Crystal structure and microwave dielectric properties of NaPb2B2V3O12(B=Mg,Zn) ceramics

Two low temperature sintered NaPb₂B₂V₃O₁₂ (B?=?Mg, Zn) ceramics with garnet structure were synthesized through conventional solid state reaction route and their crystal structure and microwave dielectric properties were investigated for the first time. Rietveld refinements of XRD patterns show both the compounds belong to cubic symmetry with space group Ia-3d. Observed number of Raman bands and group theoretical predictions also confirm cubic symmetry with space group Ia-3d for both NPMVO and NPZVO. At the optimum sintering temperature of 725?°C NPMVO has a relative permittivity of 20.6?±?0.2, unloaded quality factor (Q_uxf) of 22,800?±?1500?GHz (f?=?7.7?GHz) and temperature coefficient of resonant frequency +25.1?±?1?ppm/°C while NPZVO has relative permittivity of 22.4?±?0.2, Q_uxf of 7900?±?1500?GHz (f?=?7.4?GHz) and near zero temperature coefficient of resonant frequency of -6?±?1?ppm/°C at 650?°C. The relative permittivity of the compounds is inversely related to the corresponding Raman shifts.     

Characterization of structure and chemical bond in high-Q microwave dielectric ceramics LiM2GaTi2O8 (M = Mg,Zn)

LiM₂GaTi₂O₈ (M = Mg, Zn) ceramics with the Fd-3m space group were synthesized using the solid-state method. In comparison with Mg²⁺ that fully occupied the tetrahedral (A) site in LiMg₂GaTi₂O₈, LiZn₂GaTi₂O₈ was jointly occupied by Zn²⁺ and Li⁺ at the A site. Excellent microwave dielectric properties of Q×f = 133,400 ± 500 GHz, 101,800 ± 500 GHz, ε_r = 17.1 ± 0.2, 15.8 ± 0.2, and τ_f = ?60.1 ± 3.0 ppm/°C, ? 42.2 ± 3.0 ppm/°C for LiZn₂GaTi₂O₈ and LiMg₂GaTi₂O₈ were obtained, respectively. The large deviations (30.3% for LiMg₂GaTi₂O₈ and 19.6% for LiZn₂GaTi₂O₈) between the corrected ε_corr and theoretical ε_th were observed, which might be attributed to the underestimated Shannon’s ionic polarizability of Ti⁴⁺ in Ti-containing spinels. Their intrinsic microwave dielectric properties were discussed based on bond valence, lattice energy (U), and B-site bond energy (E). Besides, their large negative τ_f values were compensated to near-zero by CaTiO₃.     

Bond characteristics and microwave dielectric properties on Zn3-xCux(BO3)2 ceramics with ultralow dielectric loss

The crystal structure and microwave dielectric properties of Zn_3-xCu_x(BO₃)₂ (x = 0–0.12) ceramics prepared via a traditional solid-state reaction method were investigated by means of X-ray diffraction (XRD) utilizing the Rietveld refinement, complex chemical bond theory, and Raman spectroscopy. XRD showed that all samples were single phase. The samples maintained a low permittivity, even at higher Cu²⁺ contents, which is conducive to the shortening of signal delay time, and intimately related to the average bond ionicity and Raman shift. Moreover, proper Cu²⁺ substitution greatly reduced the dielectric loss associated with the lattice energy. Cu²⁺ entering the lattice optimized the temperature coefficient of resonance frequency (τ_f) values and improved the temperature stability of samples by affecting the bond energy. Optimal microwave dielectric properties were: ε_r = 6.64, Q × f = 160,887 GHz, τ_f = ?42.76 ppm/°C for Zn_2.96Cu_0.04(BO₃)₂ ceramics sintered at 850 °C for 3 h, which exhibited good chemical compatibility with silver and are therefore good candidate materials for Low temperature co-fired ceramic applications.     

