Stability of (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) and (Tb0.9Ca0.1)2Ti2O7−δ pyrochlores under redox conditions

The stability of (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) pyrochlores under different redox conditions (air, dry and wet H2) has been studied. The bulk conductivity of the terbium-containing materials is slightly higher under reducing conditions (5% H2 + 95% Ar) than in air, reaching ~ 6 × 10− 2 and 2.5 × 10− 2 S/cm at 800 °C for (Yb0.8Ca0.1Tb0.1)2Ti2O7−δ and (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ, respectively. In air–hydrogen–air cycles, the bulk conductivity returns to its original level in air: ~ 2 × 10− 2 and 1 × 10− 2 S/cm at 800 °C in (Yb0.8Ca0.1Tb0.1)2Ti2O7−δ and (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ, respectively. Exposure of (Dy0.8Ca0.1Tb0.1)2Ti2O7−δ to a flowing mixture of 5% H2 and 95% Ar for 100 h slightly reduces its conductivity: from 0.029 to 0.023 S/cm at 900 °C. The cerium-containing materials (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ and (Dy0.8Ca0.1Ce0.1)2Ti2O7−δ were found to be unstable under reducing conditions. The bulk conductivity of (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ is practically independent on the atmosphere while the grain boundary conductivity increased from 5.6 × 10− 8 in air to ~ 1 × 10− 5 S/cm at 425 °C on reducing the sample in 5%H2–Ar gas mixture. However, after two redox cycles air–5%H2–Ar–air the sintered pellet of (Yb0.8Ca0.1Ce0.1)2Ti2O7−δ fractured whereas (Dy0.8Ca0.1Ce0.1)2Ti2O7−δ degraded immediately under reducing conditions. observed that the size mismatch between Ln and Ln´ cations strongly affects the bulk to grain boundary conductivity ratio in (Ln0.8Ca0.1Ln′0.1)2Ti2O7−δ (Ln=Dy, Yb; Ln′=Ce, Tb) pyrochlores.