High-temperature phase equilibria in lithium-, sodium-, and thorium-fluoride mixtures

Abstract

The equilibrium phase diagram for the lithium-, sodium-, thorium-fluoride ternary system was determined experimentally using standard laboratory methods. Crystallization reactions in LiF NaFThF 4 mixtures were found to result in the occurrence of fourteen singular and invariant equilibrium points at the liquidus, including three eutectics at the LiFNaFThF 4 compositions, 30−56·5–13·5−13·5, 43·5−32·5−5−24 and 54·5−13·5−32 mole%, and at the temperatures of 505, 509 and 525°C. Peritectic invariant points were found at temperatures as high as 825°C. The high-temperature modification of a crystalline phase, long assumed to be NaThF 5, was identified as the hexagonal modification of the compound, 7NaF. 6ThF 4, space group, P3 c1. In this phase lithium was found to exchange for sodium, to a saturation limit where lithium substitutes at approximately two-thirds of the sodium sites. Ternary compound formation was absent in the system. The results of the present study were the basis for a revision of the NaFThF 4 phase diagram.