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 Na3ClMoxW1 – xO4 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 Na3ClMoxW1 – xO4 CSS field.