Phase equilibria in the ThF₄-PuF₃ system were measured by differential scanning calorimetry. Samples were encapsulated to prevent leakage during the measurements and to prevent fluoride contamination of the instrument. Formation of intermediate compounds and solid solutions was investigated by X-ray diffraction. A thermodynamic model based on the CALPHAD approach was developed for the measured system. Good fit between measurements and phase equilibria calculated using the model was achieved.

A thermodynamic assessment of the KF-ThF₄ binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K₂ThF₆ has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: C_p,m^o(K₂ThF₆,cr,298.15K)=(193.2±3.9) J·K^-1·mol^-1 and S_m^o(K₂ThF₆,cr,298.15K)=(256.9±4.8) J·K^-1·mol^-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF₄ and NaF-KF-ThF₄ using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF₆ are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF₄ system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF₅ and NaF-KThF₅ pseudobinaries, with good agreement.

The optimized Gibbs energies for the second-nearest neighbours exchange reactions of the liquid solution were not reported correctly in Eqs. (17)–(19) in our recent work [1]. The corresponding equations should read: [Formula presented] [Formula presented] [Formula presented]

Na_3.16(2)U^V,VI _0.84(2)O₄ is obtained from the reaction of sodium with uranium dioxide under oxygen potential conditions typical of a sodium-cooled fast nuclear reactor. In the event of a breach of the steel cladding, it would be the dominant reaction product forming at the rim of the mixed (U,Pu)O₂ fuel pellets. High-temperature X-ray diffraction measurements show that a distortion of the uranium environment in Na_3.16(2)U^V,VI _0.84(2)O₄ results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Na_n-2Mⁿ⁺O_n-1 - including Na₃SbO₄ and Na₃TaO₄, whose crystal structures are reported for the first time - has allowed us to assess the role played in the lattice expansion by the Mⁿ⁺ cation radius and the Na/M ratio. On this basis, the thermomechanical behavior of the title compound is discussed, along with those of several related double oxides of sodium and actinide elements, surrogate elements, or fission products.

The physicochemical properties of the potassium neptunate K₂NpO₄ have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L₃ edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K₂NpO₄ have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.