Colossal permittivity behavior and its origin in rutile (Mg1/3Ta2/3)xTi1-xO2

DFT calculations and giant dielectric responses in (Ni1/3Nb2/3)xTi1-xO2

The origins of dielectric responses in Ni2+ and Nb5+ co-doped TiO2 were explored considering intrinsic and extrinsic effects. DFT calculations demonstrated that Ni2+ doping induced oxygen vacancies, while Nb5+ doping generated free electrons. Theoretical predictions indicated complex defect dipoles forming in the rutile structure, contributing to overall dielectric responses. Theoretical calculations also showed a possible linear alignment of Ni2+-2Nb5+ without oxygen vacancies, especially in high doping concentrations. Experimentally, (Ni1/3Nb2/3)xTi1-xO2 ceramics (x = 1%, 2.5%, and 10%) were synthesized. The substantial dielectric response at room temperature, attributed to factors like defect dipoles and grain boundary/surface barrier layer capacitor (GBLC/SBLC) effects, increased with higher doping levels. However, in a temperature range where GBLC/SBLC effects were suppressed, the dielectric response decreased with increased doping, likely due to self-charge compensation between Ni2+-2Nb5+. Notably, (Ni1/3Nb2/3)xTi1-xO2 with x = 2.5% exhibited a high dielectric permittivity of 104 and a low loss tangent of 0.029 at 1 kHz. Moreover, the dielectric permittivity changed by less than ±15% (compared to 25 °C) at 150 °C. This work provides an understanding of the origins of dielectric responses in co-doped TiO2 and optimizes the doping concentration to achieve the best dielectric performance.

The origin of dielectric relaxation behavior in TiO2 based ceramics co-doped with Zn2+, W6+ ions under a N2/O2 sintering atmosphere

Dense (Zn_0.5W_0.5)_xTi_1-xO₂ (ZWTO_x) ceramics were fabricated using a conventional solid state reaction method with sintering under a nitrogen atmosphere (ZWTO_x-N₂) and an oxygen atmosphere (ZWTO_x-O₂), respectively. Colossal permittivity (ε > 10⁴) and low loss (tan δ < 0.1) were simultaneously achieved in ZWTO_x-N₂ ceramics, and two types of dielectric relaxation behaviors observed were interpreted to be due to interface polarization and disassociation between oxygen vacancies and trivalent titanium ions, respectively. The impedance plots suggested that the ZWTO_x-N₂ ceramics are electrical heterostructures composed of semiconductor and insulator grain boundaries, which proved that the CP performance of ZWTO_x-N₂ ceramics almost originates from the internal barrier layer capacitance (IBLC) effect. In addition, a series of anomalous dielectric behaviors such as low permittivity and low frequency dispersion were observed for ZWTO_x-O₂ ceramics; polarization (P)-electric field (E) hysteresis loop curves were obtained for ZWTO_x-O₂ ceramics, and that impedance plots have shown that the ZWTO_x-O₂ ceramics display higher insulation resistivity. Density functional theory (DFT) calculations illustrated that the Zn²⁺-W⁶⁺ ion pairs are easy to form in ZWTO_x-O₂ ceramics, which causes destruction of the local lattice and thus leads to abnormal dielectric behavior. This work will provide a new strategy for defect engineering in TiO₂ and other CP materials.

Giant Dielectric Properties of W6+-Doped TiO2 Ceramics

The effects of the sintering temperature and doping level concentration on the microstructures, dielectric response, and electrical properties of W⁶⁺-doped TiO₂ (WTO) prepared via a solid-state reaction method were investigated. A highly dense microstructure, pure rutile-TiO₂, and homogenously dispersed dopant elements were observed in all of the ceramic samples. The mean grain size increased as the doping concentration and sintering temperature increased. The presence of oxygen vacancies was studied. A giant dielectric permittivity (ε′ ~ 4 × 10⁴) and low tanδ (~0.04) were obtained in the WTO ceramic sintered at 1500 °C for 5 h. The ε′ response at a low temperature was improved by increasing the doping level concentration. The giant ε′ response in WTO ceramics can be described by the interfacial polarization at the interface between the semiconducting and insulating parts, which was supported by the impedance spectroscopy.

