with the use of data for bounding systems is important for constructing phase diagrams of ternary and multicomponent systems. Using three-dimensional vector graphics software, we have constructed a 3D model of equilibrium phase states in the pseudoternary system LiF–NaF–KBr, which is a stable composition triangle in the quaternary reciprocal system Li⁺,Na⁺,K⁺||F^–,Br^–. Based on the 3D model, we have constructed for the first time polythermal and crystallization polytherm. For two polythermal sections, we have demonstrated the presence of regions of limited sodium fluoride-based solid solutions and liquid miscibility gaps and identified the phase crystallization sequence. In the 620°C isothermal section, we have delineated liquid-phase and two- and three-phase fields. The polytherm comprises three crystallization fields: terminal solid solutions based on sodium fluoride, potassium bromide, and lithium fluoride, in which a liquid miscibility gap has been delineated. The stability of the LiF–NaF–KBr composition triangle has been confirmed by thermodynamic calculations for several temperatures of interaction in mixtures of substances in the unstable composition triangle LiBr–NaF–KF. The crystallization polytherm allows one to choose mixtures in the temperature ranges 625–650 and 625–700°C suitable for practical application as melting electrolytes of intermediate-temperature electrochemical cells and molten solvents of inorganic substances.