Raman scattering, electronic structure and microwave dielectric properties of Ba([Mg1−xZnx]1/3Ta2/3)O3 ceramics

Effects of Zn substitution for Mg on the crystal structure, lattice vibrations and microwave dielectric properties of Ba(Mg_1/3,Ta_2/3)O₃ (BMT) ceramics were investigated. Raman scattering spectra for Ba([Mg_1−xZn_x]_1/3Ta_2/3)O₃ (BMZT) ceramics, with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0, were measured at room temperature. The Raman result shows a dominance of 1:2 ordered structure at all Zn substitution contents. All Raman modes shift to lower frequencies with increasing Zn substitution. Higher Qf value correlates well with narrower width of the breathing Raman mode A_1g(4) and larger relative intensity of 1:2 long-range-ordered mode E_g(2) in BMZT solid solution. First-principle calculation was performed to investigate the electronic structure of 1:2 ordered BMT and Ba(Zn_1/3,Ta_2/3)O₃ (BZT). Covalent bond between Zn and O in BZT is much stronger than that between Mg and O in BMT due to the Zn 3d orbital. Zn substitution for Mg leads to longer and weaker Ta-O bonds, which may be one reason for the variation of Raman spectroscopy and microwave dielectric properties of BMZT system.     

Effect of grain size and secondary phase on microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 and Ba([Mg,Zn]1/3Ta2/3)O3 systems

Here we report the influence of the grain size of the Ba(Mg_1/3,Ta_2/3)O₃ (BMT) and secondary phase formed in Ba([Mg_0.8Zn_0.2]_1/3,Ta_2/3)O₃ (BMZT) ceramics on the microwave dielectric properties. BMT and BMZT ceramics were prepared by conventional mixed-oxide reaction method using high purity reagents. Samples with different grain size and secondary phase content were obtained by controlling the sintering time of the BMT and BMZT. The secondary phase comprised of BaTa₂O₆, which includes small amount of Zn, was formed due to the vaporization of ZnO from BMZT by the high temperature sintering. The Qf value of BMT increased up to 400 THz with increasing the grain size of ceramics, whereas the Qf value of BMZT decreased with increase of the BaTa₂O₆ content. This second phase also caused an increase in the temperature coefficient of the resonant frequency of BMZT.     

Origin of extrinsic dielectric loss in 1:2 ordered,single-phase BaMg1/3Ta2/3O3

Among the family of temperature-compensated microwave dielectric ceramics, BaMg_1/3Ta_2/3O₃ shows the lowest dielectric loss and remains a material of choice for state-of-the-art airborne and land-based communication systems. We report on the compositional stability range, microwave dielectric properties, and the degree of atomic order of the title compound within the BaO–MgO–Ta₂O₅ ternary diagram. In most cases an atomic order is robust to the deviation from stoichiometry with an exception of Ba-rich and/or Ta-deficient samples which favor (partial) disorder. We further demonstrate that the dense, atomically ordered BaMg_1/3Ta_2/3O₃ ceramic shows large variation of dielectric loss within a single phase composition region – a clear message that the dielectric loss in practical ceramics is dominated by extrinsic sources and that the cation order alone is insufficient to achieve a minimum dielectric loss in BaMg_1/3Ta_2/3O₃. The low-temperature dielectric relaxation studies suggest that the extrinsic dielectric loss in the title compound is due to the ‘rattling’ of the off-centered Mg²⁺ ions misplaced at the Ba sites. Controlled deviation from the BaMg_1/3Ta_2/3O₃ stoichiometry toward the Mg-deficient region leads to suppression of the extrinsic dielectric loss as a result of the reduced chemical activity of Mg ion.     

