A new electrolyte based on Tm3 +-doped δ-Bi2O3-type phase with enhanced conductivity

• The single δ-Bi2O3-type phases were successfully synthesized in (Bi1–xTmx)2O3 system if 0.08 < x ≤ 0.20.
• The promising electrolyte with extremely high ionic conductivity and enhanced stability was obtained in (Bi1 – xTmx)2O3 system if x = 0.20.

The possibility to stabilize the δ-Bi2O3-type polymorph with high ionic conductivity by doping with Tm2O3 was investigated in this study. Five microcrystalline samples were synthesized at 750 °C from α-Bi2O3 and Tm2O3 mixtures with compositions (Bi1–xTmx)2O3, where x = 0.04, 0.08, 0.11, 0.14 and 0.20. The XRD, SEM, HRTEM and SAED results showed that the targeted δ-Bi2O3-type single-phase (space group Fm3¯m) was formed for 0.11 ≤ x ≤ 0.20. The unit cell parameter of δ-Bi2O3-type phase decreases with increase in Tm content. According to DTA, no phase transitions were observed in the sample with x = 0.20, indicating that the obtained δ-Bi2O3-type phase is structurally stable from room temperature to 985 °C. Based on EIS, (Bi0.80Tm0.20)2O3 exhibits high conductivity (0.117 S cm–1 at 550 °C) with activation energies: 0.38(4) eV above 550 °C and 1.27(2) eV below 550 °C, due to the change in charge carrier mobility. The effect of thermal aging was investigated and discussed.