Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents Study of (1−x)Ba0.6Sr0.4La4Ti4O15–xTiO2 Ceramics

Microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ (x = 0, 0.01, 0.02, 0.05, and 0.1) ceramics were studied. X‐ray diffraction analysis indicates that the specimens show a hexagonal perovskite structure; however, with an increase of x to 0.1, TiO_2?δ as a secondary phase can be detected in the ceramics. The variation of TiO₂ content has a significant effect on the dielectric properties of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ at microwave frequency. The dielectric permittivity of ceramics increases from 44 to 49 with the increase of TiO₂ content. The Qf value is in the range of 39 300–53 400 GHz. However, the temperature coefficient of resonant frequency (τ_f) changes from ?7.5 to–9.4 ppm/°C, and then turns to +3.9 ppm/°C. A near zero τ_f value can be obtained by tuning the content of TiO₂ addition. TSDC was also employed to analysis the extrinsic loss mechanism. Utilizing a fixed polarization condition, the TSDC relaxation peaks are present, which are generated by oxygen vacancies. And the concentration of oxygen vacancies increases with the increase of TiO₂ content. It can be concluded that the extrinsic dielectric loss is dominated by microstructure and oxygen vacancy defects.     

Structure,Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)Ba0.6Sr0.4La4Ti4O15–xBa5Nb4O15 Solid Solutions

(1 ? x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xBa₅Nb₄O₁₅ (x = 0.05, 0.1, 0.15 and 0.2, BSLT–BN) ceramic samples were prepared by co‐firing the mixtures of Ba_0.6Sr_0.4La₄Ti₄O₁₅ and Ba₅Nb₄O₁₅ powders. Crystal structure, microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of the BSLT–BN series ceramics were investigated. X‐ray diffraction patterns reveal that all the samples exhibit a hexagonal perovskite structure, which implies that the BSLT–BN mixtures form solid solutions. With increasing Ba₅Nb₄O₁₅ content, the diffraction peaks shift to low angles and the sintering temperature of BSLT–BN decreases. Raman spectra analysis reveals the shifting and splitting of the vibration modes. The microwave dielectric properties of the well‐sintered (1 ? x)BSLT–xBN ceramics vary with Ba₅Nb₄O₁₅ content. The dielectric permittivity of the ceramics exhibits a slight decreasing trend. The quality factor varies in the range of 45 000–11 200 GHz, whereas near‐zero temperature coefficients of the resonant frequency may be achieved by changing the Ba₅Nb₄O₁₅ content. TSDC was utilized to explore the extrinsic loss mechanism associated with defects. TSDC relaxation peaks are mainly generated by oxygen vacancies, and the Ba₅Nb₄O₁₅ content has a significant influence on the TSDC spectra.     

Lattice dynamics and terahertz response of microwave dielectrics: A case study of Al-doped Ca0.6Sm0.27TiO3 ceramics

Low-loss microwave dielectric ceramics are of great significance for fifth-generation telecommunication (5G) devices used for higher frequencies, while reducing the dielectric loss in the microwave band is still a technical challenge. Lattice dynamics and terahertz response are closely related to the polarization of dielectrics, and can be used as effective methods to explore the loss mechanism of microwave dielectrics. Here, Al³⁺ was doped into the conventional high-permittivity (ε_r) Ca_1-xSm_2x/3TiO₃ (x = 0.4) ceramic system, enhancing the Q×f values significantly. Raman spectra, thermally stimulated depolarization currents (TSDC) and terahertz time-domain spectra (THz-TDS) together showed that the stiffness of cation vibration became stronger when z value was larger, indicating a smaller damping of the lattice vibration, and leading to lower extrinsic losses and higher Q×f values. The relationship between lattice dynamics, defect behavior and microwave dielectric loss will provide a significant reference for the development of low-loss microwave dielectric ceramics.     

