Phase Transformation from Cubic to Rhombohedral in Doped Bi2O3-CaO System

Abstract

In the CaO-Bi 2 O 3 system, the rhombohedral phase was easily obtained. However, the cubic phase was hardly observed in this system. It is believed that the amount of oxygen vacancies and the size of doping ions play an important role in the phase transformation of doped Bi 2 O 3 system. In this study, the effect of Nb 2 O 5 addition on the structural stability and conductivity of CaO-Bi 2 O 3 solid electrolyte was investigated. With the addition of Nb 2 O 5 into Bi 1,4 Ca 0,6 O 2.4 , the rhombohedral phase gradually disappeared. In the sample of 6 mol% Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 , all rhombohedral phase was not observed. Instead, a single cubic phase was found. The cubic structure of Bi 1.4 Co 0.6 O 2.4 was stabilized by the addition of Nb 2 O 5 . When 10 mol% Nb 2 O 5 was added into Bi 1.4 Ca 0.6 O 2.4 , a second phase, Bi 5 Nb 3 O 15 , appeared because the solubility of Nb in the cation sublattice was reached. The ionic conductivity of Bi 1.4 Ca 0.6 O 2.4 was also enhanced by the addition of Nb 2 O 5 . The conductivity of 6 mol% Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 was as high as 0.72 S/cm at 800°C. The microstructure of Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 shows good agreement with XRD result. A single and isotropic microstructure was seen from the 6 mol% Nb 2 O 5 doped sample. When the second phase appeared, the suppression of grain growth was observed in 10 mol% Nb 2 O 5 Bi 1.4 Ca 0.6 O 2.4 .