Interactions in the Li⁺,Na⁺,K⁺||F^–,Cl^– System and a 3D Model of the LiF–NaCl–KCl Stable Triangle

A 3D model of phase equilibria in the LiF–NaCl–KCl quasi-ternary system, which is the stable triangle of the Li⁺,Na⁺,K⁺||F^–_,Cl^– reciprocal four-component system, is designed. The 3D model is used to design polythermal and isothermal sections and crystallization polytherms for the first time. Two polythermal sections feature areas of sodium chloride- and potassium chloride-base terminal solid solutions. The fields of coexisting phases on the 620°C isothermal section are demarcated. The crystallization polytherm is represented by three phase fields, namely, lithium fluoride and NaCl- and KCl-base terminal solid solution fields. The stability of the LiF–NaCl–KCl triangle is confirmed by the thermodynamic calculations of interactions of the compounds involved in the LiCl–NaF–KF unstable triangle at several temperatures. The exothermicity of the exchange reaction is confirmed by the exotherm that appears on the DTA heating curve of a 50 mol % LiCl + 25 mol % NaF + 25 mol % KF powder mixture. The phases produced in the reaction, namely, LiF, NaCl- and KCl-base terminal solid solutions, are identified by X-ray powder diffraction. A low-melting region extending from the melting point of the ternary eutectic at 604°C to the 650°C isotherm is indicated on the concentration triangle, mixtures of which can be used as functional materials for various purposes.