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 NaU^VO₃/[Rn]­(5f¹), Na₄U^VIO₅/[Rn]­(5f⁰), and Na₂U^VI₂O₇/[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: α_<i>a</i> = 22.7 × 10^–6 K^–1, α_<i>b</i> = 12.9 × 10^–6 K^–1, α_<i>c</i> = 16.2 × 10^–6 K^–1, and α_vol = 52.8 × 10^–6 K^–1 for NaUO₃ in the range 298–1273 K; α_<i>a</i> = 37.1 × 10^–6 K^–1, α_<i>c</i> = 6.2 × 10^–6 K^–1, and α_vol = 81.8 × 10^–6 K^–1 for Na₄UO₅ in the range 298–1073 K; α_<i>a</i> = 6.7 × 10^–6 K^–1, α_<i>b</i> = 14.4 × 10^–6 K^–1, α_<i>c</i> = 26.8 × 10^–6 K^–1, α_β = −7.8 × 10^–6 K^–1, and α_vol = −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