Grain boundary segregation and the formation mechanism of secondary-phase in (Ce,Nb)-codoped TiO₂ ceramics

Highlights: • There exist trivalent valence state Ce³⁺ and four valence state Ce⁴⁺ in TiO₂ ceramics. • Secondary phases are originated from segregation of point defects Nb_Ti^· and Ce´_Ti. • The Ce´_Ti segregation driving force is mainly the elastic strain energy. • With sintering temperature increasing, second phases transit from CeNbO₄ to CeNbTiO₆. Secondary phases are common phenomena in doped titanium dioxide ceramics. Effects of secondary phases on the structure and performance of the doped TiO₂ ceramic are crucial. Both boundaries segregation and formation mechanism of secondary phases should be comprehensively explored to enhance properties and application. (Ce,Nb)-codoped TiO₂ ceramic samples were prepared from TiO₂, Nb₂O₅, and CeO₂ oxide powders by a traditional solid-state sintering method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and wavelength-dispersive X-ray spectroscopy (WDS) were conducted to determine the sample microstructures, crystal structures, ionic valences, and chemical composition. The point defect distributions at the grain boundaries were calculated via point defect thermodynamic analysis, and the effects of Ce⁴⁺ and Ce³⁺ on segregation at the grain boundaries were compared. The XPS analysis showed that both ions are present in the TiO₂ ceramics, and that the Ce³⁺ content increases with increasing sintering temperature. Further, secondary phases originate from segregation of the Nb_Ti^· and Ce´_Ti point defects at the grain boundaries. The driving force for segregation is primarily the elastic strain energy, and CeNbO₄ and CeNbTiO₆ secondary phases occur in the (Ce,Nb)-codoped TiO₂ ceramics. With increased sintering temperature, the secondary phase transforms from CeNbO₄ to CeNbTiO₆.