Tuning enhanced dielectric properties of (Sc³⁺-Ta⁵⁺) substituted TiO₂ via insulating surface layers

In this study, we achieved significantly enhanced giant dielectric properties (EG-DPs) in Sc³⁺-Ta⁵⁺ co-doped rutile-TiO₂ (STTO) ceramics with a low loss tangent (tanδ ≈ 0.05) and high dielectric permittivity (ε' ≈ 2.4 × 10⁴ at 1 kHz). We focused on investigating the influence of insulating surface layers on the nonlinear electrical properties and the giant dielectric response. Our experimental observations revealed that these properties are not directly correlated with the grain size of the ceramics. Furthermore, first-principles calculations indicated the preferred formation of complex defects, specifically 2Ta diamond and 2ScV_o triangular-shaped complexes, within the rutile structure of STTO; however, these too showed no correlation. Consequently, the non-Ohmic properties and EG-DPs of STTO ceramics cannot be predominantly attributed to the grain boundary barrier layer capacitor model or to electron-pinned defect-dipole effects. We also found that the semiconducting grains in STTO ceramics primarily arise from Ta⁵⁺, while Sc₃₊ plays a crucial role in forming a highly resistive outer surface layer. Notably, a significant impact of grain boundary resistance on the nonlinear electrical properties was observed only at lower co-dopant concentrations in STTO ceramics (1 at%). The combination of low tanδ values and high ε' in these ceramics is primarily associated with a highly resistive, thin outer-surface layer, which substantially influences their non-Ohmic characteristics.