The O ionic conductivity of apatite-type La9.83Si4.5Al1.5–yFeyO26±δ (y = 0 to 1.5), La10–xSi6–yFeyO26±δ (x = 0-0.77; y = 1-2), and La7–xSr3Si6O26–δ (x = 0-1) increased with increasing O content. The ion transference numbers, determined by faradaic efficiency measurements at 973 to 1223K in air, were close to unity for La9.83Si4.5Al1.5–yFeyO26+δ and La10Si5FeO26.5, and varied from 0.96 to 0.99 for other compositions. Doping of La9.83(Si, Al)6O26 with iron resulted in an increasing Fe4+ fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La10(Si, Fe)O26+δ apatites stabilized the Fe3+ state. Among the studied materials, the highest ionic and electronic transport was observed for La10Si5FeO26.5, where O interstitials were close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the O partial pressure confirmed that the ionic conduction in Fe-substituted apatites remained dominant under solid oxide fuel cell operation conditions. However, reducing PO2 leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which was similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low PO2, and probably partly suppressed the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air were 8.8 x 10–6 to 9.9 x 10–6/K at 300 to 1250K.

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes. V.V.Kharton, A.L.Shaula, M.V.Patrakeev, J.C.Waerenborgh, D.P.Rojas, N.P.Vyshatko, E.V.Tsipis, A.A.Yaremchenko, F.M.B.Marques: Journal of the Electrochemical Society, 2004, 151[8], A1236-46