Microcrystalline Hexagonal Tungsten Bronze. 2. Dehydration Dynamics.

Low-temperature (25—600°C) thermal transformations have been studied for hydrothermally prepared, microcrystalline hexagonal tungsten bronze (HTB) phases A[subx]WO[sub3+x/2]∙zH[sub2]O as a function of temperature, where A is an exchangeable cation (in this case Na[sup+] or Cis[sup+]) located in hexagonal structural tunnels. Thermal treatment of the as-prepared sodium- and cesium-exchanged transformations in vacuum were studied using synchrotron X-ray and neutron diffraction. Concurrent thermogravimetric, diffuse reflectance infrared (DRIFT), and [sup23]Na and [sup133]Cs 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—O[sub2] bond length. These changes explain why Cs[sup+] ions are locked into tunnel positions at temperature as low as 400°C, subsequently inducing a significant reduction Cs[sup+] extractability under low pH (nitric acid) conditions. The changing Cs[sup+] speciation as detected by [sup133]Cs MAS NMR showed a condensation from multiple Cs sites, presumably associated with differing modes of Cs[sup+] hydration in the tunnels, to a single Cs[sup+] environment upon thermal transformation and water removal. While similar lattice contraction was observed for the as-prepared sodium variant, the smaller radius of Na[sup+] caused it to be relatively easily removed with acid in comparison to the Cs[sup+] variant. From [sup23]Na MAS NMR studies of the parent material, complex Na[sup+] speciation was observed with dehydrated and various hydrated Na[sup+] species being indentified, and a subsequent dynamic interchange within this speciation was observed upon the thermal treatment. [ABSTRACT FROM AUTHOR]