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 expan...