Engineering the Defects and Microstructures in Ferroelectrics for Enhanced/Novel Properties: An Emerging Way to Cope with Energy Crisis and Environmental Pollution

In the past century, ferroelectrics are well known in electroceramics and microelectronics for their unique ferroelectric, piezoelectric, pyroelectric, and photovoltaic effects. Nowadays, the advances in understanding and tuning of these properties have greatly promoted a broader application potential especially in energy and environmental fields, by harvesting solar, mechanical, and heat energies. For example, high piezoelectricity and high pyroelectricity can be designed by defect or microstructure engineering for piezo- and pyro-catalyst, respectively. Moreover, highly piezoelectric and broadband (UV-Vis-NIR) light-responsive ferroelectrics can be designed via defect engineering, giving rise to a new concept of photoferroelectrics for efficient photocatalysis, piezocatalysis, pyrocatalysis, and related cocatalysis. This article first summarizes the recent developments in ferroelectrics in terms of piezoelectricity, pyroelectricity, and photovoltaic effects based on defect and microstructure engineering. Then, the potential applications in energy generation (i.e., photovoltaic effect, H2 generation, and self-powered multisource energy harvesting and signal sensing) and environmental protection (i.e., photo-piezo-pyro- cocatalytic dye degradation and CO2 reduction) are reviewed. Finally, the outlook and challenges are discussed. This article not only covers an overview of the state-of-art advances of ferroelectrics, but also prospects their applications in coping with energy crisis and environmental pollution.

Improved microstructure and significantly enhanced dielectric properties of Al3+/Cr3+/Ta5+ triple-doped TiO2 ceramics by Re-balancing charge compensation

The charge compensation mechanism and dielectric properties of the (Al x Cr0.05-x )Ta0.05Ti0.9O2 ceramics were studied. The mean grain size slightly changed with the increase in the Al3+/Cr3+ ratio, while the porosity was significantly reduced. The dielectric permittivity of the co-doped Cr0.05Ta0.05Ti0.9O2 ceramic was as low as ε'∼ 103, which was described by self-charge compensation between Cr3+-Ta5+, suppressing the formation of Ti3+. Interestingly, ε' can be significantly increased (6.68 × 104) by re-balancing the charge compensation via triple doping with Al3+ in the Al3+/Cr3+ ratio of 1.0, while a low loss tangent (∼0.07) was obtained. The insulating grains of [Cr0.05 3+Ta0.05 5+]Ti0.9 4+O12 has become the semiconducting grains for the triple-doped Al x 3+[Cr0.05-x 3+Ta0.05-x 5+][Ta x 5+Ti x 3+Ti0.9+x 4+]O12+3x/2. Considering an insulating grain with low ε' of the Cr0.05Ta0.05Ti0.9O2 ceramic, the electron-pinned defect-dipoles and interfacial polarization were unlikely to exist supported by the first principles calculations. The significantly enhanced ε' value of the triple-doped ceramic was primarily contributed by the interfacial polarization at the interface between the semiconducting and insulating parts, which was supported by impedance spectroscopy. This research gives an underlying mechanism on the charge compensation in the Al3+/Cr3+/Ta5+-doped TiO2 system for further designing the dielectric and electrical properties of TiO2-based ceramics for capacitor applications.

Origins of Giant Dielectric Properties with Low Loss Tangent in Rutile (Mg1/3Ta2/3)0.01Ti0.99O2 Ceramic

The Mg²⁺/Ta⁵⁺ codoped rutile TiO₂ ceramic with a nominal composition (Mg_1/3Ta_2/3)_0.01Ti_0.99O₂ was synthesized using a conventional solid-state reaction method and sintered at 1400 °C for 2 h. The pure phase of the rutile TiO₂ structure with a highly dense microstructure was obtained. A high dielectric permittivity (2.9 × 10⁴ at 10³ Hz) with a low loss tangent (<0.025) was achieved in the as-sintered ceramic. After removing the outer surface, the dielectric permittivity of the polished ceramic increased from 2.9 × 10⁴ to 6.0 × 10⁴, while the loss tangent also increased (~0.11). The dielectric permittivity and loss tangent could be recovered to the initial value of the as-sintered ceramic by annealing the polished ceramic in air. Notably, in the temperature range of −60–200 °C, the dielectric permittivity (10³ Hz) of the annealed ceramic was slightly dependent (<±4.4%), while the loss tangent was very low (0.015–0.036). The giant dielectric properties were likely contributed by the insulating grain boundaries and insulative surface layer effects.

