Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type and increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for and and vary in the range 0.96-0.99 for other compositions. Doping of with iron results in an increasing fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric apatites stabilize the state. Among the studied materials, the highest ionic and electronic transport is observed for where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are K −1 at 300-1250 K. © 2004 The Electrochemical Society. All rights reserved.