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The effect of replacing Sc2O3 with Yb2O3 on the structural and electrical properties of xYb2O3–(12–x)Sc2O3–88ZrO2 has been investigated. Spark plasma sintering technique is employed to fabricate dense bulk samples from the nano‐sized powders. X‐ray diffraction and transmission electron microscopy performed on pellets indicate the existence of cubic and rhombohedral phases in 12ScSZ, and a single cubic phase in all the co‐doped compositions. However, Raman spectroscopic studies suggest the presence of a metastable tetragonal t″‐phase along with rhombohedral phases in 12ScSZ, whereas a single cubic phase in all the co‐doped compositions. Significant enhancement in the conductivity of grain and grain boundary is observed on replacing Sc2O3 with Yb2O3. In the intermediate temperature range, 1Yb11ScSZ exhibits the highest, while 12ScSZ shows the lowest conductivity values, which is attributed to corresponding phases present in that range. Through co‐doping with >1 mol% Yb2O3 leads to conductivity decrease, but the value remains higher than that of 12ScSZ. A sharp conductivity change is observed in 12ScSZ and 1Yb11ScSZ samples, which is attributed to partial phase transition as well to the formation of cation‐vacancy complexes. In this work, the beneficial effect of Yb2O3 co‐doping in 12ScSZ on the phase and conductivity has been highlighted.

The present work studies the effect of thermal aging and 1 mol % Yb 2 O 3 codoping on the metastable tetragonal phases present in 8−9 mol % Sc 2 O 3 −ZrO 2 . Dense ceramic discs of xYb 2 O 3 −(y − x)Sc 2 O 3 −(100− y)ZrO 2 (x = 0 and 1 mol %; y = 8 and 9 mol %) are prepared using conventional solid-state route. XRD, TEM and Raman spectroscopy analysis confirmed the metastable t′phase in 8Sc 2 O 3 −92ZrO 2 and t″-phase in 9Sc 2 O 3 −91ZrO 2 . Although t′-phase is retained on Yb 2 O 3 codoping in 8Sc 2 O 3 −92ZrO 2 , the tetragonality reduces. However, codoping does not affect the t″-phase present in 9Sc 2 O 3 −91ZrO 2 . After aging at 650 °C for 2000 h in air, a decrease in unit-cell parameters ratio and weak XRD peak-splitting are observed in 8Sc 2 O 3 −92ZrO 2 and 1Yb 2 O 3 −7Sc 2 O 3 − 92ZrO 2 suggesting a reduction in unit-cell tetragonality, though t′-phase still exists in both the compositions. In compositions possessing t″-phase, a slight lattice contraction is detected on thermal aging. The tetragonality induced by oxygen-ion shuffling inside the unit-cell increases after aging in all the compositions. This shuffling is lower in the aged sample of 1Yb 2 O 3 − 7Sc 2 O 3 −92ZrO 2 compared to 8Sc 2 O 3 −92ZrO 2 . However, it remains similar in both 1Yb 2 O 3 −8Sc 2 O 3 −91ZrO 2 and 9Sc 2 O 3 − 91ZrO 2 , suggesting a negligible codoping effect on t″-phase. These results demonstrate the effectiveness of Yb 2 O 3 codoping in lowering the tetragonality of metastable t′-phase, which may positively affect the conductivity of ScSZ.

The present work studies the influence of 1 mol % Yb 2 O 3 codoping on the long-term conductivity stability in the metastable tetragonal phases present in 8−9 mol % Sc 2 O 3 −ZrO 2 . Dense polycrystalline samples are synthesized via conventional solid-state reaction and pressureless sintering methods. The 8 mol % dopant containing ZrO 2 compositions contain the metastable t′phase, while the t″-phase exists in the 9 mol % doped compositions. However, the grain conductivity values in all of the compositions are comparable and insensitive to the type of tetragonal phase present in the samples. The total conductivity follows a similar trend except in the 1Yb 2 O 3 −8Sc 2 O 3 −91ZrO 2 specimen, which shows a low total conductivity because of the resistive grain boundaries. Long-term thermal aging is performed on all of the tested samples at 650 °C for 2000 h in air. The in situ conductivity study suggests that the conductivity declines occur primarily in the first 650 h. The total conductivity loss at 650 °C is higher in the t′-phase (∼10−16%) compared to the t″-phase (∼3%). Even though substituting 1 mol % Yb 2 O 3 (for Sc 2 O 3 ) is beneficial in reducing the % conductivity degradation from 16 to 10% in 8Sc 2 O 3 − 92ZrO 2 , it does not affect the long-term conductivity stability in 9Sc 2 O 3 −91ZrO 2 (∼3%). Thus, the 9Sc 2 O 3 −91ZrO 2 composition exhibits excellent temporal stability of oxygen-ion conductivity with the magnitude of 20.4 mS•cm −1 after 2000 h aging at 650 °C in air.

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