Thermodynamic stability study of Li3ThF7 and Li7Th6F31 in Li-Th-F system

Abstract

The LiF-ThF4 binary salt mixture is considered as reference solvent for Molten Salt Reactor to dissolve the fissile fluoride (UF4). LiF and ThF4, interacts at elevated temperature, leads to formation of a number of complex fluoride compounds; LiThF5(s), LiTh2F9 (s), LiTh4F17 (s), Li3ThF7 (s) and Li7Th6F31 (s). The present study is focused on determination of experimental thermodynamic properties of Li3ThF7(s) and Li7Th6F31(s) in LiF-ThF4 system. The Gibbs energy of formation of Li3ThF7(s) and Li7Th6F31(s) has been measured by using following Galvanic cell setup, (−) Pt, ThOF2 (s) + Li3ThF7 (s) + LiF (s)|CaF2 (s)|NiO (s) + NiF2(s), Pt (+) [For Li3ThF7(s) ] (−) Pt, Li7Th6F31(s) + ThOF2 (s) + Li3ThF7 (s) + LiF (s)|CaF2 (s)|NiO (s) + NiF2(s), Pt (+) [For Li7Th6F31(s)] The ΔfG°m of these compounds as a function of temperature were expressed as ΔfG°m(Li3ThF7, s, T) kJ.mol−1 = (−3948.0 ± 0.9) + (0.5933 ± 0.0009) · (T/K) ΔfG°m(Li7Th6F31, s, T) kJ.mol−1 = (−17072.1 ± 1.6) + (2.6007 ± 0.0003) · (T/K) The heat capacities of these compounds were also measured using differential scanning calorimeter. Using the experimental thermodynamic data, the thermodynamic tables of Li3ThF7(s) and Li7Th6F31(s) were constructed and chemical potential diagram of Li-Th-F-O system and fluorine potential diagram of Li-Th-F2 system was calculated to study the stability domain and coexisting phases of these compounds.