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

Abstract

a b s t r a c tThe vaporization behavior and decomposition mechanism of NaUO 3 were studied using Knudsen effusion mass spectrometry. The temperature dependence of the sodium partial pressure over the ternary phase field NaUO 3 -Na 2 U 2 O 7 -UO 2 was measured for the first time: logðP Na =PaÞ ¼ 10:98ðAE0:04ÞÀ 14; 999ðAE50Þ=ðT=KÞ. The enthalpy of formation of a-Na 2 U 2 O 7 was moreover derived from second law analysis of the data at D f H o m ða-Na 2 U 2 O 7 ; cr; T ¼ 298:15 KÞ = À(3208.4 ± 5.5) kJ Á mol À1 , in very good agreement with the value reported in the literature from solution calorimetry. The existence of three polymorphs for Na 2 U 2 O 7 (a, b, and c) 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.

Polymorphism and phase transitions in Na2U2O7 from density functional perturbation theory

“…, above ∼1473 K by Gasperin 16 ) has to be rapidly quenched to room temperature for crystallographic characterization. As reported by Smith et al , 21 gradual cooling of γ-Na 2 U 2 O 7 to room temperature of what was expected to be either pure β-Na 2 U 2 O 7 or γ-Na 2 U 2 O 7 , resulted instead in a mixture of α and β phases.…”

“…1(a)). These values are in good agreement with the recent neutron scattering measurements of a = 12.7617(14) Å, b = 7.8384(10) Å, c = 6.8962(9) Å ( b / a = 0.6142, c / a = 0.5404, V = 642.78 Å 3 ), and β = 111.285(9)° at 293 K by Ijdo et al , 5 as well as with the parameters of a = 12.778(3) Å, b = 7.823(3) Å, c = 6.880(3) Å ( b / a = 0.612, c / a = 0.538, V = 640.9 Å 3 ), and β = 111.28(5)° determined from XRD data at 303 K by Smith et al 21 Previous DFT calculations by Smith et al 6 using CASTEP with GGA/PBE predicted a larger unit cell, with lattice parameters of a = 13.1322 Å, b = 7.8815 Å, c = 6.9292 Å ( b / a = 0.6002, c / a = 0.5276, V = 669.18 Å 3 ), and β = 110.994°. The predicted equilibrium cell volume in this study is ∼2.5% larger than the experimental estimate of Ijdo et al 5 – although ∼1.5% smaller than the DFT estimate of Smith et al 6 – owing to the well-known tendency of GGA functionals to overestimate bond distances.…”

“…They also reported that after complete cooling to room temperature of what they expected to be either pure β-Na 2 U 2 O 7 or γ-Na 2 U 2 O 7 , the final XRD pattern always corresponded to a mixture of α and β phases. 21 Given the above discussion, a clear understanding of the relationships between structures and relative thermodynamic stabilities of Na 2 U 2 O 7 polymorphs is still missing.…”

Elemental quantification and radioactive characterization of soil from Douala Bassa area: littoral region of Cameroon using X- and γ-rays spectrometry

“…Specification on the x-ray generator shows that it was operating on 50 kV and 50 μA of 900-seconds acquisition's time. Only experimental setup arrangements and conditions are set at the laboratory; the detector and software used are provided by another scientist (Solé et al 2007, Blagoev et al 2013, Smith et al 2015. This part of the study was performed directly on the surface of different pellets realized in the laboratory.…”

“…Measurements were performed at three points of the sample surface and the considered result is the mean of the three in the case of very low deviation. The experimental setup used to perform our experiment was proposed and well described by Wenbin (Blagoev et al 2013, Smith et al 2015 and is represented in figure 3. Once acquired the XRF spectra, the PyMCA software package was used for data analysis and treatment.…”