Giant dielectric behavior of monovalent cation/anion (Li+, F−) co-doped CaCu3Ti4O12 ceramics

Giant dielectric behavior and electrical properties of monovalent cation/anion (Li⁺, F⁻) co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction route were systematically investigated. Substitution of Li⁺ and F⁻ led to a significantly enlarged mean grain size. A reduced loss tangent (tanδ ~0.06) with the retainment of an ultra-high dielectric permittivity (ε′ ~7.7-8.8 × 10⁴) was achieved in the co-doped ceramics, while the breakdown electric field and nonlinear coefficient of CaCu₃Ti₄O₁₂ ceramics were increased by co-doping with (Li⁺, F⁻). The variations in nonlinear electrical properties and giant dielectric response, as well as the dielectric relaxation, were well explained by the Maxwell-Wagner polarization model for an electrically heterogeneous microstructure, in which a Schottky barrier height at the grain boundaries (GBs) was formed. ε′ was closely correlated to the GB capacitance. Significantly decreased tanδ value and enhanced nonlinear properties were related to a significant increase in the GB resistance, which was attributed to the significantly increased potential barrier height and conduction activation energy at the GBs. The semiconducting nature of the grains was also studied using X-ray photoelectron spectroscopy and found to originate from the presence of Cu⁺ and Ti³⁺ ions.     

Triple-doping of (Ga1/2Nb1/2)xTi1-xO2 ceramics with Al3+ for enhanced giant dielectric response with simultaneous decrease in dielectric loss

An acceptor-donor co-doped (Ga_1/2Nb_1/2)_0.1Ti_0.9O₂ ceramic is triple-doped with Al³⁺, followed by sintering at 1450 °C for 5 h to obtain (Al_xGa_1/2-xNb_1/2)_0.1Ti_0.9O₂ ceramics with improved giant dielectric properties. Homogeneous dispersion of all dopants inside the grains, along with the partially segregated dispersion of the Ga³⁺ dopant along the grain boundaries, is observed. The (Al_xGa_1/2-xNb_1/2)_0.1Ti_0.9O₂ ceramics exhibit high dielectric permittivities (ε′～4.2–5.1 × 10⁴) and low loss tangents (tanδ～0.007–0.010), as well as a low-temperature coefficients (<±15%) between ? 60 and 200 °C. At 1 kHz, tanδ is significantly reduced by ～4.4 times, while ε′ is increased by ～3.5 times, which is attributed to the higher Al³⁺/Ga³⁺ ratio. The value of tanδ at 200 °C is as low as 0.04. The significantly improved dielectric properties are explained based on internal and surface barrier-layer capacitor effects, which are primarily produced by the Ga³⁺ and Al³⁺ dopants, respectively, whereas the semiconducting grains are attributed to Nb⁵⁺ doping ions.     

Influence of Sb-doping on dielectric properties of NaCu3Ti3TaO12 ceramics and relevant mechanism(s)

A series of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics were prepared by the conventional solid-state reaction technique, and their dielectric properties, crystalline structures, microstructures and complex impedance were investigated systematically. All the ceramics show the main phases of perovskite-related crystallographic structure, and their dielectric properties change significantly with the increasing Sb-doping. All these ceramics exhibit giant dielectric-permittivity properties, and impedance spectroscopy analysis reveals that NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics are electrically heterogeneous and composed of insulating grain boundaries and semiconducting grains. Moreover, CuO secondary phase and Cu²⁺/Cu¹⁺, Ti⁴⁺/Ti³⁺, Sb⁵⁺/Sb³⁺ and Ta⁵⁺/Ta³⁺ aliovalences are found to exist in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics through XRD, EDS and XPS analysis. Therefore, CuO segregation and aliovalences of metal ions were suggested to contribute greatly to the internal barrier layer capacitance effect formation in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics. Furthermore, Sb-doping could decrease the tanδ of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics at low frequencies, and the reason was discussed.     

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of SnxTa0.025Ti0.975-xO2 ceramics

The Sn_xTa_0.025Ti_0.975-xO₂ (x%Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn⁴⁺ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO₂ phase were observed as x% increased. The mean grain size was slightly reduced (～17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x%, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x%Sn(TTO) ceramics remained unchanged owing to the same Ta⁵⁺ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x% increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ～0.02 and high ε′～10⁴ with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn⁴⁺–isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO₂ without the creation of complex defect clusters inside the grains.     

