Structural and Thermodynamic Investigation of the Perovskite Ba₂NaMoO_5.5

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 Fm3̅m 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.