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The hexavalent and heptavalent sodium neptunate compounds Na4NpO5 and Na5NpO6 have been investigated using X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, and specific heat measurements. Na4NpO5 has tetragonal symmetry in the space group I4/m, while Na5NpO6 adopts a monoclinic unit cell in the space group C2/m. Both structures have been refined for the first time using the Rietveld method. The valence states of neptunium in these two compounds, i.e., Np(VI) and Np(VII), respectively, have been confirmed by the isomer shift values of their Mössbauer spectra. The local structural properties obtained from the X-ray refinements have also been related to the quadrupole coupling constants and asymmetry parameters determined from the Mössbauer studies. The absence of magnetic ordering has been confirmed for Na4NpO5. However, specific heat measurements at low temperatures have suggested the existence of a Schottky-type anomaly at around 7 K in this Np(VI) phase. [ABSTRACT FROM AUTHOR]

The detailed crystal structure as well as the heat capacity at low temperature and standard entropy of Ba 2 MoO 5 are reported for the first time. High-resolution X-ray and neutron diffraction were employed to reveal the structural features of this compound. Ba 2 MoO 5 has a six-coordinated Mo and a strongly negative excess volume with respect to the binary oxides. X-ray absorption near edge structure (XANES) spectroscopy at the Mo K-edge shows Mo to be in the oxidation state 6+. The pre-edge peak in the XANES spectrum indicates a distorted octahedral environment, in line with the results from diffraction studies and FDMNES calculations. The standard entropy and heat capacity of Ba 2 MoO 5 at 298.15 K, determined with a thermal-relaxation technique, are calculated to be respectively 223.2 ± 7 and 184.7 ± 5 J·K -1 ·mol -1 . The obtained thermodynamic properties are discussed in the context of the literature reports on molybdate compounds.

The heat capacities of CsPbI 3 , Cs 4 PbI 6 , and Cs 3 Bi 2 I 9 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 Cs 3 Bi 2 I 9 are not fully consistent with the present findings. Moreover, the magnetic susceptibilities of the three title compounds were measured., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The structural, thermochemical, and thermophysical properties of the NaF-ThF 4 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 [ThF n ] m - complexes in the (Na,Th)F x melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF 4 and 85-95 mol % ThF 4 regions of the NaF-ThF 4 phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF 4 ) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na 2 ThF 6 and NaTh 2 F 9 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 Th 4+ 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, Cs 2 Ba(MoO 4 ) 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 α V ≈ 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 Δ f H m °(Cs 2 Ba(MoO 4 ) 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 S m °(Cs 2 Ba(MoO 4 ) 2 , cr, 298.15 K) = 381.2 ± 11.8 J K -1 mol -1 .

The physicochemical properties of the potassium neptunate K 2 NpO 4 have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L 3 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 K 2 NpO 4 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.