High permittivity and low loss dielectric ceramics in the BaO–La2O3–TiO2–Ta2O5 system

Dielectric ceramics were synthesized and characterized in the BaO–La₂O₃–TiO₂–Ta₂O₅ quaternary system for the three typical compositions: Ba₃La₃Ti₅Ta₅O₃₀, Ba₄La₂Ti₄Ta₆O₃₀ and Ba₅LaTi₃Ta₇O₃₀, which formed the filled tungsten-bronze structures. The present ceramics indicated high dielectric constant ε (127.7–148.1) and low dielectric loss tanδ (in the order of 10⁻⁴–10⁻³ at 1 MHz). Meanwhile, the temperature coefficient of dielectric constant τ_ε varied from −728 to −1347 ppm/°C with increasing Ba and Ta and decreasing La and Ti concentration in the temperature range of 20–85 °C. The present ceramics are promising candidates for high-ε and low loss dielectric ceramics, and the suppression of τ_ε should be the primary issue in the future work.     

Colossal permittivity and ultralow dielectric loss in (Nd0.5Ta0.5)xTi1-xO2 ceramics

Giant permittivity ceramic is one of the most significant classes of material to realize the miniaturization and integration of a high-performance capacitor. In this paper, to realize good giant dielectric properties, the (Nd_0.5Ta_0.5)_xTi_1-xO₂ ceramics (NTTO x = 0.005, 0.01, 0.03, 0.05) were synthesized by a standard conventional solid-state reaction. Comparing with the previous co-doped TiO₂ ceramics giant permittivity material system, NTTO ceramics perform extremely colossal permittivity and ultralow dielectric loss (1%NTTO: ε = 82052, tanδ = 0.008 at 1 kHz; 5%NTTO: ε = 170131, tanδ = 0.090 at 1 kHz). The broad distinction of the dielectric behavior between the (Nd_0.5Ta_0.5)_xTi_1-xO₂ ceramics can be explained by the impedance analysis and the calculated polarization activation energies. The main electron-pinned defect-dipole (denoted as EPDD) polarization corresponds to the ultralow loss, embodying in the maximum value of E_gb (the activation energy of the grain boundary), E_a2 (the EPDD polarization activation energies) and the minimum value of E_a1 (the total polarization activation energies). Though the interfacial polarization can cause the permittivity increase, it can also give rise to poor frequency stability and higher loss.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Surface barrier layer effect in (In + Nb) co‐doped TiO2 ceramics: An alternative route to design low dielectric loss

Giant dielectric permittivity (ε′) with low loss tangent (tanδ) was reported in (In + Nb) co‐doped TiO₂ ceramics. Either of electron‐pinned defect‐dipole or internal barrier layer capacitor model was proposed to be the origin of this high dielectric performance. Here, we proposed an effectively alternative route for designing low‐tanδ in co‐doped TiO₂ ceramics by creating a resistive outer surface layer. A pure rutile‐TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved in (In + Nb) co‐doped TiO₂ ceramics prepared by a simple sol‐gel method. Two giant dielectric responses were observed in low‐ and high‐frequency ranges, corresponding to extremely high ε′≈10⁶‐10⁷ and large ε′≈10⁴‐10⁵, respectively. After annealing in air, a low‐frequency dielectric response disappeared and could be restored by removing the outer surface of the annealed sample, indicating the dominant electrode effect in the initial sample. Annealing can cause improved dielectric properties with a temperature‐ and frequency‐independent ε′ value of ≈1.9 × 10⁴ and cause a decrease in tanδ from 0.1 to 0.035. High dielectric performance in (In_0.5Nb_0.5)_xTi_1?xO₂ ceramics can be achieved by eliminating the electrode effect and forming a resistive outer surface layer.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

Improved non-ohmic and dielectric properties of Cr3+ doped CaCu3Ti4O12 ceramics prepared by a polymer pyrolysis solution route

CaCu_3-xCr_xTi₄O₁₂ (x?=?0.00–0.20) ceramics were prepared via a polymer pyrolysis solution route. Their dielectric properties were improved by Cr³⁺ doping resulting in an optimal dielectric constant value of 7156 and a low tanδ?value of 0.092 in a sample with x?=?0.08. This might have resulted from a decrease in oxygen vacancies at grain boundaries. XANES spectra confirmed the presence of Cu⁺ ions in all ceramic samples with a decreasing Cu⁺/Cu²⁺ ratio due to an increased content of Cr³⁺ ions. All CaCu_3-xCr_xTi₄O₁₂ ceramics showed nonlinear characteristic with improvement in both the breakdown field (E_b) and its nonlinear coefficient (α). Interestingly, the highest values of α, ～ 114.4, and that of E_b, ～8455.0?±?123.6?V?cm^?1, were obtained in a CaCu_3-xCr_xTi₄O₁₂ sample with x?=?0.08. The improvement of dielectric and nonlinear properties suggests that they originate from a reduction of oxygen vacancies at grain boundaries.     

