The electrochemical properties of Mg-, Fe- or Co-substituted K2NiF4-type oxide solid solutions were investigated, and compared with those of the In-substituted system (which was known to be a protonic conductor). The samples were prepared by using conventional solid-state reaction methods. The solubility limit of Mg was equal to about 0.05 (50% of the solubility limit of 0.1 for In). The dependence of the conductivity of Sr2Ti1-xMgxO4 upon x was very similar to that of the conductivity of Sr2Ti1-xInxO4 upon x/2. The electromotive forces of H or O concentration cells were also the same for the 2 materials. These results indicated that, in the present system, the predominant factor which governed the phase relationships and protonic conduction properties was the concentration of introduced O vacancies. Single-phase samples of Fe- or Co-substituted solutions, Sr2Ti0.93Fe0.07O4 and Sr2Ti0.93Co0.07O4, were obtained. In the former samples, the dependence of conductivity upon O partial pressure was qualitatively equal to that of the In-analogue; thus indicating that the valence state of Fe was +3 (3d5) in Sr2Ti0.93Co0.07O4, and that 5 d-electrons of Fe3+ were localized. In the case of Sr2Ti0.93Co0.07O4, the dependence of the conductivity upon the O partial pressure was different to that in Sr2Ti1-xMxO4, where M was In, Mg or Fe. There existed neither pure ionic conduction regions at intermediate O partial pressures, nor n-type electronic contributions to conductivity, at low O partial pressures for Sr2Ti0.93Co0.07O4.

Conduction Properties of Mg-, Fe- or Co-Substituted Sr2TiO4 at Elevated Temperatures. T.Shimura, K.Suzuki, H.Iwahara: Solid State Ionics, 1999, 125[1-4], 313-8