New phases in the ZrO2–Bi2O3 and HfO2–Bi2O3 systems

New phases of the type M1−xBixO2−x/2 with a defect fluorite structure have been prepared where M is Zr or Hf. These apparently metastable phases were prepared by precipitation from solution followed by calcination at 600°C. For the Hf1−xBixO2−x/2 system, x ranges from x = 0.40 to x = 0.75. For the Zr1−xBixO2−x/2 system, x ranges from x = 0.50 to x = 0.75. Heating the defect fluorite Hf1−xBixO2−x/2 phases to temperatures of 700 to 950°C leads to the formation of Bi2Hf2O7 with the Bi2Sn2O7 structure. Thus, Bi2Hf2O7 is pseudocubic, but apparently face centered tetragonal with a = 21.66 Å and c = 21.84 Å. When Bi2Hf2O7 is heated to 1000°C, it decomposes into HfO2 and Bi1.94Hf0.06O3.03 with the β-Bi2O3 structure. Heating the defect fluorite Zr1−xBixO2−x/2 phases to higher temperatures never produced a Bi2Zr2O7 phase with the Bi2Sn2O7 structure. Instead, the compositional range of the defect fluorite structure steadily decreases with increasing temperature, and this phase had completely decomposed at 750°C into ZrO2 and Bi1.84Zr0.16O3.08 with the β-Bi2O3 structure.