Effect of high oxygen deficiency in nano-confined bismuth sesquioxide

Bismuth sesquioxide in its cubic form, i.e. δ-Bi2O3, is the fastest oxygen ionic conductor known with important applications in energy technologies. However, the material is unstable as it undergoes to high-density polymorphic transitions and degradation. In this work, we show that δ-Bi2O3 can be stabilized both at high and low temperatures (T < 775 °C) under low oxygen partial pressure (pO2 < 10-5 atm), where the material is nanostructured in multi-layered thin film coherent heterostructures with yttrium stabilized zirconia (YSZ). DFT calculation confirms such a form of metastability, also showing that high oxygen defect concentration favors the cubic phase. Moreover, high oxygen deficiency in the nanoionics leads to an unexpected "two-regime" conductivity with high values (σ > 1 S cm-1 at 600 °C) at high pO2 and lower ionic conductivity (σ ~ 0.1 S cm-1 at 600 °C) at low pO2. Ionic conductivity at low pO2 occurs with high activation energy (Ea > 1.5 eV), suggesting thus a drastic decrease in mobility for high concentration of defects.