Crystallographic
Correlations with Anisotropic Oxide
Ion Conduction in Aluminum-Doped Neodymium Silicate Apatite Electrolytes
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Abstract
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<sub>(28+<i>x</i>)/3</sub>Al<sub><i>x</i></sub>Si<sub>6‑<i>x</i></sub>O<sub>26</sub> (0 ≤ <i>x</i> ≤ 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 <i>c</i> 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 <i>P</i>6<sub>3</sub>/<i>m</i>, 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<sub>9.67</sub>Si<sub>6</sub>O<sub>26.5</sub> and Nd<sub>9.33</sub>Si<sub>6</sub>O<sub>26</sub>), 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