Crystallographic Correlations with Anisotropic Oxide Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes

To 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 )/3 Al x Si 6- x O 26 (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 °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 P 6 3 / m, but with the O(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., Nd 9.67 Si 6 O 26.5 and Nd 9.33 Si 6 O 26 ), 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.