Determination of Gibbs energy of formation of LiThF5, LiTh2F9, and LiTh4F17 in Li-Th-F system by using solid electrolyte galvanic cell

Abstract

The LiF-ThF4 system has been considered as reference fuel solvent for molten salt reactor. The high-temperature interaction of LiF-ThF4 leads to formation of a number of complex fluoride compounds. The study is focused on determination of experimental thermodynamic properties of the compounds in this system. The Gibbs energy of formation of LiThF5 (s), LiTh2F9 (s), and LiTh4F17 (s) has been measured by solid electrolyte galvanic cell method and the Gibbs energy of formation of these compounds as a function of temperature were expressed as $$ {\Delta}_{\mathrm{f}}G{}^{\circ}{}_{\mathrm{m}}\left({\mathrm{LiThF}}_5,\mathrm{s},T\right)\ \mathrm{kJ}\ {\mathrm{m}\mathrm{ol}}^{-1}=\left(-2738.7\pm 0.6\right)+\left(0.4050\kern0.5em \pm 0.0007\right)\cdotp \left(T/\mathrm{K}\right) $$ $$ {\Delta}_{\mathrm{f}}G{}^{\circ}{}_{\mathrm{m}}\left({\mathrm{LiTh}}_2{\mathrm{F}}_9,\mathrm{s},T\right)\ \mathrm{kJ}\ {\mathrm{m}\mathrm{ol}}^{-1}=\left(-4885.1\pm 0.8\right)+\left(0.7430\pm 0.0009\right)\cdotp \left(T/\mathrm{K}\right) $$ $$ {\Delta}_{\mathrm{f}}G{}^{\circ}{}_{\mathrm{m}}\left({\mathrm{LiTh}}_4{\mathrm{F}}_{17},\mathrm{s},T\right)\ \mathrm{kJ}\ {\mathrm{m}\mathrm{ol}}^{-1}=\left(-9126.5\pm 2.6\right)+\left(1.3801\pm 0.0034\right)\cdotp \left(T/\mathrm{K}\right) $$ The heat capacities of these compounds were also measured using differential scanning calorimeter. Using the experimental thermodynamic data, the thermodynamic tables of LiThF5 (s), LiTh2F9 (s), and LiTh4F17 (s) were constructed and the ternary phase diagram of Li-Th-F system was calculated to study the stability domain and coexisting phases of these compounds.