The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5Ϫy Fe y O 26Ϯ␦ (y ϭ 0-1.5), La 10Ϫx Si 6Ϫy Fe y O 26Ϯ␦ (x ϭ 0-0.77; y ϭ 1-2), and La 7Ϫx Sr 3 Si 6 O 26Ϫ␦ (x ϭ 0-1) 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 La 9.83 Si 4.5 Al 1.5Ϫy Fe y O 26ϩ␦ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4ϩ fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe͒O 26ϩ␦ apatites stabilize the Fe 3ϩ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , 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 (O 2 ) 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 p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) ϫ 10 Ϫ6 K Ϫ1 at 300-1250 K.

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Incorporation of alkaline-earth cations into the zircon-type lattice of Ce 1 2 x A x VO 4 (A ~Ca, Sr; x ~0-0.2) leads to higher p-type electronic conductivity, while the tetragonal unit cell volume and Seebeck coefficient decrease due to increasing concentration of electron holes localised on cerium cations. The oxygen ion transference numbers of Ce 1 2 x Ca x VO 4 in air, determined by faradaic efficiency measurements, vary in the range from 2 6 10 24 to 6 6 10 23 at 973-1223 K, increasing with temperature. The ionic conductivity is essentially independent of calcium content and decreases with reducing oxygen partial pressure. The activation energy for ionic transport in Ce(Ca)VO 4 is 90-125 kJ mol 21 . Doping with calcium enhances the stability of cerium orthovanadate at reduced oxygen pressures, shifting the phase decomposition limits down to oxygen activity values of 10 216 -10 214 atm at 1023 K. The results on structure, Seebeck coefficient, and the partial p-type electronic and oxygen ionic conductivities suggest the presence of hyperstoichiometric oxygen in the Ce 1 2 x A x VO 4 1 d lattice. The hyperstoichiometry, estimated from Seebeck coefficient data in the p(O 2 ) range from 10 219 to 0.75 atm at 923-1223 K, may achieve 2-3% of the total oxygen content and weakly depends on the temperature and oxygen pressure variations within the zircon phase existence domain. Thermal expansion coefficients of Ce 1 2 x A x VO 4 1 d ceramics in air, calculated from dilatometric data, are in the narrow range (5.6-5.9) 6 10 26 K 21 at 400-800 K.

The oxygen permeability of dense garnet-type Gd 3Ϫx A x Fe 5 O 12Ϯ␦ (A ϭ Ca,Pr;x ϭ 0-0.8) and Y 3ϪxϪy Ca x Nd y Fe 5Ϫz Ni z O 12Ϫ␦ (x ϭ 0-0.5;y ϭ 0-0.25;z ϭ 0-1.0) membranes at 1173-1273 K is limited by the bulk ambipolar conductivity. The ion transference numbers, calculated from results on the oxygen permeation and total conductivity, vary from 1 ϫ 10 Ϫ5 to 5 ϫ 10 Ϫ3 , increasing with temperature. Ionic conduction in ferrite garnets, primarily determined by the oxygen vacancy concentration, increases with acceptor dopant additions. The activation energies for the oxygen ionic and electronic transport in air are in the ranges 176-224 and 20-81 kJ/mol, respectively. The ceramic microstructure of garnet-based materials has no essential effect on their ionic conductivity, which is low compared to that of perovskite-related ferrites. The low mobility of oxygen ions, probably limited by the ion transfer along the edges of Fe-O tetrahedra in the garnet lattice, is likely to result from a crooked diffusion pathway. Decreasing the A-site cation radius leads to a higher ionic conductivity of garnet phases. The thermal expansion coefficients of ferrite garnet ceramics at 300-1200 K in air are in the range (9.4-10.9) ϫ 10 Ϫ6 K Ϫ1 .