Effect of dopants on microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been prepared with various amounts of different dopants such as oxides of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent elements. Effect of these dopants on microwave dielectric properties of BZN is investigated. Some of the dopants are found to increase quality factor Q × f and slightly alter the temperature coefficient of resonant frequency (τ_f). Annealing undoped BZN increased the quality factor. Small amounts of dopants such as oxides of Ni, In, Al, Ga, Zr, Ce, Sn, Ti, Sb, and W increased the quality factor. The doped ions substitute for the ordered B ions decreasing the order parameter. Annealing increased the quality factor for all doped BZN samples. Doping BZN with In₂O₃, Al₂O₃, WO₃ and SnO₂ decreased the order parameter but at the same time increased the quality factor indicating that order parameter alone is a poor indicator of quality factor. The quality factor is found to depend on the dopant ionic radii and its concentration. The quality factor increased when the ionic radius of the dopant is close to the ionic radius of the B site ions Zn or Nb. Microstructure studies using SEM showed that the doped high Q ceramics contained large grains.     

Enhancing giant dielectric properties of Ta5+-doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

Various strategies to improve the dielectric properties of ACu₃Ti₄O₁₂ (A = Sr, Ca, Ba, Cd, and Na_1/2Bi_1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta⁵⁺–doped Na_1/2Y_1/2Cu₃Ti₄O₁₂ (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid-state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta⁵⁺ doping ions owing to the increased Cu⁺/Cu²⁺, Cu³⁺/Cu²⁺, and Ti³⁺/Ti⁴⁺ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta⁵⁺ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.     

Effect of Ni substitution on the dielectric properties and lattice vibration of Ba(Mg1/3Ta2/3)O3

Far infrared reflectivity spectra for Ba(Mg_1/3,Ta_2/3)O₃ and Ba([Mg_1−xNi_x]_1/3,Ta_2/3)O₃ ceramics were measured and eigenfrequencies and damping constants of transverse optical modes were estimated in order to investigate the variations in the dielectric properties with Ni substitution. These ceramics were prepared by the conventional solid phase reaction method using high purity chemicals. The observed reflectivity spectra were fitted as a function of the vibration frequencies and the damping constants. The permittivity and Qf value decreased with Ni substitution. The decrease in permittivity is due to the decrease in the damping constant of 4th vibration mode of the oxygen layer vibration. The decrease in Qf value is due to the increase in the damping constant of 5th vibration mode of the oxygen layer vibration. It was inferred that the increase in the Qf value might be due to the substitution of the Ni ion for the Ta ion in the Ta layer in the BMT.     

High frequency dielectric properties of Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics

The dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ ceramics were investigated using Fourier transformed infrared and Raman spectroscopies. The real part of dielectric constant of these materials decreases when the heavier Ta atoms replaced the lighter Nb atoms, which is closely related to the shift of mode-frequency. The Q-factor of the materials depends weakly on the composition. The decrease in Q-factor with the frequency is due to the presence of a resonance mode at around 3 THz. Larger width of the breathing Raman mode, A_1g[O], correlates very well with the lower Q-value of the BMN materials, as compared with the BMT materials.     

Temperature and frequency dependence of dielectric loss of Ba(Mg1/3Ta2/3)O3 microwave ceramics

Ba(Mg_1/3Ta_2/3)O₃ ceramic possessing extremely high Q × f value of more than 300 THz at microwave frequency was developed in our previous study. It is of great interest to understand the mechanism of microwave absorption in such a practical material. In the present study we report on the temperature dependence of the dielectric loss in the Ba(Mg_1/3Ta_2/3)O₃. The mechanism of the microwave absorption is discussed using two phonons difference process. The samples were prepared by conventional solid state reaction and sintered at 1893 K in oxygen atmosphere. Dielectric properties in the microwave range were measured by Hakki & Colemann and resonant cavity methods in the temperature range of 20–300 K. Whispering gallery mode technique was used for the measurement of the dielectric properties at the millimeter wave frequency. Dielectric loss of the Ba(Mg_1/3Ta_2/3)O₃ at the microwave frequency increases with temperature between 200 and 300 K in general agreement with the theory of intrinsic dielectric loss derived from the two phonon difference process. However below 200 K, the dielectric loss has shown a distinctive behavior with a loss peak at 40 K. It was inferred that the loss peak of the Ba(Mg_1/3Ta_2/3)O₃ was caused by the local orientation polarization having dispersion at the microwave frequency.     

