Origin of giant permittivity in Ta, Al co-doped TiO2: Surface layer and internal barrier capacitance layer effects

In order to achieve optimized electric properties, Ta, Al co-doped TiO2 was researched by carefully tuning the composition of ceramics in this work. X-ray diffraction patterns and Raman spectra co-evidenced that all samples were in pure tetragonal rutile TiO2. (A(10.5)Ta(0.5))(0.08)Ti0.92O2 ceramics showed relatively good dielectric properties including a colossal permittivity (CP) over 104, a low dielectric loss (0.05) and a good temperature stability. Especially, physical origins of giant permittivity were explored by a combined analysis of impedance spectroscopy, XPS and reflection spectra. Impedance and modulus spectrum verified three relaxation peaks indicating the electronic heterogeneity of materials. Combined with nonlinear I-V behavior, it can be concluded that the origin of the CP was attributed to the internal barrier layer capacitance (IBLC) effect. In addition, based on the results of XPS and reflection spectra, significant changes of oxygen vacancy and Ti3+ concentration were observed for the samples with different thickness, which further proved the contribution of surface layer effect to low-frequency dielectric response. Finally, the CP of (Al0.5Ta0.5)(x)Ti1-xO2 ceramics were attributed to a synergy effect of the IBLC and surface layer effect.