CUBIC HEXAGONAL TRANSFORMATION OF YTTRIA-STABILIZED DELTA-BISMUTH SESQUIOXIDE, BI2-2XY2XO3 (X=0.215-0.235)

This paper describes a new phase present in the bismuth-rich portion of the system Bi//2O//3 minus Y//2O//3. The phase has a layered structure and forms a solid solution Bi//2// minus //2//xY//2//xO//3 over a limited range of x between 0. 215 and 0. 235 at 650 degree C. This low-temperature stable hexagonal phase transforms reversibly around 720 degree C into the high-temperature stable modification, which has an oxygen-deficient fluorite-type structure. The high-temperature modification stable between 730 and 825 degree C showed excellent oxide ion conductivity.