Thermodynamic properties for Sm 2 (MoO 4 ) 3 determined by calorimetric measurement and re-evaluation of heat capacities for elemental molybdenum: standard entropy, Néel temperature, solubility product

The thermodynamic properties for Sm2(MoO4)3 were investigated. Sm2(MoO4)3 is the one of the yellow phase-related substances. Yellow phases are known as hygroscopic harmful phases in the nuclear fuel glasses. The standard molar entropy, $$\Delta_{0}^{T} S_{\text{m}}^{^\circ }$$ , at 298.15 K of Sm2(MoO4)3 was determined by measuring its isobaric heat capacities, $$C_{{{p},{\text{m}}}}^{^\circ }$$ , from 2 K via the fitting functions including the Debye–Einstein formula and electronic as well as magnetic terms. The Neel temperature, TN, was estimated by extrapolating the magnetic term in the fitting function. Its standard Gibbs energy of formation, $$\Delta_{\text{f}} G_{\text{m}}^{^\circ }$$ , was determined by combining the obtained $$\Delta_{0}^{T} S_{\text{m}}^{^\circ }$$ datum with the reference datum of the standard enthalpy of formation, $$\Delta_{\text{f}} H_{\text{m}}^{^\circ }$$ , where $$\Delta_{0}^{T} S_{\text{m}}^{^\circ }$$ at 298.15 K of the elemental molybdenum as the standard state was re-evaluated by re-reviewing the literature. The unknown standard Gibbs energy of solution, $$\Delta_{\text{sln}} G_{\text{m}}^{^\circ }$$ , and the solubility product, Ks, at 298.15 K for Sm2(MoO4)3 were predicted on the basis of the data obtained in this study and the reference datum of  MoO42−(aq) and Sm3+(aq). The thermodynamic values determined in the present study are as follows: $$\Delta_{0}^{T} S_{\text{m}}^{^\circ } \left( {{\text{Sm}}_{2} \left( {{\text{MoO}}_{4} } \right)_{3} \left( {\text{cr}} \right), \, 298.15 {\text{K}}} \right)/\left( {{\text{J K}}^{ - 1} {\text{mol}}^{ - 1} } \right) = 400.14 \pm 4.00,$$ $$\Delta_{\text{f}} G_{\text{m}}^{^\circ } \left( {{\text{Sm}}_{2} \left( {{\text{MoO}}_{4} } \right)_{3} \left( {\text{cr}} \right), \, 298.15 {\text{K}}} \right)/\left( {{\text{kJ}}\;{\text{mol}}^{ - 1} } \right) = - 4048.71 \pm 4.45,$$ $$\Delta_{\text{sln}} G_{\text{m}}^{^\circ } \left( {{\text{Sm}}_{ 2} \left( {{\text{MoO}}_{ 4} } \right)_{ 3} \left( {\text{cr}} \right),\; 2 9 8. 1 5 {\text{K}}} \right)/\left( {{\text{kJ}}\;\left( {{\text{mol of MoO}}_{ 4}^{ 2- } \left( {\text{aq}} \right)} \right)^{ - 1} } \right) = 6 8. 5 6\pm 1. 8 8 ,$$ $$K_{\text{s}} \left( {\frac{1}{3} {\text{Sm}}_{2} \left( {{\text{MoO}}_{4} } \right)_{3} \left( {\text{aq}} \right),\;298.15{\text{K}}} \right) = 9.75 \times 10^{ - 13} \pm 1.95 \times 10^{ - 13} ,$$ $$T_{\text{N}} /{\text{K}} = 1. 30 \pm 0. 30,$$ $$\Delta_{0}^{T} S_{\text{m}}^{^\circ } \left( {{\text{Mo}}\left( {\text{cr}} \right), \, 298.15\,{\text{K}}} \right)/\left( {{\text{JK}}^{ - 1} {\text{mol}}^{ - 1} } \right) = 28.573 \pm 0.086.$$ These data are expected to be useful for geo-chemical simulation of diffusion of radioactive elements through underground water.