Nb+Al) codoped rutile TiO 2 ceramics with nominal composition Ti 4+ 0.995 Nb 5+ 0.005y Al 3+ 0.005z O 2 , z = (4−5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and Ti 4+ 0.90 Nb 5+ 0.05 Al 3+ 0.05 O 2 have been synthesized. The resultant samples in ceramic pellet form exhibit a colossal dielectric permittivity (>∼10 4 ) 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 Ti 3+ 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 ± ε[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 ∼ 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.

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2, i.e., (Ga(0.5)Nb(0.5))(x)Ti(1-x)O2, where Ga(3+) is from the same group as In(3+) but with a much smaller ionic radius. Colossal permittivity of up to 10(4)-10(5) with an acceptably low dielectric loss (tan δ = 0.05-0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10-40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials.

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Electron-pinned defect dipoles, in the form of highly stable triangle-diamond and/or triangle-linear dopant defect clusters with well defined relative positions for Ti reduction, are present in rutile In + Ta co-doped TiO2 for the colossal permittivity and low loss.

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