Thursday, January 08, 2009

Chem. Mater., Article ASAP

Sc2(WO4)3 and Sc2(MoO4)3 and Their Solid Solutions: 45Sc, 17O, and 27Al MAS NMR Results at Ambient and High Temperature

Namjun Kim* and Jonathan F. Stebbins


The mobilities of trivalent ions are generally considered to be low in solid oxides and thus trivalent ionic conductors are rare. Sc2(WO4)3 and related compounds have been described as Sc3+ ionic conductors. However, there has not been conclusive experimental evidence whether Sc3+ is the major or the only mobile species. Here, we report 45Sc, 17O, and 27Al MAS NMR studies at ambient and high temperature of Sc2(WO4)3-type compounds including Sc2(WO4)3, Sc2(MoO4)3 (scandium tungstate and scandium molybdate), and their solid solutions, as well as aluminum-doped Sc2(WO4)3. 45Sc MAS NMR spectra at ambient temperature of the Sc2(WO4)3−Sc2(MoO4)3 binary are consistent with random mixing of W6+ and Mo6+ on tetrahedral sites. 17O spectra show that the details of the end-member short-range structure are preserved throughout this solid solution, indicating that lattice distortion is minimized by the close similarity of the radii of the hexavalent cations. Al3+ substitutes only into the octahedral sites. At high temperatures (600−700 °C), only the 17O high spectra show noticeable line shape changes, whereas 45Sc high-temperature spectra maintain a constant peak width and shift in position only, suggesting that the oxide anion may be a more mobile species but not excluding the possibility of a minor number of mobile scandium ions. 17O high-temperature NMR data also suggest that oxide ions bonded to molybdenum ions may be more mobile than those in tungstate groups.

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