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

Smith, A. L. and Raison, P. E. and Martel, L. and Charpentier, T. and Farnan, I. and Prieur, D. and Hennig, C. and Scheinost, A. C. and Konings, R. J. M. and Cheetham, A. K. (2014) A 23Na Magic Angle Spinning Nuclear Magnetic Resonance, XANES, and High-Temperature X-ray Diffraction Study of NaUO3, Na4UO5, and Na2U2O7. Inorganic Chemistry, 53 (1). pp. 375-382. ISSN 0020-1669, ESSN: 1520-510X DOI 10.1021/ic402306c

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Abstract

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 Rn5f1 electronic structure of uranium in {NaUO3} causes a paramagnetic shift in comparison to {Na4UO5} and {Na2U2O7}, where the electronic structure is Rn5f0. 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, α? = ?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 ? 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.

Item Type:	Article
Uncontrolled Keywords:	2013AREP, IA67,
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences
Journal or Publication Title:	Inorganic Chemistry
Volume:	53
Page Range:	pp. 375-382
Identification Number:	10.1021/ic402306c
Depositing User:	Sarah Humbert
Date Deposited:	27 Feb 2014 23:23
Last Modified:	05 Mar 2014 17:18
URI:	http://eprints.esc.cam.ac.uk/id/eprint/2975

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