Crystallographic correlations with anisotropic oxide ion conduction in aluminum-doped neodymium silicate apatite electrolytes

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dc.contributor.authorAn, Ten_AU
dc.contributor.authorBaikie, Ten_AU
dc.contributor.authorWei, FXen_AU
dc.contributor.authorPramana, SSen_AU
dc.contributor.authorSchreyer, MKen_AU
dc.contributor.authorPiltz, ROen_AU
dc.contributor.authorShin, JFen_AU
dc.contributor.authorWei, Jen_AU
dc.contributor.authorSlater, PRen_AU
dc.contributor.authorWhite, TJen_AU
dc.date.accessioned2013-10-04T03:13:40Zen_AU
dc.date.available2013-10-04T03:13:40Zen_AU
dc.date.issued2013-04-09en_AU
dc.date.statistics2013-10-04en_AU
dc.description.abstractTo better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 <= x <= 2) single crystals has been examined. The measurement of ionic conductivity via AC impedance spectroscopy showed that the conductivities were anisotropic and superior along the c direction. An interesting aspect from the impedance studies was the identification of a second semicircle with capacitance similar to that of a grain boundary component, despite the fact that polarized optical microscopy and electron backscattered diffraction showed that the single crystals consisted of a single grain. This semicircle disappeared after long-term (up to 3 months) annealing of the single crystals at 950 degrees C, also leading to a reduction in the bulk conductivity. In order to explain these observations, single-crystal X-ray diffraction studies were performed both before and after annealing. These studies found the undoped crystal conformed to P6(3)/m, but with the 0(3) oxygen positions, that participate in conduction, split nonstatistically across two sites with a shortened Si-O(3) bond. Consequently, the bond valence sum (BVS) of the Si (4.20) is larger than the formal valence. Fourier difference maps of the Al-doped crystals contain regions of excess scattering, suggesting the possible lowering of symmetry or creation of superstructures. After long-term annealing, the single crystal structure determinations were of higher quality and the experimental and nominal compositions were in better agreement. From these observations, we propose that in the as-prepared single crystals there are regions of high and low interstitial content (e.g., Nd9.67Si6O26.5 and Nd9.33Si6O26), and the second semicircle relates to the interface between such regions. On annealing, Nd redistribution and homogenization removes these interfaces and also reduces the number of interstitial oxide ions, hence eliminating this second semicircle while reducing the bulk conductivity. The results therefore show for the first time that the conductivity of apatite materials containing cation vacancies is affected by the thermal history.© 2013, American Physical Society.en_AU
dc.identifier.citationAn, T., Baikie, T., Wei, F. X., Pramana, S. S., Schreyer, M. K., Piltz, R. O., Shin, J. F., Wei, J., Slater, P. R., & White, T. J. (2013). Crystallographic correlations with anisotropic oxide ion conduction in aluminum-doped neodymium silicate apatite electrolytes. Chemistry of Materials, 25 (7), 1109-1120. doi:10.1021/cm4000685en_AU
dc.identifier.govdoc5124en_AU
dc.identifier.issn0897-4756en_AU
dc.identifier.issue7en_AU
dc.identifier.journaltitleChemistry of Materialsen_AU
dc.identifier.pagination1109-1120en_AU
dc.identifier.urihttp://dx.doi.org/10.1021/cm4000685en_AU
dc.identifier.urihttp://apo.ansto.gov.au/dspace/handle/10238/4762en_AU
dc.identifier.volume25en_AU
dc.language.isoenen_AU
dc.publisherAmerican Chemical Societyen_AU
dc.subjectSolid electrolytesen_AU
dc.subjectApatitesen_AU
dc.subjectNeodymium silicatesen_AU
dc.subjectCrystal structureen_AU
dc.subjectOxygenen_AU
dc.subjectOxidesen_AU
dc.titleCrystallographic correlations with anisotropic oxide ion conduction in aluminum-doped neodymium silicate apatite electrolytesen_AU
dc.typeJournal Articleen_AU
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