Structure and dielectric characteristics of Ca(Fe1/2Ta1/2)O3 complex perovskite ceramics

Ca(Fe_1/2Ta_1/2)O₃ complex perovskite ceramics have been prepared and characterized in the crystal structure and microstructures, and the dielectric characteristics have been evaluated over a broad temperature and frequency range. There are two dielectric relaxations in low and high temperature ranges, respectively. Differing from the situation for Ba(Fe_1/2Ta_1/2)O₃, Sr(Fe_1/2Ta_1/2)O₃ and Ba(Fe_1/2Nb_1/2)O₃, O₂-annealing has little effect on the dielectric properties of Ca(Fe_1/2Ta_1/2)O₃, and the much lower dielectric constant and low loss are ascribed to the low concentration of Fe²⁺ according to the XPS measurements. The microwave dielectric properties (at 5.38 GHz) for Ca(Fe_1/2Ta_1/2)O₃ ceramics are obtained as: ?_r = 30.7, Qf = 3070 GHz.     

Temperature-Stable and Low-Loss Dielectrics in the Ca(B'1/2Ta1/2)O3 [B'=Lanthanides, Y, and In] System

The Ca(B′_1/2Ta_1/2)O₃ [B′=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] double perovskite-type ceramics have been prepared by the conventional solid-state ceramic route. The phase purity and surface morphology of the sintered ceramics have been studied by X-ray diffraction (XRD) and scanning electron microscopy methods. XRD study revealed intersubstitution between Ca and B′ ions. Ca(B′_1/2Ta_1/2)O₃ ceramics have relative permittivity (ɛ_r) in the range 23–30, normalized quality factor (Q_u×f) 20 600–59 200 GHz, and temperature coefficient of resonant frequency (τ_f) −6 to −35 ppm/°C. The dielectric properties of Ca(B′_1/2Ta_1/2)O₃ ceramics have been tailored by the addition of positive τ_f materials such as CaTiO₃, TiO₂, Ba(Y_1/2Ta_1/2)O₃, and Ba(Yb_1/2Ta_1/2)O₃, which form a solid solution or a mixture phase with the parent compound. The 0.7Ca(Y_1/2Ta_1/2)O₃–0.3Ba(Y_1/2Ta_1/2)O₃ ceramic has ɛ_r=27.5, Q_u×f=41 900 GHz, and τ_f=−1.2 ppm/°C, and the 0.6Ca(Yb_1/2Ta_1/2)O₃–0.4Ba(Yb_1/2Ta_1/2)O₃ ceramic has ɛ_r=27.7, Q_u×f=48 000 GHz, and τ_f=1.8 ppm/°C.     

Microwave properties of Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/3Nb2/3)O3 ceramics revealed by Raman scattering

Five Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperatures (form 1350 to 1550 °C), one Ba(Mg_1/3Ta_2/3)O₃ and a Ba(Mg_1/3Nb_2/3)O₃ sample were examined by Raman scattering to reveal the correlation of the 1:2 ordered perovskite structure with the microwave properties, such as dielectric constant and Q factors. The Ba(Co_1/3Nb_2/3)O₃ sample sintered at 1400 °C, which possesses the highest microwave Q value and the lowest dielectric constant among five Ba(Co_1/3Nb_2/3)O₃ samples, has the narrowest width and the highest frequency of the stretch mode of oxygen octahedron (i.e. A_1g(O) near 800 cm⁻¹). We found that the dielectric constant is strongly correlated with the Raman shift of A_1g(O) stretch modes, and the width of A_1g(O) stretch mode reflects the quality factor Q × f value in the 1:2 ordered perovskite materials. This concludes that the oxygen octahedron play an important role of the material's microwave performance. Based on the results of Q × f values and the lineshapes of A_1g(O) stretch mode, we found that the propagation of microwave energy in Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ shows weak damping behavior, however, Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperature exhibit heavily damped behavior.     

