Colossal Dielectric Permittivity in (Nb+Al) Codoped Rutile TiO 2 Ceramics: Compositional Gradient and Local Structure

Abstract

(Nb+Al) codoped rutile TiO<inf>2</inf> ceramics with nominal composition Ti4+<inf>0.995</inf>Nb5+<inf>0.005y</inf>Al3+<inf>0.005z</inf>O<inf>2</inf>, z = (4-5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and Ti4+<inf>0.90</inf>Nb5+<inf>0.05</inf>Al3+<inf>0.05</inf>O<inf>2</inf> 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<inf>r</inf> ± 1/<inf>3</inf>[011]∗ satellite reflections with the modulation wave-vector q = 1/<inf>3</inf>[011]<inf>r</inf>∗ and sharp diffuse streaking running along the G<inf>r</inf> ± 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.