Evolution of Oxygen–Ion and Proton Conductivity in Ca-Doped Ln(2)Zr(2)O(7) (Ln = Sm, Gd), Located Near Pyrochlore–Fluorite Phase Boundary

Sm(2−x)Ca(x)Zr(2)O(7−x/2) (x = 0, 0.05, 0.1) and Gd(2−x)Ca(x)Zr(2)O(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−x)Ca(x)Zr(2)O(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−x)Ca(x)Zr(2)O(7−x/2) (x = 0.05, 0.1) is ~ 1 × 10(−4) S/cm. The fluorite-like Gd(2−x)Ca(x)Zr(2)O(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−x)Ca(x)Zr(2)O(7−x/2) (x = 0.05, 0.1) to fluorite-like Gd(2−x)Ca(x)Zr(2)O(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−x)Ca(x)Zr(2)O(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−x)Ca(x))Zr(2)O(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−x)Ca(x)Zr(2)O(7−x/2) (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO(3)-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.