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

The valence state of uranium has been confirmed for the three sodium uranates NaUVO3/[Rn](5f1), Na4UVIO5/[Rn](5f0), and Na2UVI2O7/[Rn](5f0), using X-ray absorption near-edge structure (XANES) spectroscopy. Solid-state 23Na 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]5f1 electronic structure of uranium in NaUO3 causes a paramagnetic shift in comparison to Na4UO5 and Na2U2O7, where the electronic structure is [Rn]5f0. A 23Na 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.

SMITH Anna;  RAISON Philippe;  MARTEL Laura;  CHARPENTIER Thibault;  FARNAN I.;  PRIEUR Damien;  HENNIG Christoph;  SCHEINOST Andreas;  KONINGS Rudy;  CHEETHAM Anthony; 

2018-08-16

AMER CHEMICAL SOC

JRC88927

0020-1669,   

https://pubs.acs.org/doi/10.1021/ic402306c,    https://publications.jrc.ec.europa.eu/repository/handle/JRC88927,   

10.1021/ic402306c,