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.     

Crystal structure,infrared spectra and microwave dielectric properties of novel extra low-temperature fired Eu2Zr3(MoO4)9 ceramics

A new kind of low-temperature fired Eu₂Zr₃(MoO₄)₉ ceramic was fabricated by conventional solid-state reaction method. The mixed powders was calcined at 600 ℃ and the samples were sintered at 500 ℃ –700 ℃ for 4 h. X-ray diffraction analysis and Rietveld refinement indicated that Eu₂Zr₃(MoO₄)₉ belonged to a trigonal system with space group R-3c. Bond ionicity, lattice energy and bond energy of the ceramic were calculated by the complex chemical bond theory. Microwave dielectric properties were determined at microwave frequencies of 9.7–14.7 GHz by a network analyzer. Far infrared spectra indicated the main contribution to polarization for Eu₂Zr₃(MoO₄)₉ ceramics was the absorption of structural phonon oscillation. The ceramic sintered at 600 ℃ for 4 h exhibited the best dielectric properties with a relative permittivity (ε_r) of 10.75, a quality factor (Q·f) of 74,900 GHz and a temperature coefficient of resonance frequency (τ_f) of -8.88 ppm/℃.     

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.     

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.     

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.     

Correlations between structure and microwave dielectric properties of Co doped MgMoO4 ceramics

Mg_1-xCo_xMoO₄ (x = 0.01–0.15) ceramics were prepared by traditional solid-state methods. The phase composition, crystalline structure, micromorphology, and microwave dielectric properties of Mg_1-xCo_xMoO₄ ceramics were comprehensively studied. Mg_1-xCo_xMoO₄ ceramics present monoclinic wolframite structures from x = 0.01 to x = 0.15 with Co occupying the Mg-site. With the addition of Co²⁺, ε_r of Mg_1-xCo_xMoO₄ ceramics increase. Q × f is maximal at 5 mol% Co²⁺ content. The Mg_0.95Co_0.05MoO₄ ceramic exhibits an optimal microwave dielectric property: ε_r = 7, Q × f = 59247 GHz, τ_f = −68 ppm/°C. The Q × f values increase by 20% compared with the pure MgMoO₄ ceramics (~49149 GHz). Doping Co²⁺ effectively promotes the densification of ceramics and increases ε_r and Q × f. However, when the Co content exceeds 5 mol%, the decreased packing fraction and disorder distribution of ions contribute to the increase in dielectric losses. The correlations between Co²⁺ substitution and wolframite structure have been discussed by Raman spectroscopy, FT-IR spectroscopy and Rietveld refinement.     

Phase composition,Raman spectra,infrared spectra and dielectric properties of Li2MgTi1-x(Mg1/3Nb2/3)xO4 ceramics at microwave frequency

The Li₂MgTi_1-x(Mg_1/3Nb_2/3)_xO₄ (0?≤x?≤?0.5) ceramics were prepared by the conventional solid-state method. The relationship among phase composition, substitution amount and microwave dielectric properties of the ceramics was symmetrically investigated. All the samples possess the rock salt structure with the space group of Fm-3m. As the x value increases from 0 to 0.5, the dielectric constant linearly decreases from 16.75 to 15.56, which can be explained by the variation of Raman spectra and infrared spectra. The Q·f value shows an upward tendency in the range of 0?≤x?≤?0.3, but it then decreases when x?>?0.3. In addition, the temperature coefficient of resonant frequency (τ_f) is shifted toward zero with the increasing (Mg_1/3Nb_2/3)⁴⁺ addition. By comparison, the Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄ ceramics sintered at 1400?°C can achieve an excellent combination of microwave dielectric properties: ε_r=?16.19, Q·f =?160,000?GHz and τ_f =??3.14?ppm/°C.     

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).     

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.     

Crystal structure,vibration characteristics and microwave dielectric properties of low loss MgMo1-xWxO4 solid solution ceramics

Solid-phase method was used to synthesize MgMo_1-xW_xO₄ (x = 0–0.15) ceramics. The influences of substitution Mo⁶⁺ with W⁶⁺ on crystal structure, vibration characteristics and microwave dielectric properties of MgMo_1-xW_xO₄ ceramics were comprehensively studied. X-Ray diffraction illustrated all samples exhibit single-phase monoclinic wolframite structure when x = 0–0.15, in which W⁶⁺ replaces Mo⁶⁺ sites formed solid solution. W⁶⁺ effectively improves sintering properties of the MgMoO₄, the average grain size and relative density were increased. Raman characterization reveals that suitable W⁶⁺ substitution amount leads to reduction of v₁ A_g peaks FWHM and the enhancement of specific v₃ A_g peak for Mo/WO₄ tetrahedron, which improves the ordered distribution of the crystal structure. The above combined effect results in the increased Q × f value, but has little influence of W⁶⁺ substitution on ε_r and τ_f for MgMoO₄. When x = 0.09, MgMo_0.91W_0.09O₄ ceramic sintered at 1050 °C has optimal microwave dielectric performance: ε_r = 7.21, Q×f = 90,829 GHz, τ_f = ?67 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.     

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.     