Bond characteristics and microwave dielectric properties of high-Q materials in li-doped Zn3B2O6 systems

The bond characteristics, Raman spectroscopy, and microwave dielectric properties of Zn_3-xLi_2x(BO₃)₂ ceramics prepared by solid-state reaction method were investigated. According to the complex chemical bond theory, the bond ionicity and lattice energy of the B–O bond were proved to contributed more to the electric polarization and phase structure stability than that of A-site bond. Thus, the B–O bond had a dominant effect on the dielectric constant and Q × f values. The optimization of the τ_f value can be attributed to the bond valence. Moreover, the shift and full width at half maximum of the Raman peak were closely related to the dielectric constant and Q × f values, respectively. On the whole, Li⁺ substitution contributed greatly to improve the temperature stability and reducing the dielectric loss of Zn_3-xLi_2x(BO₃)₂ ceramics. Additionally, Zn_2.99Li_0.02(BO₃)₂ ceramics sintered at 850 °C exhibited satisfactory microwave dielectric properties of ε_r=6.59, Q × f=122,030 GHz, τ_f=−76.9 ppm/°C, and had good chemical compatibility with silver.     

Raman spectra,bond characteristics,and microwave dielectric properties of Gd2Mo3O12 ceramics

Gd₂Mo₃O₁₂ ceramics were prepared using the traditional solid-phase reaction method. All samples were found to possess an orthorhombic crystal structure with a space group of Pba2, as revealed by refined XRD results. The ceramic sintered at 1000 °C exhibited a high relative density of 96.95 % and superior microwave dielectric properties, including ε_r of 9.42 ± 0.05,  × f of 49258 ± 1200 GHz, and τ_f of −71.8 ± 0.7 ppm/°C. The results suggest a correlation between an increase in ε_r and higher relative density, and a more compact and uniform microstructure can lead to higher  × f value. Chemical bonding theories and Raman spectroscopy analysis reveal that Mo-O bonds, rather than Gd-O bonds, dominate the microwave dielectric properties. Furthermore, the ε_r of Gd₂Mo₃O₁₂ ceramic was closely related to bond ionicity, while  × f and τ_f were mainly determined by lattice energy and bond energy, respectively.     

Rattling effects on microwave dielectric properties of Ca3TiBGe3O12 (B = Mg,Zn) garnets

Herein two cubic garnet ceramics Ca₃TiBGe₃O₁₂ (B = Mg, Zn) were prepared with optimal sintering temperatures of 1300 ℃ and 1200 ℃, respectively. Both ceramics exhibited promising microwave dielectric properties as well as rather low coefficient of thermal expansion (α_L) of ε_r = 11.95, Q×f = 83,000 GHz, τ_f = ?34.1 ppm/℃ and α_L = 3.9 ppm/℃ for Ca₃TiMgGe₃O₁₂, and ε_r = 11.80, Q×f = 79,000 GHz, τ_f = ?42.9 ppm/℃ and α_L = 4.0 ppm/℃ for Ca₃TiZnGe₃O₁₂. Bond valence calculation reveals that Ca²⁺ at the A-site is slightly compressed, Ge⁴⁺ at C-site is rattling, and compressed Mg²⁺/Zn²⁺ coexists with rattling Ti⁴⁺ in the B-site. The large positive deviations (17.22% for Ca₃TiMgGe₃O₁₂ and 10.90% for Ca₃TiZnGe₃O₁₂) between the porosity corrected ε_r(Corr) (12.32 and 12.41) and calculated ε_r(C-M) (10.51 and 11.19) by the C-M equation were observed, which might be attributed to the combination of rattling effects in B-site and C-site. The relatively stronger rattling effect in Ca₃TiMgGe₃O₁₂ leads to a higher ε_r and a τ_f closer to zero than those of Ca₃TiZnGe₃O₁₂. Besides, the Q×f values of Ca₃TiBGe₃O₁₂ (B = Mg, Zn) ceramics are mainly affected by relative density, packing fraction, and Raman mode (A_1 g).     

