Copper-doped hexagonal barium titanate ceramics

The crystallographic phase, microstructure and dielectric properties of BaTiO3+0.02 BaO+x CuO ceramics are studied at various Cu-doping level (0⩽×⩽0.02). It is confirmed by electron paramagnetic resonance that CuTi2+ occupies Ti lattice-sites. Tetragonal and hexagonal phase coexist at room temperature for x⩾0.003 (air-sintered, 1400 °C). The portion of tetragonal phase decreases with x, leading to a decrease and broadening of the dielectric anomaly at the Curie temperature. Although, as compared to other 3d transition dopants (e.g., Mn), the hexagonal phase is stabilized at room temperature at smaller Cu-concentration, the tetragonal phase does not vanish completely even at higher doping level. Grains with exaggerated, plate-like shape (mean grain size >100 μm) are attributed to the hexagonal phase. We suggest that Jahn-Teller distortion due to the d9 electron configuration of CuTi2+ represents the driving force for the cubic–hexagonal phase transition.