In this work, novel medium-entropy Ca₃(Co_0.25Zn_0.25Mg_0.25Cu_0.25)₂SiV₂O₁₂ (CZMC) ceramics with garnet structure were designed and synthesized based on the entropy effect. A small average atomic size difference was proven to be advantageous for the formation of a garnet-structured solid solution, as evidenced via X-ray diffraction (XRD) and Rietveld refinement results. The sintering temperature was effectively reduced due to an increase in the configurational entropy (ΔS_config). The dielectric constant (ε_r) of the CZMC ceramics showed the opposite trend to that of the Raman shift at approximately 838 cm⁻¹, whereas the variation in the quality factor (Q×f) was identical to that of the relative density but opposite to that of the full width at half maximum (FWHM) of the Raman spectra. Alternating current (AC) impedance spectroscopy revealed that the conductivity of the CZMC ceramics was affected mainly by the diffusion of thermally activated oxygen vacancies. The high activation energy further indicated that the low defect concentration in the sample contributed to reducing the dielectric loss of the ceramics. The B‒O bond had the strongest contribution to the total bond energy, thus playing an important role in manipulating the temperature coefficient of the resonant frequency (τ_f) of the ceramics. Moreover, the V‒O bond significantly influenced the ε_r and the Q×f of the CZMC ceramics. Finally, superior microwave dielectric properties (ε_r = 10.89, Q×f = 59,200 GHz, and τ_f = –9.6 ppm/°C) together with a high relative density of 95.4% were achieved at 1010 °C. Therefore, extensive applications can be found in the field of millimeter wave communication for CZMC ceramics.

Complex ion substitution is gaining more attention as an appealing method of modifying the structure and performance of microwave ceramics. In this work, Li₂Zn[Ti_1−x(Ni_1/3Nb_2/3)_x]₃O₈ (LZTNNx, 0 ≤ x ≤ 0.3) ceramics were designed based on the complex ion substitution strategy, following the substitution rule of radius and valence to investigate the relationship among phase compositions (containing oxygen vacancies and Ti³⁺ ions), microstructures, and microwave dielectric characteristics of the LZTNNx ceramics. The samples maintained a single Li₂ZnTi₃O₈ solid solution phase as x ≤ 0.2, whereas the sample of x = 0.3 produced a second phase with the LiNbO₃ structure. The appropriate amount of (Ni_1/3Nb_2/3)⁴⁺ substitution could slightly improve the densification of the LZTNNx ceramics due to the formation of the Li₂ZnTi₃O₈ solid solution accompanied by a decrease in the average grain size. The presence of a new A_1g Raman active band at about 848 cm⁻¹ indicated that local symmetry changed, affecting atomic interactions of the LZTNNx ceramics. The variation of the relative dielectric constant (ε_r) was closely related to the molar volume ionic polarizability ( ${\alpha }_{\text{D}}^{\text{T}}$), and the temperature coefficient of the resonant frequency (τ_f) was related to the bond valence (V_i) of Ti. The increase in density, the absence of the Ti³⁺ ions and oxygen vacancies, and the reduction in damping behavior were responsible for the decreased dielectric loss. The LZTNN0.2 ceramics sintered at 1120 ℃ exhibited favorable microwave dielectric properties: ε_r = 22.13, quality factor (Q×f) = 97,350 GHz, and τ_f = −18.60 ppm/℃, which might be a promising candidate for wireless communication applications in highly selective electronics.

A rapid and facile approach was developed for the synthesis of ultrafine SmAlO₃ powders through the combustion of stearic acid precursors. The obtained products were characterized by typical techniques including X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to analyze the phase composition and microstructure. The dielectric characteristics of SmAlO₃ microwave ceramics, using the as-obtained products as original materials, were also studied. Compared with the conventional solid-state reaction method, the synthesis temperature was dramatically reduced to 750 ℃. The large-size sheet structure was composed of a number of micro/nano-scale crystallites, which were mostly irregular in shape due to the mutual growth and overlapping shapes of adjacent grains. The SmAlO₃ ceramics with high density and uniform microstructure were obtained after sintering at 1500 ℃ for 4 h due to the favorable sintering activity of the as-synthesized powders. In addition, desired dielectric properties at microwave frequencies (dielectric constant ε_r = 20.22, quality factor Q·f = 74110 GHz, and a temperature coefficient of resonant frequency τ_f = -74.6 ppm/℃) were achieved.