Structural dependence of microwave dielectric properties of spinel structured Mg2(Ti1-xSnx)O4 solid solutions: Crystal structure refinement,Raman spectra study and complex chemical bond theory

Mg₂(Ti_1-xSn_x)O₄ (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg₂(Ti_1-xSn_x)O₄ average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg₂(Ti_1-xSn_x)O₄ compounds were obtained (${\epsilon }_r$= 12.18, $Q\times f$?=?170,130?GHz, ${\tau }_f$?=??53.1?ppm/°C, x?=?0.2).     

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.     

Correlation between crystal structure and enhanced microwave dielectric characteristics of wolframite-structure Mg0.5Zr0.5NbO4 ceramics

An effective and simple synthesis of high-performance materials exerts a significant influence on low-cost commercial production with high production efficiency. In the present work, the effects of intrinsic factors on the microwave dielectric properties of reaction-sintering Mg_0.5Zr_0.5NbO₄ ceramics were quantified through crystal chemistry, Raman analysis, and bond characteristics. Pure-phase and low-loss Mg_0.5Zr_0.5NbO₄ ceramics were synthesized with an effective and simple reaction-sintering procedure. Detailed Raman vibrations with complete mode assignments were firstly investigated to characterize the lattice vibration. Rietveld refinement and Raman analysis confirmed the formation of pure-phase MgZrNb₂O₈ ceramics even with the reaction-sintering process. Moreover, well-distributed microstructure with enhanced densification (nearly full density) was verified through SEM and element mapping. Through the chemical bond theory, the dielectric constant was dominated by the Nb–O bond ionicity (f_iNb-O). The Q × f was strongly related to the lattice energy (U_Nb–_O) and bond energy (E_Nb-O) of the Nb–O bond, while the τ_f value was influenced by the coefficient of thermal expansion of the Mg–O bond (α_Mg-O), providing guidance for performance modification of Mg_0.5Zr_0.5NbO₄ systems. Excellent microwave dielectric properties for the samples sintered at 1350 °C: ε_r = 28.6, Q × f = 82,000 GHz (7.1 GHz) and τ_f = ?47 ppm/°C were obtained via the reaction sintering process, exhibiting enormous potential for industrial production.     

Densification,microstructure evolution,and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg²⁺ ions replaced by Ca²⁺ ions on the microwave dielectric properties of newly developed MgZrTa₂O₈ were investigated. Mg_1-xCa_xZrTa₂O₈ (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca²⁺ significantly inhibited the densification of Mg_1-xCa_xZrTa₂O₈, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa₂O₆. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg²⁺ ions by the more polarizable Ca²⁺ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ_f values was in accordance with that of relative permittivity. The best composition appeared to be Mg_0.9Ca_0.1ZrTa₂O₈, which showed excellent microwave dielectric properties of ε_r = 22.5, Q × f = 231,951 GHz, and τ_f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.     

Correlation between crystal structure,bond characteristics,Raman vibrations,and improved microwave dielectric properties of high-performance Zn0.5Zr0.5NbO4 ceramics: First principle calculation and experiment

The structure–performance mechanism provides new insights into performance modification and materials discovery. Herein, the electronic structure, Raman vibration, chemical bond factors and enhanced microwave dielectric properties of Zn_0.5Zr_0.5NbO₄ ceramics through oxygen-assisted reaction sintering were investigated by Raman spectroscopy, first-principle calculations, and complex P–V-L theory. Pure-phase Zn_0.5Zr_0.5NbO₄ ceramics were synthesized under oxygen-assisted reaction sintering, confirmed by XRD refinement and Raman analysis. Systematic vibration analysis was first introduced to provide complete mode assignments and Raman shift. Optimized microstructure with full density was obtained through morphology and EDS analysis. First-principle calculations indicated that the d orbit exerts the main contribution to Fermi energy with energy gap of 3.51 eV and Nb–O bonds may possess strong vibration, exhibiting a remarkable effect on dielectric loss. P–V-L results showed that Nb–O bonds have a significant influence on the dielectric constant and Q×f value while the Zn–O bonds dominate the τ_f value. In addition, high-performance Zn_0.5Zr_0.5NbO₄ ceramics were fabricated through oxygen-assisted reaction sintering at 1250 °C, with ε_r = 28.4, Q×f = 79,800 GHz, and τ_f = −47.7 ppm/^°C, exhibiting tremendous superiorities for commercial production.     

