Effect of ionic radius on colossal permittivity properties of (A, Ta) co-doped TiO2 (A= alkaline-earth ions) ceramics

(A, B) co-doped TiO2 ceramics attract great interests due to the excellent dielectric properties. In this work, the (A, Ta) co-doped TiO2 ceramics were prepared by a solid state reaction process. The effect of the acceptors ionic radius on the structure and properties of TiO2 ceramics was investigated. According to XRD analysis, the main phase is rutile TiO2 for all samples. Due to the larger ionic radius, it is hard to replace Ti site in TiO6 octahedron. As a result, the content of the secondary phase increased with increasing ionic radius. The dielectric properties were significantly enhanced by co-doping of alkaline-earth ions and tantalum ions, and the best dielectric constant obtained at 3% (Sr, Ta) co-doped compositions, where ε' = 2.1 × 105, tanδ = 0.21. Meanwhile, the XPS analysis suggested that the concentration of the defect dipoles exhibit a maximum in Sr-doped TiO2 ceramics. The larger ionic radius of the acceptors leads to the more stability of the defect structure. However, for Ba ions, the replacement concentration decreased due to the excessive ionic radius, which in turn reduces the defect concentration. This work is meaningful for the further investigations on TiO2-based colossal permittivity materials.