A Novel Route to Greatly Enhanced Dielectric Permittivity with Reduce Loss Tangent in CaCu3−xZnxTi4O12/CaTiO3 Composites

Dielectric and nonlinear properties of a binary compound derived from Ca₂Cu₂Ti₄O₁₂ were greatly improved by doping with Zn²⁺ to deliberately create CaCu_3–xZn_xTi₄O₁₂/CaTiO₃ composites. Ca₂Cu_1.8Zn_0.2Ti₄O₁₂ composition can exhibit an enhanced ε′, ~6,513, with a strong reduction in tanδ to ~0.015 (at 1 kHz). The nonlinear coefficient and breakdown field strength were significantly enhanced. The dielectric and nonlinear properties were described based on the effect of Zn²⁺ substitution on electrical response of internal interfaces.     

Large-size-mismatch co-dopants for colossal permittivity rutile TiO2 ceramics with temperature stability

Colossal permittivity (CP) in donor-accepter co-doped rutile TiO₂ has attracted significant interest. Here, the CP behavior of (Ta?+?La) co-doped rutile TiO₂ ceramics were studied, where the ionic radii of Ta⁵⁺ and La³⁺ are much larger than that of Ti⁴⁺. The ceramics with an extremely low doping exhibit colossal dielectric permittivity (～2.6?×?10⁴) with an acceptable low dielectric loss (<0.07) in the frequency range from 40 to 10⁶?Hz. The CP properties obtained in (Ta?+?La) co-doped TiO₂ ceramics show excellent temperature stability over a wide temperature range of 20–400?°C. The X-ray diffraction analysis and the density functional theory calculation illustrates that the La₂³⁺${\text{V}}_{\text{o}}^{??}$Ti₂³⁺ and Ta₂⁵⁺Ti³⁺Ti⁴⁺ defect complexes with the lowest energy are responsible for the enhanced dielectric properties. Moreover, the defect complex formed by large-size trivalent substitutions and oxygen vacancy is very stable, and assists in improving temperature stability of the dielectric properties of co-doped rutile TiO₂ ceramics.     

Ge4+ doped CaCu2.95Zn0.05Ti4O12 ceramics: Two-step reduction of loss tangent

CaCu₃Ti₄O₁₂ ceramics have been extensively studied for their potential applications as capacitors in recent years; however, these materials exhibit very large dielectric losses. A novel approach to reducing the dielectric loss tangent in two steps, while increasing the dielectric permittivity, is presented herein. Doping CaCu₃Ti₄O₁₂ with a Zn dopant reduces the loss tangent of the ceramic material from 0.227 to 0.074, which is due to the increase in grain boundary (GB) resistance by an order of magnitude (from 6.3× 10³ to 3.93 × 10⁴ Ω cm). Zn-doping slightly changes the microstructure and dielectric permittivity of the CaCu₃Ti₄O₁₂ ceramic, which reveals that the primary role of the Zn dopant is to tune the intrinsic properties of the GBs. Surprisingly, the addition of the Ge⁴⁺ dopant into the Zn²⁺-doped CaCu₃Ti₄O₁₂ ceramic sample led to a further decrease in the loss tangent from 0.074 to 0.014, due to enhanced GB resistance (3.1 × 10⁵ Ω cm). The grain size increased remarkably from 2–3 μm to 85–90 μm, corresponding to a significant increase in the dielectric permittivity (~1–4 × 10⁴). The large increase in GB resistance is due to the intrinsic potential barrier height at the GBs and the segregation of the Cu-rich phase in the GB region. First-principles calculations revealed that Zn and Ge are preferentially located at the Cu sites in the CaCu₃Ti₄O₁₂ structure. The substitution of the Ge dopant does not hinder the role of the Zn dopant in terms of improving the electrical properties at the GBs. These phenomena are effectively explained by the internal barrier layer capacitor model. This study provides a way of improving the dielectric properties of ceramics for their practical use as capacitors.     