Correlation of the phonon characteristics and microwave dielectric properties of the Ba(Mg1/3Ta2/3)O3 materials

The Ba(Mg_1/3Ta_2/3)O₃, BMT, materials possess the highest quality factor (Q × f) in microwave frequency regime among the microwave dielectric materials and can potentially be used for high frequency communication application. To understand the mechanism that determines microwave dielectric properties of the BMT materials, spectroscopic techniques including Raman and Fourier transform infrared (FTIR) analyses are used for investigating the phonon characteristics of the materials. The Raman-shift (Δω_0j) of the Raman peaks and the resonance frequency (ω_0j) of the FTIR peaks vary insignificantly among the samples, which correlate very well with the phenomenon that the K-values for these materials are similar with one another. In contrast, the full-width-at-half-maximum (FWHM) of the Raman peaks and the damping coefficient (γ_j) of the FTIR peaks vary markedly among the samples. The high-Q materials possess sharpest vibrational modes, viz., smallest FWHM value for Raman peaks and smallest γ_j value for FTIR peaks and vice versa. The intimate relationship between the phonon characteristics and the fine structure of the materials is confirmed.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

Extended X-ray absorption fine structure,X-ray diffraction and Raman analysis of nickel-doped Ba(Mg1/3Ta2/3)O3

Raman, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements of xBa(Ni_1/3Ta_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples with x = 0–0.03 were performed to reveal the nickel doping effect on the microwave properties. EXAFS result clearly shows that the nickel is located on the Mg lattice site. We also found that, as the nickel concentration increases, microwave dielectric constant decreases with the TaO and NiO bond distances. X-ray diffraction shows that the 1:2 ordered structure is degraded with the increasing of nickel concentration. The stretching phonon of the TaO₆ octahedra, that is A_1g(O) phonon near 800 cm⁻¹, are strongly correlated to the microwave properties of xBa(Ni_1/3Mg_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples. The large Raman shift and the large width of the A_1g(O) imply rigid but distorted oxygen octahedral structure, therefore, the effect of nickel doping lowers the dielectric constant and the Q × f value of Ba(Mg_1/3Ta_2/3)O₃ ceramic.     

Phase evolution and electrical properties of copper-electroded BaTi4O9 materials with BaO–ZnO–B2O3–SiO2 glass system in reducing atmosphere

BaTi₄O₉ (BT4) microwave dielectric ceramics using a copper electrode and containing 10 wt% BaO–ZnO–B₂O₃–SiO₂ (BZBS) glass frit were sintered under reducing atmosphere at 950 °C and were investigated on the phase evolutions, microstructures and dielectric properties of BT4 with various BaO/SiO₂ and ZnO/SiO₂ ratios of BZBS glasses. Experimental results show that the BaO/SiO₂ ratio contributes to wettability of glass with BaTi₄O₉ ceramics, and ZnO/SiO₂ ratio determines the densification of BaTi₄O₉ ceramics. The different Ba–Ti–O and Ba–Cu–O phases with various Ba/Ti and Ba/Cu ratios can be attributed to the contents of BaO in glass. Ba₄Ti₁₃O₃₀ and Ba₂Cu₃O_5+X may form when BaO contents are too high, and inducing copper diffusion due to the reactions of BaO and Cu, accompanying with degrading of the dielectric characteristics. If the ZnO contents of BZBS glasses were raised, a little bit of ZnSiO₃ and Ba₂Cu₃O_5+X phases appear without Cu diffusion due to non-reaction of ZnO and CuO. The high ZnO/SiO₂ ratio of glass reveals the lower softening point, indicating that the high ZnO glass could enhance the density and therefore increase the dielectric constant and quality factor.     

Ba1-3x/2Lax(Mg1/3Ta2/3)O3陶瓷的微波介电性能

采用固相合成法制备了Ba1-3x/2Lax(Mg1/3Ta2/3陶瓷,研究了La掺杂对钽镁酸钡的结构和微波介电性能的影响.结果表明:A位取代能改进其烧结性能.在x≤0.02时,烧结样品为单相的钙钛矿结构,B位离子1:2有序;当x>0.02时出现第二相Ba0.5TaO3.B位离子有序度随着x的增大先增加后减小,在x=0.04时出现最大值.x≤0.02时介电常数变化较小,而后其值逐渐增大.品质因数与谐振频率的乘积(Q×f)值随着x的增大先增大后减小,在x=0.02时取得最大值;谐振频率温度系数(τf)值随着x增大而增大.     