A/B-site cosubstituted Ba4Pr28/3Ti18O54 microwave dielectric ceramics with temperature stable and high Q in a wide range

High permittivity Ba₄(Pr_1-xSm_x)_28/3Ti_18-yAl_4y/3O₅₄(0.4≤x ≤ 0.7, 0≤y ≤ 1.5) ceramics were synthesized using a standard solid-state method. The effects of Sm³⁺ substitution into the A-site and Sm³⁺/Al³⁺ cosubstitution into the A/B-sites on the microstructure, crystal structure, Raman spectra, infrared reflectivity (IR) spectra and dielectric characteristics were investigated in a Ba₄Pr_28/3Ti₁₈O₅₄ solid solution. In the ceramic samples of Ba₄(Pr_1-xSm_x)_28/3Ti₁₈O₅₄(0.4≤x ≤ 0.7), Sm³⁺ partial substitution for Pr³⁺ could improve the quality factor (Qf) value and reduce the TCF value. Nevertheless, the quality factor (Qf~10,000GHz) needed further improvement and the TCF values (+12.3~+35.4 ppm/°C) were still too large. Therefore, Al³⁺ was introduced for further optimization of the TCF values and Qf values of the Ba₄(Pr_1-xSm_x)_28/3Ti₁₈O₅₄ ceramics. Sm/Al cosubstitution led to a good combination of high ε_r (ε_r ≥ 70), high Qf (Qf ≥ 12,000 GHz), and near-zero TCF (−10 < TCF < +10 ppm/°C) in a wide range (0.4≤x ≤ 0.7). Infrared reflectivity (IR) spectra indicated that A-TiO₆ vibration modes gave the primary contribution rather than Ti–O bending and stretching modes. The decrease in the degree of B-site cations order could be confirmed by Raman spectra. XPS results demonstrated that the improvement of quality factor (Qf) value was strongly related to the suppression of Ti³⁺. Excellent dielectric properties were achieved in Ba₄(Pr_1-xSm_x)_28/3Ti_18-yAl_4y/3O₅₄ microwave ceramics with x = 0.5 and y = 1.25: ε_r = 72.5, Qf = 13,900GHz, TCF = +1.3 ppm/°C.     

Crystal structure and performance modification of a novel triclinic CaMgP2O7 microwave dielectric ceramic with low sintering temperature

In this study, crystal structure and microwave dielectric properties of phosphate CaMgP₂O₇ were comprehensively investigated. As a novel microwave dielectric ceramic, CaMgP₂O₇ consists of highly dense structure with optimal microwave dielectric properties (ε_r = 7.8 ± 0.124, Q×f = 13,165 ± 836 GHz, and τ_f = −85.04 ± 1.205 ppm/℃) at a low sintering temperature (950 ℃). The Rietveld refinement of XRD patterns revealed that CaMgP₂O₇ belongs to a triclinic system with P-1 symmetry type. Moreover, the substitution of Zn²⁺ for Mg²⁺ in CaMgP₂O₇ can further reduce the sintering temperature, effectively promote the densification process, and improve the Q×f value. The effects of porosity (or density) and chemical bond characteristics on the performance of CaMg_1-xZn_xP₂O₇ ceramics were carefully analyzed as well. Outstanding performance (ε_r = 8.05 ± 0.12, Q×f = 20,670 ± 923 GHz, and τ_f = −87.59 ± 3.24 ppm/℃) can be achieved for the CaMg_0.84Zn_0.16P₂O₇ ceramic sintered at 875 ℃ for 3 h.     

Crystal structure and microwave dielectric properties of garnet-type Ca2YZr2-xTixAl3O12 ceramics for dual-band bandpass filters

Microwave dielectric ceramics with a high-quality factor are vital materials for substrates, dielectric resonators, and filters in millimeter-wave communication systems. Here, a novel microwave dielectric ceramic based on a garnet-type Ca₂YZr₂Al₃O₁₂ compound for bandpass filter was prepared using the solid-state reaction method. Sintering the Ca₂YZr₂Al₃O₁₂ ceramics at 1600 ℃ for 5 h resulted in excellent microwave dielectric properties of ε_r = 10.81 ± 0.16, Qf = 87,628 ± 4000 GHz and τ_f = ?34.3 ± 0.5 ppm/℃. Increasing the Ti⁴⁺ content of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics significantly improved the sintering process. Superior microwave dielectric properties (ε_r = 11.93 ± 0.15, Qf = 121,930 ± 2600 GHz and τ_f = ?30.8 ± 0.4 ppm/℃) were obtained for Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) because of its dense microstructure, large grain size and high lattice energy. The Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) ceramics were used to fabricate a dual-band bandpass filter with a hairpin structure that exhibited a large return loss (|S₁₁| > 12 dB) and a small insertion loss (|S₂₁| < 0.73 dB). The high performance of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics and the corresponding bandpass filter makes this material a potential candidate for millimeter-wave devices.     

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).     

The microwave dielectric properties and crystal structure of low temperature sintering LiNiPO4 ceramics

The LiNiPO₄ ceramic for the LTCC technology was prepared via the traditional solid-state reaction route and its dielectric properties were investigated for the first time. The best dielectric properties of LiNiPO₄ ceramics with a ε_r of 7.18, Q×f value of 27,754?GHz and τ_f of ?67.7?ppm/°C were obtained in samples sintered at 825?°C for 2?h. Rietveld refinement was firstly employed to study the crystal structure and dielectric properties of LiNiPO₄ ceramics. Unfortunately, the relatively large negative τ_f was unfavorable to practical applications. Therefore, we introduced TiO₂, which possessed a considerable positive τ_f, to obtain a desired τ_f value. The prepared LiNiPO₄ ceramics with 15?wt% TiO₂ sintered at 900?°C for 2?h exhibited excellent dielectric properties of ε_r～11.49, Q×f～10,792?GHz, τ_f～?2.8?ppm/°C. The Ag co-fired experiments confirmed the excellent chemical compatibility with LiNiPO₄-TiO₂ ceramics which might be potential dielectric LTCCs for high frequency applications.     

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.