Exploring the ambient metastability of yttrium substituted Bi2O3 electrolyte materials

The phase stability of (Bi1-xYx)2O3 (0.1 ≤ x ≤ 0.275) solid state electrolytes synthesized using the sol-gel method was investigated over prolonged periods under ambient conditions. The cubic polymorph was found to be metastable under ambient conditions, particularly for 0.1 ≤ x ≤ 0.15 where significant cubic-to-tetragonal transformation occurred. Spontaneous minor formation of both the rhombohedral polymorph and a bismuth subcarbonate phase was evident across the doping range but with varied reproducibility. Aging was significantly more prevalent for samples that had only been calcined (at 450 °C) as compared to those that were subsequently annealed (at 750 °C). The more highly substituted annealed materials (0.25 ≤ x ≤ 0.275) were initially phase pure cubic and far more stable, yet some rhombohedral and bismuth subcarbonate phase formation was also seen. Although the initial source of carbon which leads to the formation of the subcarbonate phase is not known at this stage, several suggestions are made. Pelletisation with additional sintering, as well as annealing powder samples under an inert atmosphere, was also shown to enhance the long-term phase stability characteristics of the cubic phase. This is the first longer term (over 2 years) ambient phase aging study of substituted bismuth oxides.