Phase relations of Bi2O3-rich Bi2O3–Er2O3 system: The appearance of a new stable orthorhombic phase (Bi2O3)0.72(Er2O3)0.28 against the known oxide-ion conductive hexagonal phase

In the system Bi2O3–Er2O3, three intermediate phases were reported to date: the δ-Bi2O3 type fcc phase, the hexagonal phase with layered structure, and the triclinic compound at 49 mol% Er2O3. The present study proved the hexagonal phase to be metastable; in equilibrium state around 650 °C, it disappears and the low-temperature stable orthorhombic phase appears at 28 mol% Er2O3, i.e., (Bi2O3)0.72(Er2O3)0.28 (a = 10.7817(7) Å, b = 16.301(1) Å, c = 5.5299(5) Å and Z = 12) which transforms to the high-temperature stable fcc phase around 825 °C upon heating. The existence of this new orthorhombic phase implies that the fcc stabilization by Er2O3 is impossible. Considerably low electrical conductivity of (Bi2O3)0.72(Er2O3)0.28 causes the time-dependent degradation of the fcc phase at lower temperatures on account of the extremely sluggish transformation of the fcc phase to (Bi2O3)0.72(Er2O3)0.28.