Dielectric properties with high dielectric permittivity and low loss tangent and nonlinear electrical response of sol-gel synthesized Na1/2Sm1/2Cu3Ti4O12 perovskite ceramic

Giant dielectric ceramic, Na_1/2Sm_1/2Cu₃Ti₄O₁₂, was successfully prepared by a modified sol-gel method. X-ray diffraction experiments indicated that a body-centered cubic structure with a space group of Im3 was obtained. Our density functional theory calculations revealed that codoping Na and Sm in the CaCu₃Ti₄O₁₂ structure resulted in charge compensation between Na and Sm ions in this structure, whereas the oxidation states of Cu and Ti were unaltered. Giant dielectric permittivity ～7.21 × 10³ - 8.04 × 10³ and low dielectric loss tangent ～0.045–0.049 were accomplished at a sintering temperature of 1050 °C for 12–18 h. Nonlinear J - E property with breakdown electric field ～5.13 – 5.78 × 10³ V/cm and nonlinear coefficient ～6.08–6.82 were also achieved. The n-type semiconducting grain originated from short-range migrations of mixed Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ charges. Finally, our charge analysis showed that the occurrence of Cu⁺ and Ti³⁺ was related to the existence of oxygen vacancy in these ceramics.     

Effects of Ta5+ doping on microstructure evolution,dielectric properties and electrical response in CaCu3Ti4O12 ceramics

The effects of Ta⁵⁺ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The mean grain size decreased with increasing Ta⁵⁺ concentration, which was ascribed to the ability of Ta⁵⁺ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu₃Ti₄O₁₂ ceramics were reduced by doping with Ta⁵⁺. This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti⁴⁺ with Ta⁵⁺ are discussed. The experimental data and variation caused by the substitution of Ta⁵⁺ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.     

Large enhancement of piezoelectric properties and resistivity in Cu/Ta co-doped Bi4Ti3O12 high-temperature piezoceramics

In this study, the electrical properties of Bi₄Ti₃O₁₂-based Aurivillius-type ceramics were tailored by a B-site co-doping strategy combining high valence Ta⁵⁺ and low valence Cu²⁺. A series of Bi₄Ti_3−x(Cu_1/3Ta_2/3)_xO₁₂ (BTCT) (x = 0, 0.005, 0.01, 0.015, 0.02, 0.025, and 0.03) ceramics were prepared by the conventional solid-state reaction method. The effect of Cu/Ta co-doping on the crystal structure, microstructure, dielectric properties, piezoelectric properties, ferroelectric properties, and electrical conductivity of these ceramics was systematically investigated. Co-doping significantly enhanced the piezoelectric properties and DC electrical resistivity of the resulting composites. The optimized comprehensive performances were obtained at x = 0.015 with a large piezoelectric coefficient (34 pC/N) and a relatively high resistivity of 9.02 × 10⁶ Ω cm at 500°C. Furthermore, the ceramic also exhibited stable thermal annealing behaviors and excellent fatigue resistance. The results of this study demonstrated great potential of the Cu/Ta co-doped Bi₄Ti₃O₁₂ ceramics for high-temperature piezoelectric device applications.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

Dielectric and mechanical properties of CaCu3Ti3.925(Nb0.5Al0.5)0.075O12 & Al reinforced epoxy-composites (0–3) for embedded capacitor applications

CaCu₃Ti_3.925(Nb_0.5Al_0.5)_0.075O₁₂ [CCTNAO] ceramics were synthesized by microwave assisted solid state reaction technique. CCTNAO ceramics possessed room temperature (RT) dielectric constant (ε_r) ～ 24,173 with tanδ ～0.149 at 1 kHz frequency. Commercially available epoxy-resin, hardener, Al-powder along with CCTNAO powder were used to prepare epoxy based 0–3 composites. Maximum ε_r ～33.37 with tanδ ～0.107 at RT were obtained for 40 vol% CCTNAO loading in epoxy. For x = 0.2 in (1-x)[0.8 Epoxy-0.2 CCTNAO]-x Al Epoxy composites, highest ε_r ～77.6 with tanδ ～ 0.15 at 1 kHz frequency were observed. Increase in ε_r with the increase of Al filler content in composites is attributed to interfacial polarization and cluster formations. Different theoretical models were discussed to explain the dielectric properties of synthesized composites. Experimentally measured values of ε_eff were in close agreement with EMT model (n = 0.13) and Yamada Model (η = 7). An empirical proposed power law ε_eff = ε_m(1+x)ⁿ, with n ～ 10 had a considerable agreement with the experimental results. Vickers hardness test study was carried out to ascertain the mechanical properties of the synthesized composites.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.     

Experimental study and DFT calculations of improved giant dielectric properties of Ni2+/Ta5+ co-doped TiO2 by engineering defects and internal interfaces

High-performance ceramics with chemical formula (Ni_1/3Ta_2/3)_xTi_1?xO₂ with excellent dielectric properties are demonstrated. The dopants of Ni²⁺ and Ta⁵⁺ in TiO₂ caused the formation of oxygen vacancies and free electrons. The (Ni_1/3Ta_2/3)_xTi_1?xO₂ exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 10⁴ with a very weak dependence on temperature (?60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of ? 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to ? 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.