Hexagonal BaTi1-xCoxO3 phase stabilized by Co dopants

The phase transition from tetragonal to hexagonal of crystalline BaTi1-xCoxO3 (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 BaTi1-xCoxO3, in good agreement with the experimental observations.