Hexagonal BaTi_1-xCo_xO₃ phase stabilized by Co dopants

The phase transition from tetragonal to hexagonal of crystalline BaTi _1-xCo_xO₃ (0.01≤x≤0.20) powders prepared by solid state reaction has been studied by x-ray diffraction and x-ray absorption fine structure (XAFS). At the low Co doping level of x≤0.03, the structure of the samples is tetragonal, and it is transformed gradually to a hexagonal structure upon increasing Co content to ≥0.05. The detailed analysis of Co K -edge XAFS indicated unambiguously that the doped Co ions are substantially incorporated into the BaTiO₃ host and likely serve as a trigger in leading the structure transition. On the basis of the formation energy calculations, we find that there exists a critical Co concentration in the x range of 0.027-0.055 for inducing the tetragonal-to-hexagonal structure transition of BaTi_1-xCo_xO₃, in good agreement with the experimental observations.