Synthesis and mixed conductivity of ammonium tungsten bronze with tunneling structures

The ammonium tungsten bronze has been prepared by soft-chemical synthesis method at ambient pressure. The hexagonal or cubic pyrochlore structure could be obtained from the same acidified tungsten system, depending upon the dissolution or not of the ammonium tungstate in the solvent before acidification. The complete dissolution leads to the hexagonal structure and the nondissolution gives the pyrochlore one. The powders were characterized by TG-mass spectroscopy, IR and SEM analyses. XPS results confirmed the presence of the mixed valence state of tungsten. Electrical properties of the compounds were studied by impedance spectroscopy from 25 to I 10 degreesC. They are characteristic of a mixed proton and electronic conductivity in both samples. IR and XRD results indicate that the bronze can be further reduced by hydrogen in the presence of water vapor at temperatures higher than 70 degreesC, resulting in a co-inserted bronze H-x(NH4)(y)WO3..H2O. It was concluded that water plays an important role on the process of the hydrogen oxidation of the as-prepared hexagonal bronze. (C) 2004 Elsevier SAS. All rights reserved.