Salts of a sandwich-type bis 9-tungstoarsenate(III) tris(oxorhenate(V)) polyoxoanion as precursors to tungsten-rhenium bronzes

Potassium and ammonium salts of the tungstoarsenate anion [((ReO)-O-V)(3)(ASW(9)O(33))(2)](9-) (1) have been isolated and characterized by elemental analysis, infrared and W-183 NMR spectroscopy. The five-line NMR spectrum indicates a sandwich structure of C-2v symmetry analogous to that found for [As2W21O69(H2O)](6-). Thermal treatment of both salts of 1 at temperatures to 900 degrees C leads to materials in which similar to 95% of the rhenium is retained, in contrast to the Keggin-derived K4PW11ReO40 which loses rhenium at similar to 500 degrees C. Based upon powder diffraction analysis, following quantitative loss of As2O3 at 420-430 degrees C, the following phases were identified in samples heated to 600 and 725 degrees C. From the potassium salt, a monoclinic K2W3O10-type bronze, and from the ammonium salt a mixture of hexagonal and tetragonal WO3 bronzes. Reflections corresponding to ReO2, ReO3, solid solutions of ReO3 in WO3, and Re2O7 were absent. The incorporation of rhenium into tungsten bronze phases suggests that technetium might behave similarly, opening a possible route to storage of Tc-99 waste. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.