The possibility to stabilize the delta-Bi2O3-type polymorph with high ionic conductivity by doping with Tm2O3 was investigated in this study. Five microaystalline samples were synthesized at 750 degrees C from alpha-Bi2O3 and Tm2O3 mixtures with compositions (Bi1-xTmx)(2)O-3, where x = 0.04, 0.08, 0.11, 0.14 and 0.20. The XRD, SEM, HRTEM and SAED results showed that the targeted delta-Bi2O3-type single-phase (space group Fm (3) over barm) was formed for 0.11 <= x <= 0.20. The unit cell parameter of delta-Bi2O3-type phase decreases with increase in Tm content. According to DTA, no phase transitions were observed in the sample with x = 020, indicating that the obtained delta-Bi2O3-type phase is structurally stable from room temperature to 985 degrees C. Based on EIS, (Bi0.80Tm0.20)(2)O-3 exhibits high conductivity (0.117 S cm(-1) at 550 degrees C) with activation energies: 0.38(4) eV above 550 degrees C and 127(2) eV below 550 degrees C, due to the change in charge carrier mobility. The effect of thermal aging was investigated and discussed. (C) 2015 Elsevier B.V. All rights reserved.