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

Abstract Secondary phases are common phenomena in doped titanium dioxide ceramics. Effects of secondary phases on the structure and performance of the doped TiO 2 ceramic are crucial. Both boundaries segregation and formation mechanism of secondary phases should be comprehensively explored to enhance properties and application. (Ce,Nb)-codoped TiO 2 ceramic samples were prepared from TiO 2 , Nb 2 O 5 , and CeO 2 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 Ce 4 + and Ce 3 + on segregation at the grain boundaries were compared. The XPS analysis showed that both ions are present in the TiO 2 ceramics, and that the Ce 3 + 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 CeNbO 4 and CeNbTiO 6 secondary phases occur in the (Ce,Nb)-codoped TiO 2 ceramics. With increased sintering temperature, the secondary phase transforms from CeNbO 4 to CeNbTiO 6 .