Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La9.83Si4.5Al1.5−yFeyO26+δ (y=0.5–1.5) leads to increasing unit cell volume, fraction of Fe4+, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973–1223 K in air, are in the range 0.986–0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10−2 Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La9.83(Si,Al)6O26+δ apatites is dominant. Due to stabilization of Fe3+, substantially worse transport properties are observed for A-site stoichiometric La10Si4Fe2O26, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe4+ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La10−x(Si,Al)6O26+δ ceramics, calculated from dilatometric data in air, are 8.9×10−6 to 9.9×10−6 K−1 at 300–1250 K.