The structural, thermochemical, and thermophysical properties of the NaF-ThF4 fuel system were studied with experimental methods and molecular dynamics (MD) simulations. Equilibrium MD (EMD) simulations using the polarizable ion model were performed to calculate the density, molar volume, thermal expansion, mixing enthalpy, heat capacity, and distribution of [ThFn]m- complexes in the (Na,Th)Fx melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF4 and 85-95 mol % ThF4 regions of the NaF-ThF4 phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF4) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na2ThF6 and NaTh2F9 compounds were synthesized and subsequently analyzed with the use of X-ray diffraction. The heat capacities of both compounds were measured in the temperature ranges (2-271 K) and (2-294 K), respectively, by thermal relaxation calorimetry. Finally, a CALPHAD model coupling the structural and thermodynamic data was developed using both EMD and experimental data as input and a quasichemical formalism in the quadruplet approximation. Here, 7- and 8-coordinated Th4+ cations were introduced on the cationic sublattice alongside a 13-coordinated dimeric species to reproduce the chemical speciation, as calculated by EMD simulations and to provide a physical description of the melt.

The existence of a novel double-molybdate phase with a palmierite-type structure, Cs2Ba(MoO4)2, is revealed in this work, and its structural properties at room temperature have been characterized in detail using X-ray and neutron diffraction measurements. In addition, its thermal stability and thermal expansion are investigated in the temperature range 298-673 K using high-temperature X-ray diffraction, leading to the volumetric thermal expansion coefficient αV ≈ 43.0 × 10-6 K-1. The compound's standard enthalpy of formation at 298.15 K has been obtained using solution calorimetry, which yielded ΔfHm°(Cs2Ba(MoO4)2, cr, 298.15 K) = -3066.6 ± 3.1 kJ· mol-1, and its standard entropy at 298.15 K has been derived from low-temperature (2.1-294.3 K) thermal-relaxation calorimetry as Sm°(Cs2Ba(MoO4)2, cr, 298.15 K) = 381.2 ± 11.8 J K-1 mol-1.

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 low-temperature heat capacity of (U_1−yTh_y)O₂ and ²³⁸Pu-doped UO₂ samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO₂ in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the ²³⁸Pu-doped UO₂ around 25 K. The magnetic behaviour of UO₂ exposed to the high alpha dose from the ²³⁸Pu isotope was studied over time and it was found that 1.6% ²³⁸Pu affects the magnetic transition substantially, even after short period of time after annealing. In both systems the antiferromagnetic transition changes intensity, shape and Néel temperature with increasing Th-content and radiation dose, respectively, related to the increasing disorder on the crystal lattice resulting from substitution and defect creation.

The low-temperature heat capacity of (U _1-y,Am_y)O _2−x solid solution with y = 0.0811 and 0.2005 and x = 0.01–0.03 was determined from a minimum of 12.52 K up to 297.1 K and from 9.77 K up to 302.3 K, respectively, using hybrid adiabatic relaxation calorimtry. The low temperature heat capacity results of the investigated system revealed the absence of the magnetic transition specific for UO₂ in the temperature region of 30 K. Since there are no experimental data available for AmO₂ in this temperature region, the results obtained for the intermediate compositions are validated based on the experimental data of UO₂ end-member and the low-temperature heat capacity computation of AmO₂. In the measured temperature interval, excess heat capacity was observed for the two investigated intermediate compositions, which is concluded to be dominated by self-radiation effects at very low temperature.

The enthalpy of formation at 298.15 K and low temperature heat capacity of Cs3Na(MoO4)2 have been measured for the first time in this work using solution calorimetry and thermal-relaxation calorimetry in the temperature range T = (1.9–299.6) K, respectively. The solution calorimetry measurements, performed in 2 M HNO3 solution, have yielded an enthalpy equal to ΔrHmΔrHm(298.15 K) = (6.79 ±± 1.72)  kJ··mol−1 for the reaction:
3/2Cs2MoO4(cr)+1/2Na2MoO4(cr)=Cs3Na(MoO4)2(cr)3/2Cs2MoO4(cr)+1/2Na2MoO4(cr)=Cs3Na(MoO4)2(cr)
Combining with the enthalpies of formation of Cs2MoO4(cr) and Na2MoO4(cr), also determined in this work in 0.1 M CsOH and 0.1 M NaOH solutions, respectively, the standard enthalpy of formation of Cs3Na(MoO4)2 at 298.15 K has been determined as View the MathML sourceΔfHmo(Cs3Na(MoO4)2, cr, 298.15 K) = −(2998.5 ±± 3.0) kJ··mol−1. The heat capacity and entropy values of Cs3Na(MoO4)2 at 298.15 K have been derived as View the MathML sourceCp,mo(Cs3Na(MoO4)2,cr,298.15K)=(296.3±3.3) J··K−1··mol−1 and View the MathML sourceSmo(Cs3Na(MoO4)2,cr,298.15K) (467.2±6.8) J··K−1··mol−1. Combining the newly determined thermodynamic functions, the Gibbs energy of formation of Cs3Na(MoO4)2 at 298.15 K has been derived as View the MathML sourceΔfGmo(Cs3Na(MoO4)2,cr,298.15K)=-(2784.6±3.4) kJ··mol−1. Finally, the enthalpies, entropies and Gibbs energies of formation of Cs3Na(MoO4)2 from its constituting binary and ternary oxides have been calculated.

The structure of α-Cs₂Mo₂O₇ (monoclinic in space group P2₁/c), which can form during irradiation in fast breeder reactors in the space between nuclear fuel and cladding, has been refined in this work at room temperature from neutron diffraction data. Furthermore, the compounds' thermal expansion and polymorphism have been investigated using high temperature X-ray diffraction combined with high temperature Raman spectroscopy. A phase transition has been observed at T_tr(α→β)=(621.9±0.8) K using Differential Scanning Calorimetry, and the structure of the β-Cs₂Mo₂O₇ phase, orthorhombic in space group Pbcm, has been solved ab initio from the high temperature X-ray diffraction data. Furthermore, the low temperature heat capacity of α-Cs₂Mo₂O₇ has been measured in the temperature range T=(1.9–313.2) K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The heat capacity and entropy values at T=298.15 K have been derived as C_p,m ^o(Cs₂Mo₂O₇,cr,298.15K)=(211.9±2.1)JK⁻¹mol⁻¹ and S_m ^o(Cs₂Mo₂O₇,cr,298.15K)=(317.4±4.3)JK⁻¹mol⁻¹. When combined with the enthalpy of formation reported in the literature, these data yield standard entropy and Gibbs energy of formation as Δ_fS_m ^o(Cs₂Mo₂O₇,cr,298.15K)=−(628.2±4.4)JK⁻¹mol⁻¹ and Δ_fG_m ^o(Cs₂Mo₂O₇,cr,298.15K)=−(2115.1±2.5)kJmol⁻¹. Finally, the cesium partial pressure expected in the gap between fuel and cladding following the disproportionation reaction 2Cs₂MoO₄=Cs₂Mo₂O₇+2Cs(g)+ 1/2 O₂(g) has been calculated from the newly determined thermodynamic functions.

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.

The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na₂NpO₄ are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a λ-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm^-1 above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of α-Na₂NpO₄.