Defect Complex Effect in Nb Doped TiO2 Ceramics 
with Colossal Permittivity
Donor-doped NbxTi1−xO2 (x = 1%, 2%, 4%, 6%, and 8%) ceramics with giant permittivity (>104) and a very low dielectric loss (∼0.05) were sintered under flowing N2 at 1400°C for 10 h. By increasing Nb doping concentration, two different dielectric responses were evidenced in the frequency dependence of dielectric properties of Nb doped TiO2 ceramics, which corresponded to the space charge polarization and the electron-pinned defect-dipoles effect, respectively. Especially, combined with the x-ray photoelectron spectroscopy results, the electron-pinned defect-dipoles induced by the $$2({\hbox{Nb}}^{5 + } )_{\rm{Ti}}^{ \bullet } \to 4({\hbox{Ti}}^{3 + } )^{\prime}_{\rm{Ti}} \leftarrow {\hbox{V}}_{\rm{o}}^{ \bullet \bullet }$$2(Nb5+)Ti∙→4(Ti3+)Ti′←Vo∙∙ defect complex were further confirmed to give rise to both their high εr and low tan δ in the high frequency range for the NbxTi1−xO2 ceramics with x > 4%.