Hole-Pinned Defect Clusters for a Large Dielectric Constant up to GHz in Zinc and Niobium Codoped Rutile SnO2

High permittivity materials for a gigahertz (GHz) communication technology have been actively sought for some time. Unfortunately, in most materials, the dielectric constant starts to drop as frequencies increase through the megahertz (MHz) range. In this work, we report a large dielectric constant of ∼800 observed in defect-mediated rutile SnO₂ ceramics, which is nearly frequency and temperature independent over the frequency range of 1 mHz to 35 GHz and temperature range of 50-450 K. Experimental and theoretical investigations demonstrate that the origin of the high dielectric constant can be attributed to the formation of locally well-defined Zn²⁺-Nb⁴⁺ defect clusters, which create hole-pinned defect dipoles. We believe that this work provides a promising strategy to advance dipole polarization theory and opens up a direction for the design and development of high frequency, broadband dielectric materials for use in future communication technology.

Colossal dielectric permittivity, reduced loss tangent and the microstructure of Ca1-x Cd x Cu3Ti4O12-2y F2y ceramics

Ca1-x Cd x Cu3Ti4O12-2y F2y (x = y = 0, 0.10, and 0.15) ceramics were successfully prepared via a conventional solid-state reaction (SSR) method. A single-phase CaCu3Ti4O12 with a unit cell ∼7.393 Å was detected in all of the studied ceramic samples. The grain sizes of sintered Ca1-x Cd x Cu3Ti4O12-2y F2y ceramics were significantly enlarged with increasing dopant levels. Liquid-phase sintering mechanisms could be well matched to explain the enlarged grain size in the doped ceramics. Interestingly, preserved high dielectric permittivities, ∼36 279-38 947, and significantly reduced loss tangents, ∼0.024-0.033, were achieved in CdF2 codoped CCTO ceramics. Density functional theory results disclosed that the Cu site is the most preferable location for the Cd dopant. Moreover, F atoms preferentially remained close to the Cd atoms in this structure. An enhanced grain boundary response might be a primary cause of the improved dielectric properties in Ca1-x Cd x Cu3Ti4O12-2y F2y ceramics. The internal barrier layer capacitor model could well describe the colossal dielectric response of all studied sintered ceramics.

High-Performance Giant Dielectric Properties of Cr3+/Ta5+ Co-Doped TiO2 Ceramics

The effects of the sintering temperature on microstructures, electrical properties, and dielectric response of 1%Cr3+/Ta5+ co-doped TiO2 (CrTTO) ceramics prepared using a solid-state reaction method were studied. The mean grain size increased with an increasing sintering temperature range of 1300-1500 °C. The dielectric permittivity of CrTTO ceramics sintered at 1300 °C was very low (ε' ∼198). Interestingly, a low loss tangent (tanδ ∼0.03-0.06) and high ε' (∼1.61-1.9 × 104) with a temperature coefficient less than ≤ ±15% in a temperature range of -60 to 150 °C were obtained. The results demonstrated a higher performance property of the acceptor Cr3+/donor Ta5+ co-doped TiO2 ceramics compared to the Ta5+-doped TiO2 and Cr3+-doped TiO2 ceramics. According to a first-principles study, high-performance giant dielectric properties (HPDPs) did not originate from electron-pinned defect dipoles. By impedance spectroscopy (IS), it was suggested that the giant dielectric response was induced by interfacial polarization at the internal interfaces rather than by the formation of complex defect dipoles. X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Ti3+, resulting in the formation of semiconducting parts in the bulk ceramics. Low tanδ and excellent temperature stability were due to the high resistance of the insulating layers with a very high potential barrier of ∼2.0 eV.