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• 作者：A. L. Smith ; P. E. Raison ; L. Martel ; T. Charpentier ; I. Farnan ; D. Prieur ; C. Hennig ; A. C. Scheinost ; R. J. M. Konings ; A. K. Cheetham
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：January 6, 2014
• 年：2014
• 卷：53
• 期：1
• 页码：375-382
• 全文大小：379K
• ISSN：1520-510X

文摘

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 鈭?9.1 (NaUO₃), 15.1 (Na₄UO₅), and 鈭?4.1 and 鈭?9 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: 伪_a = 22.7 脳 10^鈥? K^鈥?, 伪_b = 12.9 脳 10^鈥? K^鈥?, 伪_c = 16.2 脳 10^鈥? K^鈥?, and 伪_vol = 52.8 脳 10^鈥? K^鈥? for NaUO₃ in the range 298鈥?273 K; 伪_a = 37.1 脳 10^鈥? K^鈥?, 伪_c = 6.2 脳 10^鈥? K^鈥?, and 伪_vol = 81.8 脳 10^鈥? K^鈥? for Na₄UO₅ in the range 298鈥?073 K; 伪_a = 6.7 脳 10^鈥? K^鈥?, 伪_b = 14.4 脳 10^鈥? K^鈥?, 伪_c = 26.8 脳 10^鈥? K^鈥?, 伪_尾 = 鈭?.8 脳 10^鈥? K^鈥?, and 伪_vol = 鈭?17.6 脳 10^鈥? K^鈥? for Na₂U₂O₇ in the range 298鈥?73 K. The 伪 to 尾 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.