Insights on the phase transitions, stability and conductivity in the Bi2O3-WO3 system

Equilibrium phases of the Bi2O3-WO3 system, synthesized using the citrate-gel method with slow cooling, have been systematically investigated for samples with 22–27 mol% WO3. Annealing temperature and content of WO3 were shown to play a significant role in the phases present. Both powder X-ray diffraction and Raman spectroscopy revealed that the room temperature equilibrium phases were 7Bi2O3-WO3 and 7Bi2O3-2WO3, with the latter being dominant. Variable temperature Raman spectroscopy showed that both these phases were present up to 850 °C, with the possible formation of a third unidentified phase at higher temperatures. The ionic conductivities of the mixed-phase materials were between that for the pure 7Bi2O3-WO3 and 7Bi2O3-2WO3 phases and at 700 °C was only about three times lower than that of the pure defect fluorite phase of the 22 mol% WO3 samples. The Arrhenius plots showed no sudden increase in conductivity between 300 and 750 °C providing evidence that no major phase change occurred in this temperature range.