Neutron diffraction measurements of the double molybdate Cs3Na(MoO4)2 have been performed for the first time in this work and the crystal structure refined using the Rietveld method. The thermal expansion of this trigonal phase, in space group P3¯m1, measured using high temperature X-ray diffraction (XRD), remains moderate: αa=31·10−6K−1 and αc=24·10−6K−1 in the temperature range T = (298−723) K. The melting temperature of this compound has been determined at Tfus= (777 ± 5) K using Differential Scanning Calorimetry (DSC). No phase transition was detected, neither by DSC, nor by high temperature XRD or high temperature Raman spectroscopy, which disagrees with the literature data of Zolotova et al. (2016), who reported a reversible phase transition around 663 K. Finally, thermodynamic equilibrium calculations have been performed to assess the probability of formation of Cs3Na(MoO4)2 inside the fuel pin of a Sodium cooled Fast Reactor by reaction between the cesium molybdate phase Cs2MoO4, which forms at the pellet rim at high burnup, the fission product molybdenum (either as metallic or oxide phase), and the liquid sodium coolant in the accidental event of a breach of the stainless steel cladding and sodium ingress in the failed pin.@en

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 Ttr(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 Cp,m o(Cs2Mo2O7,cr,298.15K)=(211.9±2.1)JK−1mol−1 and Sm 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 ΔfSm o(Cs2Mo2O7,cr,298.15K)=−(628.2±4.4)JK−1mol−1 and ΔfGm 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.@en

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 Ttr(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 Cp,m o(Cs2Mo2O7,cr,298.15K)=(211.9±2.1)JK−1mol−1 and Sm 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 ΔfSm o(Cs2Mo2O7,cr,298.15K)=−(628.2±4.4)JK−1mol−1 and ΔfGm 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.@en

Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba2NaMoO5.5 have been performed in this work. The cubic crystal structure in space group Fm3¯ m has been refined using the Rietveld method. X-ray absorption near-edge structure spectroscopy (XANES) measurements at the Mo K-edge have confirmed the hexavalent state of molybdenum. The local structure of the molybdenum octahedra has been studied in detail using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mo-O and Mo-Ba distances have been compared to the neutron diffraction data with good agreement. The coefficient of thermal expansion measured in the temperature range of 303-923 K, using high temperature X-ray diffraction (HT-XRD) (αV = 55.8 × 10-6 K), has been determined to be ∼2 times higher than that of the barium molybdates BaMoO3 and BaMoO4. Moreover, no phase transition nor melting have been observed, neither by HT-XRD nor Raman spectroscopy nor differential scanning calorimetry, up to 1473 K. Furthermore, the standard enthalpy of formation (ΔfHm°) for Ba2NaMoO5.5(cr) has been determined to be -(2524.75 ± 4.15) kJ mol-1 at 298.15 K, using solution calorimetry. Finally, the margin for safe operation of sodium-cooled fast reactors (SFRs) has been assessed by calculating the threshold oxygen potential needed, in liquid sodium, to form the quaternary compound, following an interaction between irradiated mixed oxide (U,Pu)O2 fuel and sodium coolant.@en