Variable-Temperature ¹⁷O NMR Study of Oxygen Motion in the Anionic Conductor Bi₂₆Mo₁₀O₆₉

Variable-temperature 17 O NMR spectroscopy, spanning a temperature range from −238 to 1000 °C, has been used to investigate mechanisms for ionic conduction in Bi 26 Mo 10 O 69, a material that contains both MoO 4 2− tetrahedra and [Bi 12 O 14 ] 8− ∞ columns. Two 17 O NMR resonances are observed that are assigned to oxygen atoms in the MoO 4 2− tetrahedra and in the [Bi 12 O 14 ] 8− ∞ columns. On the basis of the nutation curves for the two groups of resonances, extremely rapid, but local, reorientational motion of the MoO 4 2− units occurs at −70 °C and above (with a frequency of >50 kHz), whereas the Bi−O oxygen ions are rigid in this temperature regime. This is confirmed by both an analysis of the line broadening of the 17 O MoO 4 2− satellite transitions (under MAS) and the spin−lattice relaxation ( T 1 ) times of these sites, the T 1 times indicating that the MoO 4 2− reorientation rates rapidly increase, reaching >100 MHz at 400 °C. Line narrowing of the MoO 4 2− central-transition resonance indicates that exchange between the tetrahedral units, a motion required for long-range anionic conduction, is much slower, involving only jump rates of approximately 1 kHz at 200 °C. Both the changes in line width of the MoO 4 2− resonance, and the jump in the T 1 times of the oxygen atoms in the [Bi 12 O 14 ] 8− ∞ columns at around the triclinic-monoclinic phase transition temperature (310 °C) are consistent with a mechanism for motion involving all the oxygen atoms. The predicted conductivity based on the [Bi−O] T 1 times is now of the order of that extracted from ac impedance measurements reported by Vannier et al. ( J. Solid State Chem. 1996, 122, 394). On the basis of this detailed NMR analysis, we propose that motion at ambient temperatures primarily involves the MoO 4 2− tetrahedral rotation: exchange between these sites is very slow. At higher temperatures (above 310 °C), the conduction process now appears to involve the oxygen atoms coordinated to Bi 3+, in the [Bi 12 O 14 ] 8− ∞ columns, and most likely in the partially vacant O[19] site. The involvement of these sites allows for long-range conduction processes that do not involve concerted, multiple Mo−O bond breakages.