Phase Equilibria in the NaF–Na₃ClMoO₄–Na₃ClWO₄ Stable Triangle of the NaF–NaCl–Na₂MoO₄–Na₂WO₄ Four-Component System

Abstract

Theoretical and experimental studies of the NaF–Na₃ClMoO₄–Na₃ClWO₄ stable secant triangle of the NaF–NaCl–Na₂MoO₄–Na₂WO₄ four-component system have been carried out. The review of the literature concerned with the boundary elements of the title system has shown two options for the numbers and compositions of crystallizing phase: in case the solid solutions are stable, two solid phases would be formed, and in case of their decay, three solid phases would. The experimental tools used to study the NaF–Na₃ClMoO₄–Na₃ClWO₄ system were differential thermal analysis (DTA) and X-ray powder diffraction (XRD). The melting point and composition of the mixture that correspond to point d, which lies on monovariant curve e₁e₂ connecting binary eutectics, have been determined. X-ray powder diffraction showed two solid phases, namely, NaF and Na₃ClMo_xW_{1 – x}O₄ continuous solid solutions (CSS), in an alloy of point d composition. The nonexistence of invariant points in the title system has been shown. The CSS based on Na₃ClMoO₄ and Na₃ClWO₄ are stable. The projection of the phase assemblage of the system on the composition triangle is represented by two crystallization fields of the terminal components: a sodium fluoride field, which is the dominant one, and a Na₃ClMo_xW_{1 – x}O₄ CSS field.