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

(Nb+Al) codoped ruffle TiO2 cerarnics With nominal composition (Ti0.9954+Nb0.005y5+Al0.005z3+O2), z = (4-5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and (Ti0.904+Nb0.0055+Al0.053+O2) have been synthesized. The resultant samples in ceramic pellet form exhibit a colossal dielectric permittivity (>similar to 10(4)) with an acceptably low dielectric loss (similar to 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 Gr + 1/3[011]* satellite reflections with the modulation wave-Vector q = 1/3[011](r)* and sharp diffuse streaking running along the Gr + epsilon[011] * 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 similar 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.