A comprehensive thermodynamic assessment of the Cs-Pb system was performed with the CALPHAD method using the experimental thermodynamic and phase diagram data available in literature supplemented by density functional theory (DFT) calculations. The exact nature of the stable compounds in the phase diagrams reported in the literature is uncertain, except for CsPb and Cs₄Pb₉ whose crystalline structures are well known. Therefore, DFT calculations were performed to calculate the energy of formation at 0 K of different possible compounds with various crystalline structures. The enthalpies of formation of the compounds CsPb, Cs₄Pb₉, and CsPb₄, found to be the stable ones by DFT, were then used in the CALPHAD model. The result of this process has enabled the development of a more refined phase diagram comparing to experimental ones, providing more comprehensive insights into the phase equilibria in this system. Moreover, the CALPHAD model succeeded in describing the peculiar behavior of the heat capacity of the liquid phase, by using an ionic two-sublattice model (Cs+1)_P(Pb−1,Va,Pb)_Q which takes into account the short-range ordering taking place at the equimolar composition CsPb, related to the formation of clusters Cs₄Pb₄, modelled as (Cs+1) (Pb−1). The model allows for the prediction of important thermodynamic properties, which are of interest for a range of applications, including lead-cooled fast reactors and perovskite-based photovoltaics.

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.

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 quaternary compound Cs₂Pb(MoO₄)₂ was synthesized and its structure was characterized using X-ray and neutron diffraction from 298 to 773 K, while thermal expansion was studied from 298 to 723 K. The crystal structure of the high-temperature phase β-Cs₂Pb(MoO₄)₂ was elucidated, and it was found to crystallize in the space group R3̅m (No. 166), i.e., with a palmierite structure. In addition, the oxidation state of Mo in the low-temperature phase α-Cs₂Pb(MoO₄)₂ was studied using X-ray absorption near-edge structure spectroscopy. Phase diagram equilibrium measurements in the Cs₂MoO₄-PbMoO₄ system were performed, revisiting a previously reported phase diagram. The equilibrium phase diagram proposed here includes a different composition of the intermediate compound in this system. The obtained data can serve as relevant information for thermodynamic modeling in view of the safety assessment of next-generation lead-cooled fast reactors.

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 thermochemistry of the ternary system CsI-PbI₂-BiI₃, of interest for applications in photovoltaics, memory devices, and nuclear applications, among other things, is investigated in this work. The binary phase diagrams CsI-PbI₂ and CsI-BiI₃ were subjected to renewed experimental investigation, and the compounds CsPbI₃, Cs₄PbI₆, and Cs₃Bi₂I₉ were found to be the only stable phases in the investigated temperature window. The liquidus lines and invariant equilibria were determined. The phase equilibria in the BiI₃-PbI₂ system were measured for the first time by using Differential Scanning Calorimetry (DSC). The end-members form a solid solution over the entire composition range. The pseudobinary section CsPbI₃-Cs₃Bi₂I₉ of the CsI-PbI₂-BiI₃ ternary system was moreover measured by DSC, as well as the ternary eutectic points. A thermodynamic model of the complete CsI-PbI₂-BiI₃ system was developed by using the Compound Energy Formalism (CEF) for the solid phases and the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) for the liquid phase. The binary systems were modeled first, and no ternary interaction parameters were found necessary to reproduce accurately the phase equilibria in the ternary system. With our model, the whole liquidus surface of the field CsI-PbI₂-BiI₃ is described for the first time.