Li+ enrichment to improve the microwave dielectric properties of Li2ZnTi3O8 ceramics and the relationship between structure and properties

Herein, Li⁺-enriched Li_(1+x)2ZnTi₃O₈ ceramics are prepared via the solid-phase methods. As x increases, the unit cell volume gradually increases, while the grain size initially increases and then decreases gradually. The Li_(1+0.06)2ZnTi₃O₈ ceramics exhibit the best dielectric properties: ε_r = 25.92, Q × f = 109534 GHz (@7.37 GHz, which is a 48 % increase compared with the stoichiometric counterpart.), and τ_f = ?8.21 ppm/°C. The complex chemical bond theory and Raman spectroscopy reveal that Ti-O bonds have a significant effect on the dielectric properties. An optimal Li⁺ enrichment leads to an overall reduction in the distortion of the Li/ZnO₄ tetrahedra, resulting in a reduction in τ_f. First-principles calculations demonstrate that a suitable excess of Li⁺ leads to an increase in the band-gap as well as an enhanced electron cloud density in the internal space of the Li1/ZnO₄ tetrahedra, thereby increasing the Q × f. In summary, Li⁺-enriched Li_(1+0.06)2ZnTi₃O₈ ceramics are promising for a wide array of applications in microwave communications.     

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.     

Mg3B2O6 microwave dielectric ceramics fabricated by combining cold sintering with post-annealing process

It is difficult to get pure-phase Mg₃B₂O₆ (abbreviated as MBO) ceramics by the traditional high-temperature solid-state reaction method. In this paper, pure-phase MBO ceramics were successfully densified and obtained by combining the cold sintering and post-annealing process. The relative density of MBO ceramics was ∼80% cold sintered at 150°C/90 min/800 MPa, which was further improved to ∼91% by post-annealing at 900°C, 400°C lower than that of the traditional high-temperature sintering process (∼1300°C). X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman results demonstrated that the secondary phase of MgO was effectively eliminated, and dense microstructure was observed by the cold-sintering process plus post-annealing treatment. Finally, the microwave dielectric properties of MBO were evaluated with ε_r: 5.15–6.37, Q×f: 5942–16 686 GHz, τ_f: −48.45–69.72 ppm/°C.     

Crystal structure,bond energy,Raman spectra,and microwave dielectric properties of Ti-doped Li3Mg2NbO6 ceramics

Crystal structure exerts dominant influence on the microwave dielectric performance enabling satisfying the demands for 5G communication system. In this study, the Ti-doped Li₃Mg₂Nb_1-xTi_xO_6-x/2 (x = 0.0-0.1) ceramics were prepared by the solid-state reaction procedure. Crystal structure refinement and microstructure analysis indicate pure phase with orthorhombic structure and homogeneous microstructure with grain size (~14 μm). The relative permittivity was affected by the relative density, cell volume, and polarizability. The Q × f value was dominated by the Nb-O bond energy and grain size. The τ_f value was correlated with the NbO₆ octahedral distortion and Nb-O bond valence. Particularly, the composition (x = 0.04) exhibited remarkable microwave dielectric performance: ε_r = 15.88, Q × f = 131 000 GHz and τ_f = −26.8 ppm/°C, providing a promising candidate for millimeter-wave applications.     

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.     

