3C-6H phase transition in BaTiO3 induced by Fe ions: an electron paramagnetic resonance study

X-ray diffraction (XRD) patterns and electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) of BaTiO3+0.04 BaO+0.5xFe2O3 (0.0005≤x≤0.02) ceramics were studied to investigate the development of the hexagonal phase (6H modification) of Fe-doped material in dependence upon doping level x and sintering temperature Ts. Three axially symmetric EPR spectra assigned to Fe3+ ions substituted at Ti lattice sites corresponding to the different distorted octahedra in tetragonal and hexagonal modification are observed at room temperature. The presence of a hexagonal phase is shown by the XRD pattern and the EPR spectra. The 6H modification begins to occur at a nominal Fe concentration of between x = 0.005 and 0.01 and increases with increasing sintering temperature. BaTiO3 ceramics with x = 0.02 sintered at Ts = 1400 deg. C is hexagonal. As discussed in the case of other 3d ions we propose the Jahn-Teller (JT) distortion as the driving force for the cubic-hexagonal transition at high temperatures whereas a sufficiently high concentration of oxygen vacancies is the second condition for this transformation process. In the case of Fe-doped BaTiO3 a minimum concentration of Fe4+ in the ceramic grains could trigger the transmutation into the hexagonal phase.