The immense potential of colossal permittivity (CP) materials for use in modern microelectronics as well as for high-energy-density storage applications has propelled much recent research and development. Despite the discovery of several new classes of CP materials, the development of such materials with the required high performance is still a highly challenging task. Here, we propose a new electron-pinned, defect-dipole route to ideal CP behaviour, where hopping electrons are localized by designated lattice defect states to generate giant defect-dipoles and result in high-performance CP materials. We present a concrete example, (Nb+In) co-doped TiO₂ rutile, that exhibits a largely temperature- and frequency-independent colossal permittivity (> 10(4)) as well as a low dielectric loss (mostly < 0.05) over a very broad temperature range from 80 to 450 K. A systematic defect analysis coupled with density functional theory modelling suggests that 'triangular' In₂(3+)Vo(••)Ti(3+) and 'diamond' shaped Nb₂(5+)Ti(3+)A(Ti) (A = Ti(3+)/In(3+)/Ti(4+)) defect complexes are strongly correlated, giving rise to large defect-dipole clusters containing highly localized electrons that are together responsible for the excellent CP properties observed in co-doped TiO₂. This combined experimental and theoretical work opens up a promising feasible route to the systematic development of new high-performance CP materials via defect engineering.

High-quality brookite flowers were fabricated via a facile solution chemistry technique. The synthetic conditions to the flower-like brookite were monitored by a series of time-resolved experiments and further optimized by adjusting the concentrations of the Na + and OHspecies involved in the reaction system. Careful sample characterizations by the combined techniques of X-ray diffraction, Raman, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectra indicate the formation of highly phase-pure and well-crystallized brookite with an extremely low defect concentration. Different from the natural brookite mineral with an indirect transition (Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154), the present high-quality brookite flowers showed a direct transition with a bandgap energy of 3.4 ( 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ( 0.1 eV for rutile and indirect band gap of 3.2 ( 0.1 eV for anatase. Room-temperature alternative current impedance measurements indicate that the permittivity for the brookite flowers is 93 at 40 Hz, which is much higher than that for anatase but slightly lower than rutile as opposed to what is theoretically predicted in the literature. Strikingly, the flower shape also enables high quality brookite TiO 2 with a high structural stability up to 900 °C in air, impossibly accessible when using other preparation methods. These observations pave the way for high-quality brookite flowers to find a broad class of technological uses.

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.