Dielectric and ferroelectric properties of Ta-modified Bi3.25La0.75Ti3O12 ceramics

B-site modified Bi_3.25La_0.75Ti_3-xTa_xO₁₂ ceramics were prepared by the conventional solid-state reaction method. The influence of Ta₂O₅ on microstructure and electric properties of the ceramics was investigated. The results demonstrated that Ta⁵⁺ ions were dissolved into the perovskite lattice and homogeneously distributed in the matrix without forming any minority phase. The conduction mechanism and dielectric response behavior were transformed with Ta substation, which is triggered by varied structural distortion characteristics and defect diploes. The Curie temperature decreased gradually with increasing Ta content and a relaxor-like behavior was observed for x = 0.09 sample. The internal bias field is decreased with Ta doping, because the substitution of Ta⁵⁺ at B-site contributes to release the involved oxygen vacancies in defect diploes. Moreover, further increasing Ta content causes a reduction in the oxygen vacancies located at lattice misfits, resulting in a decrease of coercive fields. An improved ferroelectric properties were obtained for x = 0.09 sample with a relatively lower coercive field and a larger spontaneous polarization.     

Influence of Ti4+ substitution for Ta5+ on the crystal structure,Raman spectra,and microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 ceramics

This paper systematically investigated the influence of Ti⁴⁺ substitution for Ta⁵⁺ on the phase composition and microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1, 0.2, and 0.3) ceramics with hexagonal tungsten bronze-like structures. X-ray diffraction and Rietveld refinement results indicated that single-phase Ba₃Ta₄Ti₄O₂₁ could be obtained only with the x values of 0.1 and 0.2, and a secondary phase was detected at an x value of 0.3. The valence state of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.2) ceramics was analyzed through X-ray photoelectron spectroscopy. Increasing Ti⁴⁺/Ta⁵⁺ ratios could reduce sintering temperature and improve the microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ solid solutions. However, the dielectric properties, particularly the quality factor, of Ba₃Ta_4-4xTi_4+5xO₂₁ ceramics deteriorated severely as a result of oxygen vacancy defects caused by the transition of the valence state from Ti⁴⁺ to Ti³⁺ when x = 0.2 and the coexistence of the secondary phase when x = 0.3. Infrared reflectivity spectroscopy was performed to explore the intrinsic dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics. The measured and extrapolated microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics sintered at 1240 °C for 6 h were ε_r ~ 46.5, Q × f = 13,900 GHz, τ_f ~ +49.4 ppm/°C, and ε_r ~ 44, Q × f = 34,850 GHz.     

Effects of Mn-dopant on phase,microstructure and electrical properties in Bi3.25La0.75Ti3O12 ceramics

Lanthanum-doped bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂ or BLT) is important ferroelectric materials for FeRAMS, which need further improved by substituting isovalent cations to assist the elimination of defects such as oxygen vacancy. In this work, fabrication and investigation of substituting Mn⁴⁺ for Ti⁴⁺ ion on B-site of Bi_3.25La_0.75Ti₃O₁₂ ceramics were carried out. X-ray diffraction patterns of BLTMn ceramics indicated orthorhombic structure with lattice distortion, especially for samples with higher concentration of MnO₂ dopant. Microstructural investigation showed that all ceramics composed mainly of plate-like grains. An increase in MnO₂ doping content increased diameter and thickness of grains but reduced density of the ceramics. Electrical conductivity was found to decrease while dielectric constant increased with Mn⁴⁺ doping concentration.     

Impedance spectroscopy,B-site cation ordering and structure–property relations of (1 − x) La[Al0.9(Mg0.5Ti0.5)0.1]O3–x CaTiO3 ceramics for 5G dielectric waveguide filters

Dense (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ ceramics were synthesized via solid-state reaction. The crystal structure and microwave dielectric properties of the ceramics were systematically investigated. Rietveld refinement revealed that when x ≤ 0.2, the ceramics had a rhombohedral structure with an R-3c space group. When x ≥ 0.5, the ceramics had an orthorhombic structure with a Pbnm space group. Selected area electron diffraction and Raman spectroscopy analyses proved that the microwave dielectric ceramics had a B-site order, which accounted for the great improvement in microwave dielectric properties. The content of oxygen vacancies was identified through X-ray photoelectron spectroscopy, and the change rule of Q × f was closely related to oxygen vacancy content. The perturbation of A-site cations had an important influence on dielectric constant. Specifically, with the increase in Ti⁴⁺ content, the perturbation effect of the A-site cations was enhanced and dielectric constant increased. When x = 0.65, the temperature coefficient of resonant frequency of the (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ microwave dielectric ceramics was near zero. The optimal microwave dielectric properties of 0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃–0.65CaTiO₃ were ε_r = 44.6, Q × f = 32,057 GHz, and τ_f = +2 ppm/°C.     

