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

The mobilities of trivalent ions are generally considered to be low in solid oxides and thus trivalent ionic conductors are rare. Sc-2(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 Sc-45, O-17, and Al-27 MAS NMR studies at ambient and high temperature of Sc-2(WO4)(3)-type compounds including Sc-2(WO4)(3), Sc-2(MoO4)(3) (scandium tungstate and scandium molybdate), and their solid solutions, as well as aluminum-doped Sc-2(WO4)(3). Sc-45 MAS NMR spectra at ambient temperature of the Sc-2(WO4)(3)-Sc-2(MoO4)(3) binary are consistent with random mixing of W6+ and Mo6+ on tetrahedral sites. O-17 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 degrees C), only the O-17 high spectra show noticeable line shape changes, whereas Sc-45 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. O-17 high-temperature NMR data also suggest that oxide ions bonded to molybdenum ions may be more mobile than those in tungstate groups.