Variable-temperature O-17 NMR study of oxygen motion in the anionic conductor Bi26Mo10O69

Holmes, Lesley and Peng, Luming and Heinmaa, Ivo and O'Dell, Luke A. and Smith, Mark E. and Vannier, Rose-Noelle and Grey, Clare P. (2008) Variable-temperature O-17 NMR study of oxygen motion in the anionic conductor Bi26Mo10O69. Journal of Materials Chemistry, 20 (11). pp. 3638-3648. ISSN 1364-5501

Abstract

Variable-temperature O-17 NMR spectroscopy, spanning a temperature range from -238 to 1000 degrees C, has been used to investigate mechanisms for ionic conduction in Bi26Mo10O69, a material that contains both MoO42- tetrahedra and [Bi12O14](infinity)(8-) columns. Two O-17 NMR resonances are observed that are assigned to oxygen atoms in the MoO42- tetrahedra and in the [Bi12O14](infinity)(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 degrees 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 MoO42- satellite transitions (under MAS) and the spin-lattice relaxation (T-1) times of these sites, the T-1 times indicating that the MoO42- reorientation rates rapidly increase, reaching > 100 MHz at 400 degrees 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 degrees C. Both the changes in line width of the MoO42- resonance, and the jump in the T-1 times of the oxygen atoms in the [Bi12O14](infinity)(8-) columns at around the triclinic-monoclinic phase transition temperature (310 degrees 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 MoO42- tetrahedral rotation: exchange between these sites is very slow. At higher temperatures (above 310 degrees C) the conduction process now appears to involve the oxygen atoms coordinated to Bi3+, in the [Bi12O14](infinity)(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.