Interstitial Oxide Ion Distribution and Transport Mechanism in Aluminum-Doped Neodymium Silicate Apatite Electrolytes

Rare
earth silicate apatites are one-dimensional channel structures
that show potential as electrolytes for solid oxide fuel cells (SOFC)
due to their high ionic conductivity at intermediate temperatures
(500–700 °C). This advantageous property can be attributed
to the presence of both interstitial oxygen and cation vacancies,
that create diffusion paths which computational studies suggest are
less tortuous and have lower activation energies for migration than
in stoichiometric compounds. In this work, neutron diffraction of
Nd(28+x)/3AlxSi6–xO26 (0 ≤ x ≤ 1.5) single crystals identified the locations
of oxygen interstitials, and allowed the deduction of a dual-path
conduction mechanism that is a natural extension of the single-path
sinusoidal channel trajectory arrived at through computation. This
discovery provides the most thorough understanding of the O2– transport mechanism along the channels to date, clarifies the mode
of interchannel motion, and presents a complete picture of O2– percolation through apatite. Previously reported crystallographic
and conductivity measurements are re-examined in the light of these
new findings.

稀土硅酸磷灰石构成了一维的通道结构，因其在中温范围（500–700 °C）内具备优异的离子电导率，展现出作为固态氧化物燃料电池（SOFC）电解质的巨大潜力。此优势特性可归因于晶格间隙氧和阳离子空位的共存，它们共同构筑了扩散路径。计算研究表明，这些路径相较于化学计量化合物中的路径更为平直，且具有更低的迁移活化能。在本研究中，通过中子衍射对Nd(28+x)/3AlxSi6–xO26（0 ≤ x ≤ 1.5）单晶的测定，确定了氧间隙的位置，并推导出一种双路径传导机制，该机制是计算得出的单路径正弦波通道轨迹的自然延伸。这一发现迄今为止对通道中O2-传输机制的深入理解提供了最全面的解释，明确了通道间运动的模式，并呈现了磷灰石中O2-渗透的完整图像。在新的发现背景下，先前报道的晶体学和电导率测量数据得到了重新审视。