Sm2−xCaxZr2O7−x/2 (x = 0, 0.05, 0.1) and Gd2−xCaxZr2O7−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 Sm2−xCaxZr2O7−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 Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) is ~ 1 × 10−4 S/cm. The fluorite-like Gd2−xCaxZr2O7−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 Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) to fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2−xCaxZr2O7−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 (Ln2−xCax)Zr2O7−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 Ln2−xCaxZr2O7−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.

No abstract

Arrhenius plots of the total conductivity in (1 and 3) dry and (2 and 4) wet air of two Yb6MoO12 polymorphs: (1 and 2) bixbyite (Ia3̄(206)) Yb6MoO12 and (3 and 4) rhombohedral (R3̄) Yb6MoO12; Yb6MoO12 rhombohedral (R3̄) → Yb6MoO12 bixbyite (Ia3̄(206)) at T ∼ 1600 °C.