Influence of trivalent Gd and Dy codoping on the structure and electrical conductivity of pyrochlore-type Sm ₂Zr ₂O ₇

SmDy _1-xGd _xZr ₂O ₇ (0 ≤ x ≤ 1.0) ceramics are prepared by solid state reaction method at 1973 K for 10 h in air. The crystal structure and microstructure of SmDy _1-xGd _xZr ₂O ₇ ceramics are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All the SmDy _1-xGd _xZr ₂O ₇ ceramics (x = 0.2, 0.4, 0.6, 0.8, 1.0) exhibit a pyrochlore structure except for SmDyZr ₂O ₇ ceramic. SmDyZr ₂O ₇ ceramic has a defective fluorite structure. The incorporation of Gd ³⁺ facilitates a structural evolution from a defective fluorite into an ordered pyrochlore-type structure. Electrical conductivity of SmDy _1-xGd _xZr ₂O ₇ ceramics has been investigated by impedance spectroscopy over a frequency range of 1 Hz to 8 MHz from 673 to 1173 K. The grain conductivity of SmDy _1-xGd _xZr ₂O ₇ ceramics follows the Arrhenius behavior. The grain conductivity of SmDy _1-xGd _xZr ₂O ₇ ceramics increases obviously with increasing Gd ³⁺ content in the temperature range of 673-1073 K, and the optimal grain conductivity value is 1.90 × 10 ^-2 S cm ^-1 in the composition of SmGdZr ₂O ₇ ceramic at 1173 K. All the SmDy _1-xGd _xZr ₂O ₇ ceramics are purely oxide ionic conductors in the test temperature range. Highlights: Phase transition from fluorite to pyrochlore occurs in SmDy _1-xGd _xZr ₂O ₇ ceramics. Electrical conductivity of Sm ₂Zr ₂O ₇ is significantly improved by codoping strategy. The grain conductivity of SmGdZr ₂O ₇ ceramic is 1.90 × 10 ^-2 S cm ^-1 at 1173 K. SmDy _1-xGd _xZr ₂O ₇ ceramics are purely oxide-ion conductors in test temperature range.