Thermodynamic investigation of Na2U2O7 using Knudsen effusion mass spectrometry and high temperature X-ray diffraction

The vaporization behavior and decomposition mechanism of NaUO3 were studied using Knudsen effusion mass spectrometry. The temperature dependence of the sodium partial pressure over the ternary phase field NaUO3-Na2U2O7-UO2 was measured for the first time: log(PNa/Pa)=10.98(±0.04)-14,999(±50)/(T/K). The enthalpy of formation of α-Na2U2O7 was moreover derived from second law analysis of the data at ΔfHmo(α-Na2U2O7,cr,T=298.15K) = −(3208.4  ± 5.5) kJ · mol−1, in very good agreement with the value reported in the literature from solution calorimetry. The existence of three polymorphs for Na2U2O7 (α, β, and γ) was confirmed using high temperature X-ray diffraction. The two phase transitions occur above about T = 600 K, and around T = 1323 K, respectively. Our results were finally assessed with respect to similar vaporization studies carried out in neighboring ternary phase fields of the Na-U-O phase diagram.