Microcrystalline hexagonal tungsten bronze. 2. Dehydration dynamics

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American Chemical Society
Low-temperature (25−600°C) thermal transformations have been studied for hydrothermally prepared, microcrystalline hexagonal tungsten bronze (HTB) phases AxWO3+x/2·zH2O as a function of temperature, where A is an exchangeable cation (in this case Na+ or Cs+) located in hexagonal structural tunnels. Thermal treatment of the as-prepared sodium- and cesium-exchanged phases in air were monitored using a conventional laboratory-based X-ray diffractometer, while thermal transformations in vacuum were studied using synchrotron X-ray and neutron diffraction. Concurrent thermogravimetric, diffuse reflectance infrared (DRIFT), and 23Na and 133Cs magic angle spinning (MAS) NMR spectroscopic studies have also been undertaken. For the cesium variant, cell volume contraction occurred from room temperature to about 350°C, the regime in which water was “squeezed” out of tunnel sites. This was followed by a lattice expansion in the 350−600°C temperature range. Over the entire temperature range, a net thermal contraction was observed, and this was the result of an anisotropic change in the cell dimensions which included a shortening of the A−O2 bond length. These changes explain why Cs+ ions are locked into tunnel positions at temperatures as low as 400°C, subsequently inducing a significant reduction in Cs+ extractability under low pH (nitric acid) conditions. The changing Cs+ speciation as detected by 133Cs MAS NMR showed a condensation from multiple Cs sites, presumably associated with differing modes of Cs+ hydration in the tunnels, to a single Cs+ environment upon thermal transformation and water removal. While similar lattice contraction was observed for the as-prepared sodium variant, the smaller radius of Na+ caused it to be relatively easily removed with acid in comparison to the Cs+ variant. From 23Na MAS NMR studies of the parent material, complex Na+ speciation was observed with dehydrated and various hydrated Na+ species being identified, and a subsequent dynamic interchange within this speciation was observed upon thermal treatment. © 2009, American Chemical Society
Tungsten, Dehydration, Hexagonal configuration, Bronze, Thermalization, Cations
Luca, V., Griffith, C. S., & Hanna, J. V. (2009). Microcrystalline hexagonal tungsten bronze. 2. Dehydration dynamics. Inorganic Chemistry, 48(13), 5663-5676. doi:10.1021/ic801295c