Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba₂NaMoO_5.5 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) (α_V = 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 (Δ_fH_m°) for Ba₂NaMoO_5.5(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.

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: α_a=31·10⁻⁶K⁻¹ and α_c=24·10⁻⁶K⁻¹ in the temperature range T = (298−723) K. The melting temperature of this compound has been determined at T_fus= (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.

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.