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.