Ti4+ modified MgZrNb2O8 microwave dielectric ceramics with an ultra-high quality factor

Ti⁴⁺-modified MgZrNb₂O₈ (MgZr_1-xTi_xNb₂O₈, x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized using the traditional solid-state reaction method. Pure MgZr_1–xTi_xNb₂O₈ was detected without any secondary phase via the X-ray diffraction patterns. According to the sintering behavior and the surface morphology results, the introduction of Ti⁴⁺ reduced the sintering temperature and promoted the grain growth. The correlations between the dielectric properties and the crystal structure were analyzed through the Rietveld refinement and Raman spectroscopy. The slight shifts of the Raman peaks, corresponding to different vibration modes, were induced by the substitution of Ti⁴⁺ for Zr⁴⁺ and related to the improved quality factor. In general, the sample of MgZr_0.9Ti_0.1Nb₂O₈ sintered at 1320°C for 4 h exhibited promising microwave dielectric properties with an ultra-high Q × f value of 130 123 GHz (at 7.308 GHz, 20°C), which is potential for 5G communication applications.     

Crystal structure and enhanced microwave dielectric properties of Ta5+ substituted Li3Mg2NbO6 ceramics

Composition and structure play dominant roles in realizing the microwave dielectric properties that are necessary for the ever-increasing demands of the Internet of Things and related communication technologies. In the present study, the substitution of Ta⁵⁺ in Li₃Mg₂Nb_1−xTa_xO₆ ceramics and its effect on the structural characteristics and microwave dielectric performances is systematically studied. All the substituted compositions were determined to be pure phase orthorhombic Li3Mg2NbO6 structure of space group Fddd. Furthermore, a NbO₆ octahedral distortion, Nb-O bond valence, packing fraction and polarizability were calculated to explore the structure-property-performance paradigm in the context of microwave dielectric performance. Scanning electron microscopy revealed homogeneous microstructures, with the introduction of Ta⁵⁺ promoting grain growth. Raman spectra indicated that the variation of the band (blue shift and red shift) at 771 cm⁻¹ was highly correlated with the variation in unit cell volume. The polarizability significantly impacted ɛ_r values. The Q × f values were strongly influenced by the packing fraction and grain size. The changes in the NbO₆ octahedral distortion and Nb–O bond valence impacted the τ_f values. The Li₃Mg₂Nb_0.98Ta_0.02O₆ composition displayed the most dramatic improvements in microwave dielectric properties: ε_r = 15.58, Q × f = 113 000 GHz and τ_f  = −4.5 ppm/°C, providing a potential candidate for next generation microwave and millimeter-wave applications.     

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.     

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.     

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.     

Crystal structure,phonon modes,and bond characteristics of AgPb2B2V3O12 (B = Mg,Zn) microwave ceramics

AgPb₂B₂V₃O₁₂ (B = Mg, Zn) ceramics with low sintering temperature were synthesized via the conventional solid-state reaction route. Rietveld refinements of the X-ray diffraction patterns confirm cubic symmetry with space group . The number of observed vibrational modes and those predicted by group theoretical calculations also confirm the space group. At the optimum sintering temperature of 750°C/4 hours, AgPb₂Mg₂V₃O₁₂ has a relative permittivity of 23.3 ± 0.2, unloaded quality factor () of 26 900 ± 500 GHz (), and temperature coefficient of resonant frequency of 19.3 ± 1 ppm/°C, while AgPb₂Zn₂V₃O₁₂ has the corresponding values of 26.4 ± 0.2, 28 400 ± 500 GHz () and –18.4 ± 1 ppm/°C at 590°C/4 hours. Microwave dielectric properties of a few reported garnets and Pb₂AgB₂V₃O₁₂ (B = Mg, Zn) ceramics were correlated with their intrinsic characteristics such as the Raman shifts as well as width of A_1g Raman bands. Higher quality factor was obtained for lower full width at half-maxima (FWHMs) values of A_1g modes. The increase in B-site bond valence contributes to high and low |τ_f| with the substitution of Zn²⁺ by Mg²⁺. Furthermore, the high ionic polarizability and unit cell volume with Zn²⁺substitution contribute to increased relative permittivity.     