Characterization of microwave and terahertz dielectric properties of single crystal La2Ti2O7 along one single direction

New generation wireless communication systems require characterisations of dielectric permittivity and loss tangent at microwave and terahertz bands. La₂Ti₂O₇ is a candidate material for microwave application. However, all the reported microwave dielectric data are average value from different directions of a single crystal, which could not reflect its anisotropic nature due to the layered crystal structure. Its dielectric properties at the microwave and terahertz bands in a single crystallographic direction have rarely been reported. In this work, a single crystal ferroelectric La₂Ti₂O₇ was prepared by floating zone method and its dielectric properties were characterized from 1 kHz to 1 THz along one single direction. The decrease in dielectric permittivity with increasing frequency is related to dielectric relaxation from radio frequency to microwave then to terahertz band. The capability of characterizing anisotropic dielectric properties of a single crystal in this work opens the feasibility for its microwave and terahertz applications.     

Crystal structure and microwave dielectric properties of La2(Zr1−xTix)3(MoO4)9 (0 ≤ x ≤ 0.1) ceramics

La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics were prepared by the traditional solid-state reaction method. XRD analysis showed that La₂(Zr_1−xTi_x)₃(MoO₄)₉ (0 ≤ x ≤ 0.1) ceramics belonged to a trigonal system. Based on the chemical bond theory, the consequences of bond energy, bond ionicity, lattice energy, and thermal expansion coefficient of ceramics on microwave dielectric properties were discussed. As Ti⁴⁺ addition was increased, the reduction in dielectric constant was ascribed to the fact that the polarizability of Ti⁴⁺ is smaller than Zr⁴⁺, and the downward trend was related to the bond ionicity. Besides, the tendency of Q·f value depended on the packing fraction and the lattice energy. The improvement in τ_f value, the increase in bond energy, and the decrease in the coefficient of thermal expansion were all correlated. The far-infrared spectra implied that the absorptions of structural phonon oscillation were the main reason for the maximum polarization contribution. La₂(Zr_0.92Ti_0.08)₃(MoO₄)₉ ceramics sintered at 750°C for 4 hours exhibited the best dielectric properties (ε_r = 10.33, Q·f = 80 658 GHz, and τ_f = 3.48 ppm/°C).     

Microwave dielectric properties,low-temperature sintering mechanism,and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li₂Zn₃Ti₄O₁₂-xSr₃(VO₄)₂ (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO₃ and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τ_f value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti³⁺, and V⁴⁺, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li₂Zn₃Ti₄O₁₂-0.3Sr₃(VO₄)₂ ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of ε_r = 17.8, Q × f = 41,891 GHz, and τ_f = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.     

Microwave dielectric properties of Ca1.15RE0.85Al0.85Ti0.15O4 (RE=Nd,La, Y) ceramics prepared by the reaction sintering method

Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics were prepared by a reaction sintering method. The sintering behavior, phase composition, microstructure and microwave dielectric performances of ceramics were investigated. X-ray diffraction patterns illustrated that both the Ca_1.15Nd_0.85Al_0.85Ti_0.15O₄(CNAT) and Ca_1.15Y_0.85Al_0.85Ti_0.15O₄(CYAT) ceramics are single-phase structures, and the Ca_1.15La_0.85Al_0.85Ti_0.15O₄(CLAT) ceramic contain LaAlO₃ and CaO phases. The apparent morphology and elemental distribution of the ceramic samples were analyzed by using scanning electron microscope and energy dispersive spectrometer. When the sintering temperature is 1500 °C, the CNAT and CYAT ceramics have the best microwave dielectric properties with ε_r = 19.2, Q × f = 74924 GHz, τ_f = ?1.21 ppm/°C and ε_r = 17.5, Q × f = 27440 GHz, τ_f = ?5.79 ppm/°C, respectively. And the best microwave dielectric properties of ε_r = 17.5, Q × f = 22568 GHz, τ_f = ?14.69 ppm/°C were obtained for the CLAT ceramic sintered at 1525 °C. The reaction sintering method provides a low-cost, economical and straightforward method for the preparation of the Ca_1.15RE_0.85Al_0.85Ti_0.15O₄ (RE = Nd, La, Y) ceramics, which has promising potential for application.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Investigation of the microwave dielectric properties of the Li4x/3Zn2−2xTi1+2x/3O4 system by P–V–L theory and vibration spectroscopy

Li_4x/3Zn_2–2xTi_1+2x/3O₄ microwave dielectric ceramics with a spinel phase were prepared via a high-temperature solid-phase method. P–V–L theory, vibration spectra, and XPS were utilized to establish the links between the intrinsic and extrinsic factors and the microwave dielectric properties. According to the characterization, the change in permittivity (ε_r) was ascribed to the increase in the average bond ionicity of Ti–O(Af_iTi-O) and the polar mode of the lattice vibration; the change in quality factor(Q × f) resulted from the change in the Ti–O lattice energy (AU_Ti-O) and existence of oxygen vacancy; the increase in temperature coefficient of the resonance frequency (τ_f) was triggered by the increase in the Ti–O bond energy. The Li_0.6Zn_1.1Ti_1.3O₄ ceramics (x = 0.45) sintered at 1125 °C finally obtained optimal microwave dielectric constants of ε_r = 17.3, Q × f = 76,318 GHz and τ_f = -58 ppm/°C.     

Effects of ionic polarizability and crystal structure on microwave dielectric properties of RE2O3 (RE = La,Eu) ceramics with opposite τf and high Q

Hexagonal La₂O₃ and monoclinic Eu₂O₃ ceramics were prepared, and their microwave dielectric properties were investigated. La₂O₃ sintered at 1400 °C exhibited promising microwave dielectric properties of ε_r = 18.6, Q×f = 71,400 GHz, and a negative τ_f of − 35.1 ppm/°C, while Eu₂O₃ sintered at 1500 °C possessed relative lower ε_r and Q×f values of 17.9 and 35,000 GHz, respectively, with an abnormally positive τ_f of + 19.6 ppm/°C. The difference in their microwave dielectric properties is mainly due to lattice-induced strain, which can be characterized by bond valence. To investigate the degradation of RE₂O₃ (RE = La, Eu) ceramics in air, a series of La_2−xEu_xO₃ (x = 0.5, 1, and 1.5) ceramics were prepared. The results of the present study suggest that the introduction of Eu³⁺ effectively prevents the decomposition of La₂O₃.     

Giant permittivity up to 100 MHz in La and Nb co-doped rutile TiO2 ceramics

Dielectric spectroscopy was carried out for reduced and stoichiometric La_0.0025Nb_0.0025Ti_0.995O₂ ceramics synthesized by sintering in different atmospheres. A giant permittivity (~1 × 10⁴) was obtained at a frequency of 100 MHz and temperature range from 170 to 350 K. Three dielectric relaxation mechanisms were observed within the temperature range of 10-300 K via dielectric spectroscopy. A low temperature dipole relaxation peak (in the temperature range of 10-30 K) in the spectra was identified to be associated with the giant permittivity specifically measured at 100 MHz. The origin of such giant permittivity was attributed to dipole orientation polarization. Hopping polaron and interfacial effect contributed to giant permittivity. After annealing treatment, all the relaxation contributions were weakened. Low dielectric loss was attributed to high resistance of grain and grain boundaries. Annealing in ambient conditions led to decreased relaxation times which gives the signature of decreased concentration of oxygen vacancies and Ti³⁺. Dipoles which were related to oxygen vacancies and Ti³⁺, resulted in giant permittivity up to 100 MHz.     

