Phase Equilibria in the System NaF‐ThF4‐UF4

Abstract

Phase equilibria in the condensed system NaF‐ThF4‐UF4 were determined by cooling‐curve and thermal‐gradient quenching experiments from the liquidus to ∼450°C. A phase diagram of the ternary system was constructed, showing three singular points and eleven invariant points, including two eutectics, occurring in association with the primary phases of ten solid solutions and two pure solids. The eutectic mixture having the composition 75.5NaF‐10.5ThF4‐14UF4 (mole %) melts at 575°C, the lowest liquidus temperature in the system. Except for NaF, NaF‐ThF4, and ThF4‐UF4 solid solutions, phases crystallizing from molten NaF‐ThF4‐UF4 mixtures are solid solutions formed by interchange of U4+ and Th4+ ions in NaF‐ThF4 and NaF‐UF4 compounds. No ternary compounds were found. Phase diagrams of the systems NaF‐ThF4 and NaF‐UF4, revised in the course of this study to include the newly identified phases, 7NaF.2ThF4 and 7NaF.2UF4, are presented. X‐ray diffraction and optical data are given for the pure crystal phases, 4NaF.ThF4, 7NaF.2ThF4, 7NaF.2UF4, 3NaF.UF4, and for 3NaF.2ThF4‐5NaF.3UF4 solid solutions. The system is unique among the fluoride systems reported in containing so wide a variety of substitutional solid solutions and in containing a continuous solid solution formed of binary compound pairs 3NaF.2ThF4 and 5NaF.3UF4 which are isomorphous but of different stoichiometry.