Structural Characterization and NMR Study of NaNbWO6 and Its Proton-Exchanged Derivatives

详细信息    查看全文

• 作者：Alois Kuhn ; Marí ; a T. Azcondo ; Ulises Amador ; Khalid Boulahya ; Isabel Sobrados ; Jesú ; s Sanz ; Flaviano Garcí ; a-Alvarado
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：June 25, 2007
• 年：2007
• 卷：46
• 期：13
• 页码：5390 - 5397
• 全文大小：473K
• 年卷期：v.46,no.13(June 25, 2007)
• ISSN：1520-510X

文摘

The structural characterization of NaNbWO₆, prepared by the ceramic route, has been performed. Electron diffractionhas shown the presence of two related phases in a 1:1 ratio, whose lattice parameters correspond to those of thewell-known tetragonal tungsten bronzes (TTB) and those of a monoclinically distorted phase. In addition to basicunit cells, the morphology of the two phases has been found to be similar, but they present a slight difference inthe W/Nb ratio. ¹H and ²³Na magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of NaNbWO₆and its proton-exchanged derivatives have been interpreted on the basis of the ideal TTB structure. The averagestructure and the morphology remain unchanged in Na_{1 - x}H_xNbWO₆ derivatives. ¹H and ²³Na MAS-NMRspectroscopies have been used to monitor changes produced during exchange processes. It has been shown thatthe exchange of Na ions is mainly produced, but not exclusively, at tetragonal channels. However, a large amountof Na ions at the pentagonal channels do not exchange with protons, suggesting that these ions are needed tostabilize the TTB-like structure. A tentative distribution of sodium ions in the most-exchanged oxide, deduced fromNMR results, ~(Na_0.46)_p(Na_0.08)_sH_0.46NbWO₆, has been proposed. NMR spectra of Na_{1 - x}H_xNbWO₆ indicate thattwo different OH groups are formed upon exchanging. The study of samples hydrated with D₂O allowed us toconclude that deuterons of adsorbed water exchange with protons of the two OH groups. The proton-deuteronexchange is slow at room temperature but is strongly enhanced at 90 C. This observation relates to the protonconductivity displayed by exchanged products under a humid atmosphere.