Inorganic Chemistry - up to Issue 14 (July 2007)

Kuhn, A.; Azcondo, M. T.; Amador, U.; Boulahya, K.; Sobrados, I.; Sanz, J.; Garcia-Alvarado, F.
"Structural Characterization and NMR Study of NaNbWO6 and Its Proton-Exchanged Derivatives." Inorg. Chem. 2007, 46(13), 5390-5397.

Abstract: The structural characterization of NaNbWO6, prepared by the ceramic route, has been performed. Electron diffraction has shown the presence of two related phases in a 1:1 ratio, whose lattice parameters correspond to those of the well-known tetragonal tungsten bronzes (TTB) and those of a monoclinically distorted phase. In addition to basic unit cells, the morphology of the two phases has been found to be similar, but they present a slight difference in the W/Nb ratio. 1H and 23Na magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of NaNbWO6 and its proton-exchanged derivatives have been interpreted on the basis of the ideal TTB structure. The average structure and the morphology remain unchanged in Na1 - xHxNbWO6 derivatives. 1H and 23Na MAS-NMR spectroscopies have been used to monitor changes produced during exchange processes. It has been shown that the exchange of Na ions is mainly produced, but not exclusively, at tetragonal channels. However, a large amount of Na ions at the pentagonal channels do not exchange with protons, suggesting that these ions are needed to stabilize the TTB-like structure. A tentative distribution of sodium ions in the most-exchanged oxide, deduced from NMR results, ~(Na0.46)p(Na0.08)sH0.46NbWO6, has been proposed. NMR spectra of Na1 - xHxNbWO6 indicate that two different OH groups are formed upon exchanging. The study of samples hydrated with D2O allowed us to conclude that deuterons of adsorbed water exchange with protons of the two OH groups. The proton-deuteron exchange is slow at room temperature but is strongly enhanced at 90 C. This observation relates to the proton conductivity displayed by exchanged products under a humid atmosphere.