A ²³Na Magic Angle Spinning Nuclear Magnetic Resonance, XANES, and High-Temperature X-ray Diffraction Study of NaUO₃, Na₄UO₅, and Na₂U₂O₇

Abstract

The valence state of uranium has been confirmed for the three sodium uranates NaU(V)O3/[Rn](5f(1)), Na4U(VI)O5/[Rn](5f(0)), and Na2U(VI)2O7/[Rn](5f(0)), using X-ray absorption near-edge structure (XANES) spectroscopy. Solid-state (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) measurements have been performed for the first time, yielding chemical shifts at -29.1 (NaUO3), 15.1 (Na4UO5), and -14.1 and -19 ppm (Na1 8-fold coordinated and Na2 7-fold coordinated in Na2U2O7), respectively. The [Rn]5f(1) electronic structure of uranium in NaUO3 causes a paramagnetic shift in comparison to Na4UO5 and Na2U2O7, where the electronic structure is [Rn]5f(0). A (23)Na multi quantum magic angle spinning (MQMAS) study on Na2U2O7 has confirmed a monoclinic rather than rhombohedral structure with evidence for two distinct Na sites. DFT calculations of the NMR parameters on the nonmagnetic compounds Na4UO5 and Na2U2O7 have permitted the differentiation between the two Na sites of the Na2U2O7 structure. The linear thermal expansion coefficients of all three compounds have been determined using high-temperature X-ray diffraction: αa = 22.7 × 10(-6) K(-1), αb = 12.9 × 10(-6) K(-1), αc = 16.2 × 10(-6) K(-1), and αvol = 52.8 × 10(-6) K(-1) for NaUO3 in the range 298-1273 K; αa = 37.1 × 10(-6) K(-1), αc = 6.2 × 10(-6) K(-1), and αvol = 81.8 × 10(-6) K(-1) for Na4UO5 in the range 298-1073 K; αa = 6.7 × 10(-6) K(-1), αb = 14.4 × 10(-6) K(-1), αc = 26.8 × 10(-6) K(-1), αβ = -7.8 × 10(-6) K(-1), and αvol = -217.6 × 10(-6) K(-1) for Na2U2O7 in the range 298-573 K. The α to β phase transition reported for the last compound above about 600 K was not observed in the present studies, either by high-temperature X-ray diffraction or by differential scanning calorimetry.

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

“…c transitions. The a-Na 2 U 2 O 7 material was heated under helium up to T = 1073 K as reported in another publication [25]. These data were re-assessed in light of the recent structure determination by IJdo et al who assigned a monoclinic structure, in space group P2 1 =a, and not P2 1 to the a form of the compound.…”

“…High temperature X-ray diffraction measurements performed on the NaUO 3 material under helium [25] have evidenced the formation of Na 2 U 2 O 7 and UO 2 between T = (1273 and 1373) K. The corresponding decomposition reaction can be written as:…”

“…NaUO 3 crystallizes in the orthorhombic system, in space group Pbnm. The structure of a-Na 2 U 2 O 7 was recently re-assessed by IJdo et al using neutron diffraction, and found monoclinic (Z = 4), in space group P2 1 =a [24], and not P2 1 as reported in [25]. The ICP-MS analyses yielded sodium to uranium molar ratios of (0.999 ± 0.011) at/at and (1.029 ± 0.011) at/at for NaUO 3 and a-Na 2 U 2 O 7 , respectively.…”

“…(U, Pu) O 2 mixed oxide fuel is currently the preferred option for SFRs, and as such, the structural [1,2], thermodynamic [3][4][5][6][7][8], magnetic [9][10][11], and spectroscopic [12][13][14][15][16][17][18][19] properties of the Na-U-O and Na-Pu-O ternary compounds have been investigated in the past. The potential products of interaction are numerous: (Na 3 UO 4 , NaUO 3 , Na 2 UO 4 , Na 4 UO 5 ,Na 2 U 2 O 7 , Na 6 U 7 O 24 ) [20][21][22][23]1,24,25], and (Na 2 PuO 3 , Na 3 PuO 4 , Na 6 PuO 5 , Na 4 Pu 2 O 5 , Na 4 PuO 5 , Na 5 PuO 6 ) [1,2,26] for the first and second systems, respectively, but not all well characterized.…”

“…The sodium potential was measured for the first time over the ternary phase field NaUO 3 -Na 2 U 2 O 7 -UO 2 , and the enthalpy of formation of a-Na 2 U 2 O 7 was determined from second and third law treatment of the data. The polymorphism of Na 2 U 2 O 7 was moreover investigated using high temperature X-ray diffraction: a-Na 2 U 2 O 7 was synthesized as described in another publication by reaction between depleted uranium dioxide ( 238 UO 2 from JRC-ITU stocks) and sodium carbonate (Na 2 CO 3 99.95%, Sigma) mixed in stoichiometric amounts [25]. The purity of the samples were checked using X-ray powder diffraction at room temperature and ICP-MS analysis (table 1).…”