Fast oxygen diffusion in bismuth oxide probed by quasielastic neutron scattering☆
Introduction
The δ-phase of bismuth oxide, Bi2O3, which is stable between ca. 730 °C and the melting temperature of ca. 825 °C, is known for its highest reported ionic conductivity, on the scale of 1–3 S cm− 1. The 25% oxygen deficiency compared to regular fluorite structure is associated with high mobility of oxygen in this compound [1], [2]. Along with other diffraction techniques, neutron diffraction has played a major role in extensive investigation of Bi2O3 δ-phase structure, due to a sizable neutron coherent scattering cross-section of oxygen. Various structural models proposed for the δ-phase differ on possible displacements of oxygen anion from the tetrahedral cites of 8c symmetry, and whether the oxygen vacancies are ordered, or disordered [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. At the same time, little information is available on oxygen mobility in bismuth oxide, with the first oxygen tracer diffusion measurement reported only recently [28]. Here we report measurement of oxygen diffusivity in the δ-phase of Bi2O3 on the Angstrom length scale. The elementary anion diffusion jumps that we probe using quasielastic neutron scattering (QENS) underline the long-range macroscopic oxygen diffusivity, which on the greater length scale can be affected by nanoscopic structural non-homogeneities [29] and microscopic grain boundaries. QENS is commonly applied in studies of proton diffusion in solids (particularly proton-conducting oxides; see [30] for a recent review) and, much more rarely, diffusion of cations and anions in fast ionic conductors, but, to our knowledge, this may be the first QENS study of oxygen solid state translational diffusion. The only previous QENS study of oxygen mobility known to us probed localized rather than translational oxygen jumps [31]. One possible reason might be the common belief that very high temperatures are needed for oxygen diffusivities to enter the range readily accessible to QENS (above ca. 10− 11 m2/s). This is not a concern for Bi2O3, where oxygen diffusivities on the 10− 9 m2/s scale have been reported in the δ-phase [28], [32]. Another possible reason for the lack of previous QENS studies of oxygen mobility might be the purely coherent neutron scattering cross-section of oxygen; the coherent QENS theory is much less developed than incoherent QENS theory. Nevertheless, the QENS data that we measured from Bi2O3 allow simple interpretation. We believe that various oxygen conductors might be amenable to diffusivity studies by QENS to measure not only the atomic scale diffusion coefficient but also the elementary diffusion jump length of oxygen in the lattice.
Section snippets
Experimental
Bi2O3 powder of 99.999% purity was purchased from Sigma Aldrich. The powder was loaded in an open quartz sample holder with an inner diameter of 12.7 mm and a height of ca. 30 mm and mounted in a MICAS vacuum furnace, which controlled the temperature set points with an accuracy of better than a degree Celsius. Prior to measurements with neutrons, the sample mounted in the furnace was outgassed in vacuum at 300 °C for 48 h. This was necessary because oxide powders, either as received, or
Results and discussion
Representative spectra collected at 810 °C at a scattering momentum transfer Q = 1.1 Å− 1 are shown in Fig. 2, along with the fitting curves. The conclusion, which holds for all temperatures and Q values, is that background spectra subtraction significantly reduces, but does not eliminate the elastic line near zero energy transfer. Thus, an elastic line with a variable spectral weight, x(Q), needs to be included in the QENS data fits with the following model scattering function:
Conclusion
Despite the limitations imposed by the Bragg peaks, the presence of which seems to distort the data fits, we consistently obtained the diffusion jump length in agreement with the tetrahedral-tetrahedral site distance in the δ-phase of Bi2O3 (2.83 Å) from the QENS data collected below the first Bragg peak. The diffusion activation energy of 45 kJ/mol (0.47 eV) was obtained from the trustworthy data fits, and the diffusivity value renormalized to the single-particle tracer diffusion was ca. 7 × 10− 10 m2
Acknowledgements
We are grateful to Rebecca Mills for help and discussion of sample environment. We appreciate helpful and inspirational discussion of the data with Niina Jalarvo. The neutron scattering experiments at Oak Ridge National Laboratory's (ORNL) Spallation Neutron Source were supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). ORNL is managed by UTBattelle, LLC, for the U.S. DOE under Contract No. DE-AC05-00OR22725.
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