Phase equilibria in the fused salt systems LiF-ThF4 and NaF-ThF4

Abstract

Detailed phase equilibrium diagrams are presented for the binary systems LiF-ThF4 and NaF-ThF4. Data for the determination of phase boundaries and solid state transitions were obtained from thermal analysis of heating and cooling curves and by quenching after equilibration. Phases were identified with the use of the X-ray diffractometer and the petrographic microscope. Four compounds, 3LiF·ThF4, 7LiF·6ThF4, LiF·2ThF4 and LiF·4ThF4, have been identified in the system LiF-ThF4. These melt incongruently with the exception of 3LiF·ThF4. Five compounds, 4NaF·ThF4, 2NaF·ThF4, 3NaF·2ThF4, NaF·ThF4 and NaF·2ThF4 have been identified in the system NaF-ThF4. These melt incongruently with the exception of 2NaF·ThF4 and 3NaF·2ThF4 which melt congruently. A metastable solid phase having the composition 7NaF·6ThF4 has been identified. The compounds 4NaF·ThF4 and 3NaF·2ThF4 do not exist as equilibrium phases at low temperatures. The compound 4NaF·ThF4 decomposes on cooling into NaF and 2NaF·ThF4 at 565°; the compound 3NaF·2ThF4 decomposes on cooling into 2NaF·ThF4 and NaF·ThF4 at 693°. Optical measurements and X-ray diffraction data have been obtained for each of the solid phases observed in the systems LiF-ThF4 and NaF-ThF4.