Detecting Different Oxygen-Ion Jump Pathways in Bi₂WO₆with 1- and 2-Dimensional¹⁷O MAS NMR Spectroscopy

Bismuth tungsten oxide (Bi 2 WO 6 ) is an m = 1 Aurivillius phase and some of its doped compounds show moderately high oxygen ion conductivity. These materials are structurally related to the more disordered bismuth vanadium oxide Aurivillius phases (or BIMEVOXes), which exhibit extremely high conductivities. We demonstrate that a combination of one- and two-dimensional, variable-temperature, 17 O MAS NMR spectroscopy may be used to probe ionic motion in the Nb 5+ -doped bismuth tungstates and resolve different oxide-ion conduction mechanisms that occur over a very wide range of different time scales (10 - 1 to 10 - 4 s). The use of 17 O two-dimensional exchange spectroscopy dramatically increases the range of oxide-ion conductors that can be investigated by MAS NMR. Motion commences in the (WO 4 ) n 2 n - perovskite layers, with slow exchange between the Bi 2 O 2 2+ and (WO 4 ) n 2 n - layers occurring at higher temperatures, as detected by 2D NMR. Simulation of the 1D 17 O NMR spectra of Bi 2 W 1 - x Nb x O 6 with x = 0.05 and 0.1 shows an increase in ionic conductivity from x = 0.05 to 0.1 consistent with the anionic conductivity of these materials measured by ac impedance spectroscopy.