Structure and microwave dielectric properties of Li2Mg3Ti1-x(Al1/2Nb1/2)xO6 ceramics

Aiming to establish relationships between intrinsic structure factors and dielectric characteristics, a series of Li₂Mg₃Ti_1-x(Al_1/2Nb_1/2)_xO₆ (x = 0.0, 0.04, 0.08, 0.12, 0.16, 0.20) ceramics were synthesized to investigate the influences of (Al_1/2Nb_1/2)⁴⁺ substitution on the dielectric properties of Li₂Mg₃TiO₆ ceramics. The XRD and SEM results revealed that the pure rock salt phase (space group: Fm-3m) with a dense microstructure could be obtained with increasing the (Al_1/2Nb_1/2)⁴⁺ concentration, which is accompanied by an increase in the grain size from 11.69 to 22.81 μm. Meanwhile, some intrinsic factors, such as the average ionic polarizability, bond energy, packing fraction and lattice energy were calculated according to the complex chemical bond theory and refinement results. The unusual change in the dielectric constant (ε_r) was explained by the combined effects of the average ionic polarizability and relative density. The variation in the quality factor (Q × f) was ascribed to the packing fraction and lattice energy. The temperature coefficient of the resonant frequency (|τ_f|) reduced gradually with the increase in the octahedral bond energy, which enhanced the system thermal stability. Particularly, the Li₂Mg₃Ti_0.92(Al_1/2Nb_1/2)_0.08O₆ sample exhibited outstanding dielectric characteristics:ε_r = 15.256, Q × f = 174,300 GHz and τ_f = −19.97 ppm/°C.     

Crystal structure,Raman spectra,and modified dielectric properties of pure-phase ZnZrNb2O8 ceramics at low temperature

Low-temperature co-fired ceramics technology (LTCC) exhibits enormous superiorities in packaging, integration, and interconnection. However, the complex compositions of low-melting point sintering aids may react with ceramic matrix, which increases the difficulties of phase control and tape casting. In this work, the Li₂CO₃–B₂O₃–Bi₂O₃–SiO₂ (LBBS) sintering aid was adopted to sinter ZnZrNb₂O₈ ceramics with single phase at low temperatures. The LBBS glass could be used to fabricate pure-phase ZnZrNb₂O₈ ceramics at a low sintering temperature, promote the grain growth, and increase the densification of ZnZrNb₂O₈ ceramics. Furthermore, the unit cell volume, NbO₆ octahedral distortion, Raman shift, and FWHM changed along with LBBS addition, thereby affecting the microwave dielectric properties. Remarkably, ZnZrNb₂O₈ ceramics doped with 0.75 wt.% LBBS at 950°C were chemically compatible with the silver electrode and exhibited excellent microwave dielectric characteristics: ε_r = 27.1, Q × f = 54 500 GHz, and τ_f = −48.7 ppm/°C, providing candidates for LTCC applications.     

Crystal structure,chemical bond characteristics,and microwave dielectric properties of novel Sr2CaMoO6 ceramic at different sintering temperatures

This work prepared a novel Sr₂CaMoO₆ (SCM) ceramic through the conventional solid-state process. The crystal structure, chemical bond characteristics, and microwave dielectric properties of SCM ceramic were investigated for the first time. The X-ray diffraction patterns and Rietveld refinement indicate that SCM ceramic formed an orthorhombic phase with the space group Pmm2 (25) at temperatures above 1300 °C. The lattice vibrational modes of SCM ceramic were obtained through the Fourier transformed infrared spectrum (FTIR). The measured dielectric constant and dielectric loss of SCM ceramic is close to the theoretical values obtained by FTIR. According to the P–V–L theory, the chemical bond characteristics of SCM ceramic were calculated. The high ionicity and lattice energy of the Mo–O bond positively affect the properties of SCM ceramic. The outstanding microwave dielectric properties of ε_r = 19.37, Q·f = 32,970 GHz (at 5.96 GHz), and τ_f = ?32.56 ppm/°C were obtained in SCM ceramic sintered at 1450 °C. This work reveals the crystal structure of SCM ceramic, establishes the relationship between the properties and chemical bonds of the ceramic, and lays a foundation for studying the dielectric properties of related ceramic systems.