Variable-Temperature 17O NMR Study of Oxygen Motion in the Anionic Conductor Bi26Mo10O69
Lesley Holmes, Luming Peng, Ivo Heinmaa, Luke A. O’Dell, Mark E. Smith, Rose-Noelle Vannier, and Clare P. Grey*
Variable-temperature 17O NMR spectroscopy, spanning a temperature range from −238 to 1000 °C, has been used to investigate mechanisms for ionic conduction in Bi26Mo10O69, a material that contains both MoO42− tetrahedra and [Bi12O14]8−∞ columns. Two 17O NMR resonances are observed that are assigned to oxygen atoms in the MoO42− tetrahedra and in the [Bi12O14]8−∞ columns. On the basis of the nutation curves for the two groups of resonances, extremely rapid, but local, reorientational motion of the MoO42− 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 17O MoO42− satellite transitions (under MAS) and the spin−lattice relaxation (T1) times of these sites, the T1 times indicating that the MoO42− reorientation rates rapidly increase, reaching >100 MHz at 400 °C. Line narrowing of the MoO42− 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 MoO42− resonance, and the jump in the T1 times of the oxygen atoms in the [Bi12O14]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] T1 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 MoO42− 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 Bi3+, in the [Bi12O14]8−∞ columns, and most likely in the partially vacant O site. The involvement of these sites allows for long-range conduction processes that do not involve concerted, multiple Mo−O bond breakages.