Structural and chemical bond characteristics of microwave dielectric ceramics Sm3-xBixGa5O12

Herein, the Bi substitution for Sm in garnet Sm_3-xBi_xGa₅O₁₂ (x = 0–0.4) microwave dielectric ceramics is reported. A single garnet-structure phase could be achieved when Bi³⁺ content is in the range of 0 ≤ x ≤ 0.3. The addition of Bi³⁺ effectively reduces the sintering temperature of Sm₃Ga₅O₁₂ ceramics from 1440 °C to 1140 °C. Furthermore, the relationship between the structure and properties of Sm_3-xBi_xGa₅O₁₂ ceramics was investigated by Raman, SEM, TEM, and complex chemical bonding theory. The dielectric constant of Sm_3-xBi_xGa₅O₁₂ (0 ≤ x ≤ 0.3) ceramics increases slightly from 12.68 to 13.35, which is closely related to an increase in the polarizability and the bond susceptibility χ^μ. The Q × f increases from 107,617 GHz to 137,069 GHz, which is related to the lattice energy and the Raman FWHM values. The τ_f value of Sm_3-xBi_xGa₅O₁₂ gradually shifted in the positive direction with the increase of Bi content and the best performance (ε_r = 13.35, Q × f = 137,069 GHz and τ_f = ?15.37 ppm/°C) was obtained at x = 0.3.     

Structure evolution of wolframite-type ZnZrC2O8 (C = Nb,Ta) ceramics in relation to microwave dielectric properties

The relationship among the sintering behavior, crystal structure, chemical bonding properties, and dielectric properties of wolframite-type ZnZr(Nb_1−xTa_x)₂O₈ (0.0 ≤ x ≤ 1.0) ceramics was investigated with the progressive replacement of Nb⁵⁺ by Ta⁵⁺. The optimum sintering temperature increases from 1225 to 1375°C with increasing Ta⁵⁺ content. The ε_r value falls from 27.34 to 22.34 due to a gradual decrease in bond ionicity and a shift in the Raman vibration modes toward higher wave numbers. The Q × f increases from 63 604 GHz (@6.71 GHz) to 115 631 GHz (@7.89 GHz), which is since the increase in the total lattice of chemical bonds. Moreover, the reduction in grain boundary area and the gradual lowering of the full width at half maximum of the Raman vibration modes contribute to the reduction in dielectric losses. First-principles calculations illustrate that the growth in bandgap and electron cloud density in the internal space of the [Zn/ZrO₆] octahedron leads to a reduction in dielectric loss. Furthermore, the reduced degree of oxygen octahedral distortion causes a change in τ_f from −46.56 to −37.40 ppm/°C.     

Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si,Ge) ceramics

Novel BaCa₂M₃O₉ (M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa₂M₃O₉ (M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa₂Ge₃O₉ ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa₂M₃O₉ (M = Si, Ge) exhibited a triclinic structure with a P$\overline{1}$ space group and good microwave dielectric properties. The ε_r, Q × f, and τ_f values of BaCa₂M₃O₉ (M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M–O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa₂Ge₃O₉ ceramic due to its high r_c (Ge–O) and low intrinsic dielectric loss. The BaCa₂Si₃O₉ ceramic exhibited small |τ_f| value (‒36.4 ppm/°C) due to its large E_Si-O. Excellent microwave dielectric properties (ε_r = 8.31, Q × f = 61 800 GHz, and τ_f = ‒58.7 ppm/°C) were obtained for the BaCa₂Ge₃O₉ ceramic.     

Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

Study on structure,microstructure and microwave dielectric characteristics of CaV2O6 and (Ca0.95M0.05)V2O6 (M=Zn,Ba) ceramics

In this study, the (Ca_0.95M_0.05)V₂O₆ (M = Zn, Ba) and the CaV₂O₆ ceramics were synthesized through a solid-state reaction method, and the effects of Zn²⁺ and Ba²⁺ substitution on the structure, sintering temperature, densification, microstructure and microwave dielectric properties of CaV₂O₆ ceramic were analysed. The XRD patterns of the sintered samples indicated a single-phase of CaV₂O₆ in all temperatures. Substitution of Zn²⁺ caused a lower sintering temperature and improved the densification of the CaV₂O₆ ceramic. While the dielectric properties of the (Ca_0.95Ba0_.05)V₂O₆ compound were not desirable, the (Ca_0.95Zn_0.05)V₂O₆ sample sintered at 650°C for 4 hours showed significant dielectric properties, with ε_r = 10.29, Q × f ~  53 000 GHz (at 15.5 GHz) and τ_f = −72.37 ppm/°C. Moreover, the chemical compatibility of the CaV₂O₆ ceramic with Al electrode was examined.     

Crystal structure,infrared-reflectivity spectra,and microwave dielectric properties of novel Ce2(MoO4)2(Mo2O7) ceramics

Novel Ce₂(MoO₄)₂(Mo₂O₇) (CMO) ceramics were prepared by a conventional solid-state method, and the microwave dielectric properties were investigated. X-ray diffraction results illustrated that pure Ce₂(MoO₄)₂(Mo₂O₇) structure formed upon sintering at 600 °C-725 °C. [CeO₇], [CeO₈], [MoO₄], and [MoO₆] polyhedra were connected to form a three-dimensional structure of CMO ceramics. Analysis based on chemical bond theory indicated that the Mo–O bond critically affected the ceramics’ performance. Furthermore, infrared-reflectivity spectra analysis revealed that the primary polarisation contribution was from ionic polarisation. Notably, the optimum microwave dielectric properties of ε_r = 10.69, Q·f = 49,440 GHz (@ 9.29 GHz), and τ_f = −30.4 ppm/°C were obtained in CMO ceramics sintered at 700 °C.     

Crystal structure,Raman spectra and microwave dielectric properties of novel temperature-stable LiYbSiO4 ceramics

Olivine−type structure microwave dielectric ceramics LiYbSiO₄ with near-zero τ_f were fabricated by solid-state reaction process for the first time. The relationships among structural parameters, sintering behavior, vibrational modes and microwave dielectric properties for the ceramics were studied. The variation in ε_r could be correlated with Raman shift. The variation in Q×f values was inversely correlated to FWMH and average cation covalency. The τ_f values were explained with the bond valence sum of cations. Single phase LiYbSiO₄ ceramics could be obtained at the range of 1100–1140 °C and showed promising microwave dielectric properties with ε_r = 7.36 – 7.42, Q×f = 19081 – 25276 GHz and τ_f = +4.52 – +8.03 ppm/°C.     

Crystal structure,chemical bonding and infrared reflectivity of microwave dielectric SrIn2O4 ceramics

In this work, the orthorhombic structured SrIn₂O₄ ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (ε_r ∼ 12.3, Q × f = 96,900 GHz, τ_f ∼ −61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr²⁺ exhibits a compressed state, while the In³⁺ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy E_dc (0.94 eV) of SrIn₂O₄ was obtained by the dielectric spectroscopy, indicating that the E_dc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (ε_r ∼ 11.2, Q × f = 148,900 GHz) of SrIn₂O₄ were obtained by infrared reflectivity spectroscopy.     

Structure,chemical bond and microwave dielectric characteristics of novel Li3Mg4NbO8 ceramics

A novel Li₃Mg₄NbO₈ compound was fabricated through the process of solid-state reaction. The crystal structure, sinterability and microwave dielectric properties of the Li₃Mg₄NbO₈ ceramics were investigated. XRD refinement and Raman spectra results ascertained that the Li₃Mg₄NbO₈ compound crystallized into an orthorhombic Li₃Mg₂NbO₆-like structure with space group Fddd. The ε_r value was strongly impacted by the relative density and average ionic polarization. The Q × f value was mainly affected by the relative density and average grain size. The Li₃Mg₄NbO₈ ceramics sintered at 1150 ℃ showed outstanding microwave dielectric performance: ε_r = 13.8 ± 0.14, Q × f = 103 400 ± 3500 GHz (at 9.6 GHz), τ_f = −36.0 ± 1 ppm/℃. Additionally, the bond characteristics were calculated for a better understanding of the structure-property correlation for Li₃Mg₄NbO₈ ceramics.     

