Novel medium-entropy Ca₃(Co_0.25Zn_0.25Mg_0.25Cu_0.25)₂SiV₂O₁₂ microwave dielectric ceramics with high Q×f and near-zero τ_f

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.