Low temperature phase transition phenomena in Ba- and Pb-substituted La2Mo2O9 oxide ion conductors

The low-temperature phase transition phenomena in Ba- and Pb-substituted La(2)Mo(2)O(9)s have been investigated by means of thermal analysis, TEM observation and conductivity measurements. The low-temperature stable phase appears even in Ba- and Pb-substituted La2Mo2O9 by annealing the supercooled beta-phase at around 400 degrees C as the case of Bi-substituted system. After annealing the samples at different temperatures and time, the observed endothermic heat at beta'-beta transition was fitted to Avrami's equation to discuss the kinetics of the transformation. From the activation energies of the obtained rate constant, it was found that beta'-phase formation proceeds easier in Ba- and Pb-substituted La2Mo2O9 rather than in Bi-substituted system. HR-TEM observation suggested the superstructure of beta'-phase, and the thermal hysteresis in conductivity also supported the formation of beta'-phase as detected by thermal analysis.