The low-temperature heat capacity and temperature-dependent DC magnetic susceptibility of neptunium-doped UO2 samples (U1−y[jls-end-space/]Npy[jls-end-space/])O2 with y = 0.01, 0.03 and 0.05 were measured. A strong effect was observed on the magnetic anomaly typical for UO2[jls-end-space/], both the Néel temperature and transition entropy. By comparison with data for other (U1−y[jls-end-space/], My[jls-end-space/])O2 solid solutions, it is suggested that this effect can be explained by the lattice strain resulting from substitution on the anion sublattice, and electron spin interaction.

The chemistry following cladding failure in Lead-cooled Fast Reactors involves the interaction between lead (Pb) coolant and the Joint Oxyde Gain (JOG)-phase, mostly composed of dicesium molybdate (Cs₂MoO₄). A thermodynamic analysis of coolant-JOG phase chemical interaction as studied via the scenario of Pb-Cs₂MoO₄ chemical interaction is reported. Measurements of the standard thermodynamic properties of α -Cs₂Pb(MoO₄)₂ are presented. The enthalpy of formation of α -Cs₂Pb(MoO₄)₂ is measured to be -(2570.7 ± 2.3) kJ · mol⁻¹ using solution calorimetry, while the standard entropy is determined to be (399 ± 12) J · K⁻¹·mol⁻¹ using thermal-relaxation calorimetry. Thermodynamic calculations show that Cs₂Pb(MoO₄)₂ is in several cases thermodynamically stable under conditions typical for operation of Lead-cooled Fast Reactors. This means Cs₂Pb(MoO₄)₂ can form in cladding failure scenarios.

The structural, thermodynamic, and magnetic properties of Na₂CrCl₄ have been investigated to provide fundamental insights into this ternary halide relevant to chloride-based molten salt reactor systems. Room-temperature powder X-ray and neutron diffraction confirm a monoclinic (P2₁/c) structure and phase purity. Neutron diffraction measurements at 4.6 K reveal additional magnetic reflections indexed with (Formula presented), indicating the onset of long-range antiferromagnetic order. Low-temperature heat capacity measurements in the range 2–300 K show a pronounced λ-type anomaly at T_N = 8.5 ± 0.5 K, with an associated magnetic entropy S_mag = 11.9 ± 0.4 J K^–1 mol ^–1 consistent with antiferromagnetic ordering of high-spin Cr²⁺ (S = 2), a second-order phase transition. The standard molar entropy at 298.15 K, S_m^°(298.15 K) = 256.8 ± 7.7 J K^–1 mol ^–1, is slightly lower than previous CALPHAD assessments of the NaCl-CrCl₂ system. Magnetic susceptibility measurements also confirm antiferromagnetic behavior, with a Curie–Weiss fit giving μ_eff = 5.57 ± 0.05 μ_B and θ_CW = −15.0 ± 1.0 K. Compared to the related ferromagnetic chlorides K₂CrCl₄, Rb₂CrCl₄, and Cs₂CrCl₄, Na₂CrCl₄ exhibits a distinctly lower ordering temperature and antiferromagnetic structure, likely due to variations in lattice geometry and exchange interactions. These results provide the first experimental thermodynamic parameters for Na₂CrCl₄, contributing to refining phase diagrams and corrosion models in chloride salt systems.

This work provides an in-depth analysis of the extensive research and development activities on americium-based ceramics for space applications, particularly as heat source for radioisotope power generation. Our pioneering efforts focus on synthesizing and characterizing various americium ceramics with fluorite, monazite, perovskite, zircon, and pyrochlore structures, and assessing their potential for use in Radioisotope Power Systems (RPSs). This study identifies uranium-stabilised cubic americium oxide as the best candidate among the ceramic forms analysed, due to its superior stability and performance under extreme conditions relevant to space missions. The review emphasises the unique facilities and methodologies employed, including remote-handling techniques and advanced material characterization, to overcome the challenges posed by the high radiation dose and specific activity of ²⁴¹Am when working with gram quantities.

The heat capacity of technetium metal has been measured from 2.1 K to 293 K using relaxation calorimetry and the enthalpy increment up to 1700 K using drop calorimetry. The low-temperature calorimetry measurements revealed a superconducting transition temperature of T_C = (7.76 ± 0.08) K. The zero-degree Debye temperature(θ_E) and the electronic heat capacity coefficient (γ_e) of the normal state were derived as (307 ± 5) K and (4.22 ± 0.20) mJ·K⁻²·mol⁻¹, respectively. The standard entropy of the superconducting standard state was derived as S_m^° (298.15) = (36.8 ± 1.3) J·K⁻¹·mol⁻¹. The fitting of enthalpy-increment data together with high-temperature heat capacity data reported in literature yielded a heat capacity equation up to 1700 K.

This work examines the thermochemistry of the chromium difluoride CrF₂ corrosion product in the molten [Formula presented] fuel salt system. Through a combination of experimental investigations and thermodynamic modeling assessment, the study elucidates the thermodynamic properties, phase diagram equilibria, and overall thermodynamic behavior of CrF₂ corrosion product, following dissolution from a structural material to the molten salt fuel environment. In this work, two different synthesis methods were developed for pure CrF₂, further allowing to experimentally measure the phase equilibria in the [Formula presented], [Formula presented], and [Formula presented] systems. Then, thermodynamic models were developed using the CALPHAD method based on the quasichemical model in the quadruplet approximation.

The heat capacities of CsPbI₃, Cs₄PbI₆, and Cs₃Bi₂I₉ were studied using low-temperature thermal relaxation calorimetry in the temperature range of 1.9-300 K. The three compounds are insulators, with no electronic contribution to the heat capacity. None of them show detectable anomalies in the studied temperature window. Thermodynamic properties at standard conditions are derived. Previously reported results on Cs₃Bi₂I₉ are not fully consistent with the present findings. Moreover, the magnetic susceptibilities of the three title compounds were measured.

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.

Thermodynamic measurements on BaMoO₄, BaMoO₃ and BaMo₃O₁₀ are reported, that served as input for the development of a thermodynamic model of the Ba-Mo-O system using the CALPHAD methodology. The valence states of molybdenum in BaMoO₄ and BaMoO₃ were confirmed to be VI and IV, respectively, from X-ray Absorption Near Edge Structure Spectroscopy measurements at the Mo K-edge. The heat capacity at low temperatures of these compounds was obtained from thermal-relaxation calorimetry. Phase equilibrium data in the BaMoO₄-MoO₃ section were also measured, and the transition enthalpy associated with the peritectic decomposition of BaMo₃O₁₀ was determined using Differential Scanning Calorimetry. The developed thermodynamic model used the compound energy formalism for intermediate compounds, and an ionic two-sublattice model for the liquid phase. The optimized Gibbs energies were assessed with respect to the known thermodynamic and phase equilibrium data. A good agreement is generally obtained, but a number of ill-defined data were also identified.

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 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.

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 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.