Effect of water/water vapor on microstructure and phase stability of (Y_0.25Bi_0.75)₂O₃ solid electrolytes

Highly conductive cubic (Y_0.25Bi_0.75) ₂O₃ tends to transform to rhombohedral (Y _0.25Bi_0.75)₂O₃ when annealed at 600 °C for more than 200 h. Although the rhombohedral phase of (Y _0.25Bi_0.75)₂O₃ was known to be the stable phase at temperatures ≤600 °C, it was found that the annealed (Y_0.25Bi_0.75)₂O₃ was not thermodynamically stable in the water-containing environment. From x-ray diffraction and transmission electron microscopy analysis, it was observed that the annealed (Y_0.25Bi_0.75)₂O₃ easily decomposed into monoclinic α-Bi₂O₃ and yttrium hydroxide at a temperature as low as 50 °C. The monoclinic α-Bi₂O₃ further reacted with CO₂ and formed Bi₂O₂CO₃. Consequently, the annealed (Y_0.25Bi_0.75)₂O₃ degraded and became flaky powder. Scanning electron microscopy micrographs of water-reacted (Y_0.25Bi_0.75)₂O₃ also showed surface swelling and peeling. Such surface deterioration was caused by a large volume increase during the water reaction. Similar reaction was also observed when the annealed (Y_0.25Bi_0.75)₂O₃ was exposed in the humidified air at 300 °C. As the temperature was raised to 500 °C, little reaction was observed between water vapor and (Y _0.25Bi_0.75)₂O₃. The better stability of (Y_0.25Bi_0.75)₂O₃ at elevated temperature was observed.