Grain boundary segregation and secondary-phase transition of (La,Nb)-codoped TiO2 ceramic

The formation and transition of secondary phases in (La, Nb)-codoped TiO2 ceramics were investigated using samples prepared by traditional solid-state sintering from anatase TiO2, Nb2O5, and La2O3 oxide powders. From the microstructure, crystal structure and chemical composition of these samples, as measured by SEM, XRD and EDS, the formation mechanism of the secondary phase is determined though point defect thermodynamical analysis. The results show that LaNbO4 and LaNbTiO6 secondary phases originate from segregation of the point defects NbTi· and La′Ti at grain boundaries, with elastic strain energy providing the main driving force for this segregation. An increase in sintering temperature causes more of the LaNbO4 secondary phase to transition to LaNbTiO6. A ternary phase diagram of La2O3–Nb2O5–TiO2 was plotted based on binary phase diagrams of La2O3–Nb2O5, Nb2O5–TiO2 and La2O3–TiO2, which can be used to estimate the relative content of secondary phases in samples sintered at different temperatures.