Phase composition,crystal structure,and microwave dielectric properties of LiIn1-xAlxO2 ceramics

A series of LiIn_1-xAl_xO₂ (x =0.05, 0.10, 0.15, 0.20, 0.25) microwave dielectric ceramics with low permittivity were synthesized via a solid-state reaction method. XRD, Raman spectra, and SEM analysis reveal that a single LiInO₂ tetragonal structure phase could be obtained at the x < 0.10, and with the x increased further to 0.15–0.25, the diffraction peaks of the secondary phase LiAlO₂ were detected. In the LiIn_1-xAl_xO₂ ceramics, the τ_f was closely related to the ε_r, and the relative density, microstructure, and microwave dielectric properties were effectively improved by the Al³⁺ substitution for In³⁺. Bond valence theory analysis demonstrates that the Al³⁺ entered the In³⁺ site exhibits a strength rattling effect, which is beneficial to the increase of ε_r. While Al³⁺ substitution for In³⁺ simultaneously lowers the average ionic polarizability, resulting in a decrease in ε_r. A near-zero τ_f (0.74 ppm/°C) combined with ε_r approximately 12.83, Q × f = 58 200 GHz, was obtained in LiIn0_.85Al0_.15O₂ ceramic sintered at 970°C.     

Vibrational spectroscopic and crystal chemical analyses of double perovskite Y2MgTiO6 microwave dielectric ceramics

Structure-property relationships of Y₂MgTiO₆, a type of double perovskite materials, were investigated systematically. Rietveld refinement of XRD patterns confirmed that Y₂MgTiO₆ belongs to the P2₁/c space group and has a structure analogous to that of monoclinic Dy₂MgTiO₆. In accordance with the observed number of vibrational bands and group theoretical predictions, O and Y ions at the 4e site dominated the Raman peaks. The IR reflectivity spectrum indicated that the major contributions to the relative permittivity and intrinsic loss were modes lower than 500 cm⁻¹. The measured relative permittivity closely approached the calculated values determined according to P-V-L theory, the Clausius-Mossotti equation and IR fitted results. The larger bond energy for Mg–O bonds than for Ti–O bonds corresponds to a more stable octahedron of [MgO₆], as verified by the octahedral distortion. Satisfactory microwave dielectric properties of Y₂MgTiO₆ ceramics were as follows: ε_r = 20.4 ± 0.8, Q × f = 42 060 ± 310 GHz, τ_f = −52 ± 3 ppm/°C, sintered at 1450°C.     

Structure,bond characteristics and Raman spectra of CaMg1-xMnxSi2O6 microwave dielectric ceramics

The CaMg_1-xMn_xSi₂O₆(x = 0–0.08)ceramics were reported here for the first time. The relationships among structural characteristics, vibrational modes and dielectric properties for the ceramics were researched based on complex chemical bond theory and Raman vibrational spectroscopy. The formation of a single phase with clinopyroxene structure when x = 0 to 0.08 was detected by X-ray diffraction. The monotonous increase of ${\epsilon }_r$ is ascribed to the average bond covalency, polarizability and Raman shift. The $Q\times f$ value is influenced by total lattice energy and full width at half maximum of Raman spectra which are both connected with the intrinsic loss. The variation of ${\tau }_f$ is related to thermal expansion coefficient and M1-site bond valence. Furthermore, the CaMg_0.98Mn_0·02Si₂O₆ ceramic sintered at 1300 °C possessed optimal microwave dielectric properties of ${\epsilon }_r$ = 8.01, $Q\times f$ = 83469 GHz and ${\tau }_f$ = −45.27 ppm/°C.