Highly conductive cubic (Y0.25Bi0.75) 2O3 tends to transform to rhombohedral (Y 0.25Bi0.75)2O3 when annealed at 600 °C for more than 200 h. Although the rhombohedral phase of (Y 0.25Bi0.75)2O3 was known to be the stable phase at temperatures ≤600 °C, it was found that the annealed (Y0.25Bi0.75)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 (Y0.25Bi0.75)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 (Y0.25Bi0.75)2O3 degraded and became flaky powder. Scanning electron microscopy micrographs of water-reacted (Y0.25Bi0.75)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 (Y0.25Bi0.75)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 0.25Bi0.75)2O3. The better stability of (Y0.25Bi0.75)2O3 at elevated temperature was observed.
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