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.