The class of oxygen-ion-conducting rare-earth pyrochlores has been considerably extended. New solid electrolytes, Ln₂Ti₂O₇ (Ln = Dy-Lu) and Ln₂Hf₂O₇ (Ln = Eu, Gd) pyrochlores, are intrinsic ionic conductors at elevated temperatures, as are the well known Ln₂Zr₂O₇ (Ln = Sm-Gd) zirconates, which suggests that oxygen ion conduction in the rare-earth pyrochlore family has a general character. The thermodynamic order-disorder transitions that yield a PII cation- and anion-disordered pyrochlore phase possessing high oxygen ion conductivity occur throughout the rare-earth pyrochlore family: Ln₂M₂O₇ (Ln = Sm-Lu; M = Ti, Zr, Hf). The composition-structure-oxygen-ionic conductivity relationship is analyzed for Ln₂(M_{2 − x} Ln _x )O_{7 − δ} (Ln = Sm-Lu; M = Ti, Zr, Hf) with x from 0 to 0.81.

Arrhenius plots of the total conductivity in (1 and 3) dry and (2 and 4) wet air of two Yb ₆ MoO ₁₂ polymorphs: (1 and 2) bixbyite ( Ia 3̄(206)) Yb ₆ MoO ₁₂ and (3 and 4) rhombohedral ( R 3̄) Yb ₆ MoO ₁₂ ; Yb ₆ MoO ₁₂ rhombohedral ( R 3̄) → Yb ₆ MoO ₁₂ bixbyite ( Ia 3̄(206)) at T ∼ 1600 °C.

The tolerance factor is a good criterion to understand the structural transitions in Ln_2−xCa_xScMO_7−δ (M = Nb, Ta).

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.