Phase Equilibria in the System NaF/BeF₂/ThF₄

The equilibrium phase diagram of the condensed system NaF/BeF₂/ThF₄ is presented. Liquid/solid phase transitions were found in the temperature range from ∼300 to 1100 C. The phase diagram of the system is based on data obtained from thermal analysis and thermal-gradient-quenching experiments; phase identifications were made employing the polarizing light microscope and the x-ray diffractometer. Thirteen crystalline phases are precipitated as primary phases from molten NaF/BeF₂/ThF₄ solutions: 4NaF-ThF₄, 7NaF·2ThF₄, 2NaF·ThF₄, 3NaF·2ThF₄, α- and β -NaF·ThF₄, NaF-2ThF₄, 2NaF·BeF₂, NaF·BeF₂, NaF·BeF₂, ·3ThF₄, NaF, BeF₂, and ThF₄ solid solution. The character, location, and temperature of the fourteen invariant and singular points in the system were established. The ternary system is comprised of two subsystems, NaF/2NaF·ThF₄/2NaF·BeF₂ and 2NaF·ThF₄/ThF₄/BeF₂/2NaF·BeF₂, separated by the quasibinary section, 2NaF·BeF₂/2NaF·ThF₄. The lowest liquidus temperatures in the respective subsystems were found at the eutectics, 72NaF/22BeF₂/6ThF₄ (mole %), m.p. 509 C, and at 55NaF/43BeF₂/2ThF₄ (mole %), m.p. 320 C.