Sm_2−xCa_xZr₂O_7−x/2 (x = 0, 0.05, 0.1) and Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore–fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100–900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700–950 °C. The Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen–ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) is ~ 1 × 10⁻⁴ S/cm. The fluorite-like Gd_2−xCa_xZr₂O_7−x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) to fluorite-like Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd_2−xCa_xZr₂O_7−x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln_2−xCa_x)Zr₂O_7−δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln_2−xCa_xZr₂O_7−x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO₃-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

Conductivity, structure and thermochemical stability of compositions in the following series (100-x)La0.6Sr0.4CoO3•xCeO2 (LSCC), (100-x)La0.8Sr0.2MnO3•xCeO2 (LSMC) and (100-x)La0.95Ni0.6Fe0.4O3•xCeO2 (LNFC), where x= 0, 2 and 10 mol%, were studied. CeO2 has slightly higher solubility limit in La0.6Sr0.4CoO3 and La0.8Sr0.2MnO3 compared to La0.95Ni0.6Fe0.4O3, forming LSCC2 and LSMC2 single phase solid solutions. All compositions containing more than 2 mol% ceria were two phase and consisted of a perovskite constituent with rhombohedral structure and ceria with cubic structure. Small additions of CeO2 influence the conductivity of the modified perovskites in a different way. Conductivity in the LSCC series dramatically decreased. Adding up to 10 mol % CeO2 slightly changed the total conductivity in the LSMC series in spite of the presence of the secondary phase. The conductivity of the perovskites as a function of oxygen partial pressure (0.21-10-4 atm) is discussed.