Microcrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium
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Date
2009-07-06
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American Chemical Society
Abstract
The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2·zH2O has been studied using X-ray and neutron diffraction techniques, 1D and 2D 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D 23Na MAS NMR has identified three sodium species within the HTB tunnels—species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers 137Cs and 85Sr have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a−b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework. © 2009, American Chemical Society
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Keywords
Tungsten, Ion exchange, Cesium, Strontium, Bronze, Hexagonal configuration
Citation
Griffith, C. S., Luca, V., Hanna, J. V., Pike, K. J., Smith, M. E., & Thorogood, G. S. (2009). Microcrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium. Inorganic Chemistry, 48(13), 5648-5662. doi:/10.1021/ic801294x