Dielectric loss and damping constants of lattice vibrations in Ba(Mg1/3,Ta2/3)O3 ceramics

Far-infrared reflectivity spectra for Ba(Mg_1/3,Ta_2/3)O₃ ceramics (prepared from high quality oxide reagents) sintered at 1600 °C for 4 and 50 h were measured at room temperature, to determine eigen frequencies and damping constants of lattice vibration in order to understand why the Q·f value at microwave frequency was improved by sintering time. The observed reflectivity spectra were fitted to 16 IR active modes predicted by factor group analysis in order to estimate the fitting parameters. Differences in reflectivity and damping constants were confirmed between the two samples by observation of the measurement spectra and their spectrum fitting. The fitting parameters were used to simulate dielectric loss at low frequency. Results of the simulation suggested a contribution of 3rd, 4th and 5th vibration modes to dielectric loss at low frequency.     

Microwave dielectric properties of multi-ions Ba(Zn,Ta)O3-based perovskite ceramics

In this study, Ba(Zn_1/3Ta_2/3)O₃-based complex perovskite compounds, including Ba(Zn_1/3Ta_2/3)O₃, Ba(Zn_1/3Ta_1/3Nb_1/3)O₃, Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, were prepared and characterized. There was no second phase formation shown in the XRD patterns. Though it has been suggested that substitutions of multiple ions over A-site or B-site of the Ba(Zn_1/3Ta_2/3)O₃ ceramics may not be beneficial to their microwave dielectric properties, the Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramics in this study were found to perform in a fairly acceptable manner. The Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1575 °C for 6 h) and the Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1550 °C for 6 h) reported the following characteristics after annealing at 1400 °C for 10 h: 24.9 and 27.0 for dielectric constants (?_r), 83,000 and 32,100 GHz for quality factors (Q × f) values and −12.8 and −22.6 ppm/°C for temperature coefficients of resonance frequency (τ_f).     

Crystal structure of corundum type Mg4(Nb2–xTax)O9 microwave dielectric ceramics with low dielectric loss

New microwave dielectric ceramics, i.e. Mg₄(Nb_2−xTa_x)O₉ (MNT) solid solutions, were synthesized and their microwave dielectric properties and crystal structure were investigated in this study. From the discrete variational Xα (DV-Xα) method, it was found that the Ta–O bonds in the TaO₆ octahedron become more covalent than Nb–O bonds in the NbO₆ octahedron; this result leads to the decrease of the ionicity in the Ta⁵⁺ ion. The dielectric constants of MNT were slightly decreased from 12.4 to 11.5; this result might be due to the covalent interaction of Ta–O bonding. The quality factors of the samples were found to exhibit high value (Q·f≒350 000 GHz for x=2) which is comparable to those of Al₂O₃.     

Effect of temperature and stoichiometry on the long-range 1:2 cation order in BaZn1/3Ta2/3O3

We report on the compositional stability range, the degree of atomic order and Raman and optical spectra of the off-stoichiometric BaZn_1/3Ta_2/3O₃ (BZT) within the BaO–ZnO–Ta₂O₅ ternary diagram. Almost all off-stoichiometric BZT compositions equilibrated at 1200?°C show significant degree of the long-range 1:2 cation order ranging from 60% to 80%. Ceramics equilibrated at 1550?°C and annealed at 1450?°C show strong effect of composition on the 1:2 order. The regions where an 1:2 atomic order is robust to the deviation from stoichiometry include the off-stoichiometric compositions along the BZT–Ba₄Ta₂O₉, BZT–Ba₃Ta₂O₈, BZT–BaTa₂O₆, BZT–Ta₂O₅, BZT–ZnTa₂O₆ and BZT–Zn₄Ta₂O₉ (pseudo) tie lines. At the same time ceramics formulated along the BZT–BaO, BZT–ZnO, BZT–BaZnO₂, BZT–Ba₂ZnO₃ tie lines and BZT–‘Ba₃ZnO₄’ pseudo tie line show complete disorder. There is a very close correlation between the degree of the 1:2 order on one hand and the unit cell volume and lattice distortion on the other hand. The ordered BZT show contraction of the unit cell whereas disordered ceramics show expansion of the unit cell in the off-stoichiometric region. The pronounced signatures of the order-disorder phase transition in the Raman and optical spectra are discussed.