Role of a low level of La2O3 dopant on the tetragonal-to-monoclinic phase transformation of ceria-yttria co-stabilized zirconia

Yttria, yttria-ceria and yttria-ceria-lanthana stabilized zirconia powders were prepared by coprecipitation. Their tetragonal-to-monoclinic phase (t -> m) transformation was investigated by calcining the powders in a temperature range of 400-1400 degrees C for 2 h. The results show that after doping with 0.1 to 0.3 mol.% La2O3 and calcining at 1400 degrees C in air, unusual redox behaviours of cerium were detected in the 1.5 mol.% Y2O3+5.5 mol.% CeO2 co-stabilized zirconia. Grain refinement and a sharp reduction in oxygen vacancy concentration were observed simultaneously. The t -> m transition was not found in the 0.1 mol.% La2O3 doped zirconia but appeared in the cases with a higher dopant content. The changes are discussed with regards to the grain size, valence change of cerium, presence of oxygen vacancies, and segregation of the dopants at grain boundaries.