The substitution of manganese with nickel in LaSr 2 Mn 2 O 7− δ , where the solubility limit corresponds to approximately 25% Mn sites, enhances the Ruddlesden–Popper phase stability at elevated temperatures and atmospheric oxygen pressure. The total conductivity of LaSr 2 Mn 2− y Ni y O 7− δ (y=0–0.4) decreases with nickel additions, whilst the average thermal expansion coefficients calculated from dilatometric data in the temperature range 300–1370K increase from (11.4–13.7)×10 −6 K −1 at y=0 up to (12.5–14.4)×10 −6 K −1 at y=0.4. The conductivity and Seebeck coefficient of LaSr 2 Mn 1.6 Ni 0.4 O 7− δ , analyzed in the oxygen partial pressure range 10 −15 –0.3atm at 600–1270K, display that the electronic transport is n-type and occurs via a small polaron mechanism. Reductive decomposition is observed at the oxygen pressures close to Ni/NiO boundary, namely ∼2.3×10 −11 atm at 1223K. Within the phase stability domain, the electronic transport properties are essentially p(O 2 )-independent. The steady-state oxygen permeability of dense LaSr 2 Mn 1.6 Ni 0.4 O 7− δ membranes is higher than that of (La,Sr)MnO 3− δ , but lower if compared to perovskite-like (Sr,Ce)MnO 3− δ . Porous LaSr 2 Mn 1.6 Ni 0.4 O 7− δ cathodes in contact with apatite-type La 10 Si 5 AlO 26.5 solid electrolyte exhibit, however, a relatively poor electrochemical performance, partly associated with strong cation interdiffusion between the materials.