Microwave dielectric properties of La3Ti2TaO11 ceramics with perovskite-like layered structure

The first characterization of the microwave dielectric properties of the La₃Ti₂TaO₁₁ ceramics is presented. An ordinarily sintered ceramic at 1560 °C exhibits good microwave dielectric properties with ?_r = 46, Q × f = 7500 GHz and τ_f = ?47 ppm/°C. An alternative approach to tailor the temperature coefficient of resonate frequency of La₃Ti₂TaO₁₁ ceramics is also presented. Textured La₃Ti₂TaO₁₁ ceramics were fabricated using spark plasma sintering (SPS). By controlling the sintering temperature, orientation degree increased together with the steadily increase in ?_r and Q × f. A noteworthy change in τ_f from ?43.1 ppm/°C to ?13.6 ppm/°C with increasing orientation degree was observed. These results suggest that grain-orientation control was an effective way to tailor the microwave dielectric properties of La₃Ti₂TaO₁₁ ceramics.     

A new BaB2O4 microwave dielectric ceramic for LTCC application

To satisfy the requirements of miniaturization and integration of microwave devices, microwave dielectric ceramics with low sintering temperatures and good microwave dielectric properties are particularly important for LTCC materials. In this study, low-cost BaB₂O₄ ceramics were prepared with different Ba/B ratios using a solid-phase method. Combined with the Raman spectra, the effects of the Raman shift and FWHM of the vibration peaks on the microwave dielectric properties were determined. As a novel microwave dielectric ceramic, BaB₂O₄ consists of a highly dense structure with optimal microwave dielectric properties (ε_r = 4.06, Q×f = 23845 GHz, and τ_f = −7.2 ppm/℃) at a low sintering temperature (840 ℃). In addition, BaB₂O₄ ceramic is chemically compatible with Ag, making it a promising candidate substrate for microwave communications.     

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).     

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La₂Ti₂O₇ (LTO) ceramics have been enhanced through the substitution of La³⁺ ions by Pr³⁺ ions. Almost all doped Pr³⁺ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La_1.9Pr_0.1Ti₂O₇ ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO₆ oxygen octahedrons’ distortions in grains is 1.6 eV for La_1.8Pr_0.2Ti₂O₇ ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d₃₃ = 3.0 pC/N of La_2-xPr_xTi₂O₇ ceramics maintains stable when the Pr³⁺ doping content x ranging from 0.1 to 0.3.     

Phase composition and microwave dielectric properties of Ca0.128Ba0.032Sm0.46Li0.3TiO3 ceramics with alumina addition

Ca_0.128Ba_0.032Sm_0.46Li_0.3TiO₃-x wt.% Al₂O₃ microwave dielectric ceramics with temperature stability were prepared by the traditional solid-state reaction method. The effects of Al₂O₃ doping on phase composition, micromorphology, and microwave dielectric properties of Ca_0.128Ba_0.032Sm_0.46Li_0.3TiO₃ ceramics were investigated. X-ray diffraction results showed that no other phase was introduced by doping, and the diffraction peaks of all the ceramics could be indexed by standard JCPDS cards of No.42?0423 (CaTiO₃) and No.43?0235 (BaSm₂Ti₄O₁₂). To further study the effects of Al₂O₃ doping on microstructure and lattice vibration of Ca_0.128Ba_0.032Sm_0.46Li_0.3TiO₃ ceramics, the structural refinement and Raman spectroscopy techniques were also adopted. Doping Al₂O₃ decreased the lattice constant of CaTiO₃-phase solid solution, modified the morphology and improved microwave dielectric properties of the ceramics. When sintered at 1300 °C for 4 h, the ceramics with 0.9 wt.% Al₂O₃ showed excellent microwave dielectric properties: ε_r = 87.9 ± 0.4, Q × f = 6671 ± 103 GHz, and τ_f = -1.2 ± 0.9 ppm/℃.