Effects of Zn2+/Si4+co-doping on the cation distribution, crystal structure, and microwave dielectric characteristics of spinel-structured MgAl2O4 ceramics

Investigating the relationship between the structure and dielectric properties of cation-mixed spinel ceramics is crucial for overcoming the limitations of conventional microwave materials. In this study, MgAl2-x(Zn0.5Si0.5)(x)O-4 (x = 0, 0.02, 0.04, 0.06, and 0.08) ceramics were prepared by a solid-state reaction method. The co-substitution for Al with Zn2+/Si4+ induces Al3+ cations for preferential occupation of the 8a site in the oxygen tetrahedra, forming a solid-solution ceramic at x <= 0.06. Enhanced ionic diffusion and compressive stress contribute to a dense microstructure (rho > 95 %) with uniform grain-size distribution after heating at 1650 degrees C for 4 h. Raman and Al-27 NMR spectra reveal a relationship between cation distribution and covalency, consistent with the results of Rietveld refinement. The increases in cation disorder and degree of inversion are accompanied by an enhanced covalency of the Al-O bonds, which reduces intrinsic dielectric loss. In addition, the combination of Shannon ion polarizability and bond valence theory suggests that the increases in permittivity (epsilon(r)) and the temperature coefficient of resonant frequency (tau(f)) are related to the high polarizability of the dopant ions and the abnormal polarizability derived from the decreasing bond valence of the Al2-O site. The sample at x = 0.06 exhibits a low epsilon(r) of 8.31, high Q x f of 194,159 GHz (at similar to 11.35 GHz), and tau(f) of -30.8 ppm/degrees C, indicating superior microwave application prospects as compared with traditional Mg-Al spinel ceramics.