Colossal Dielectric Permittivity in (Nb+Al) Codoped Rutile TiO₂ Ceramics: Compositional Gradient and Local Structure

(Nb+Al) codoped rutile TiO₂ ceramics with nominal composition Ti⁴⁺_0.995Nb⁵⁺_0.005yAl³⁺_0.005zO₂, z = (4-5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and Ti⁴⁺_0.90Nb⁵⁺_0.05Al³⁺_0.05O₂ have been synthesized. The resultant samples in ceramic pellet form exhibit a colossal dielectric permittivity (>-10⁴) with an acceptably low dielectric loss (-10^-1) after optimization of the processing conditions. It is found that a conventional surface barrier layer capacitor (SBLC) effect, while it contributes significantly to the observed colossal permittivity, is not the dominant effect. Rather, there exists a subtle chemical compositional gradient inward from the pellet surface, involving the concentration of Ti³⁺ cations gradually increasing from zero at the surface without the introduction of any charge compensating oxygen vacancies. Instead, well-defined G_r ± ¹/₃[011]∗ satellite reflections with the modulation wave-vector q = ¹/₃[011]_r∗ and sharp diffuse streaking running along the G_r ± ε011]∗ direction from electron diffraction suggest that the induced additional metal ions appear to be digested by a locally intergrown, intermediate, metal ion rich structure. This gradient in local chemical composition exists on a scale up to submillimeters, significantly affecting the overall dielectric properties. This work suggests that such a controllable surface compositional gradient is an alternative method to tailor the desired dielectric performance.