Effect of CoO substitution on the microwave dielectric properties of rock-salt structured LiMg6AlO8 ceramics

LiMg 6-x Co x AlO 8 (x = 0.01, 0.03, 0.05, 0.07) ceramics were successfully synthesized using the traditional solid-state reaction method. Among all the studied compositions, the sample with x = 0.03 sintered at 1360 °C exhibited the optimal microwave dielectric properties: a permittivity ( ε r ) of 9.98 ± 0.02, a high-quality factor ( Q  ×  f ) reaching 147,664 ± 4,205 GHz, and a temperature coefficient of resonant frequency ( τ f ) of − 23 ± 1 ppm/°C. X-ray diffraction (XRD) combined with Rietveld refinement analysis confirmed that the primary crystalline phase is a LiMg 6-x Co x AlO 8 ceramic with a rock-salt structure of space group Fm-3 m (225). Additionally, a small amount of Co 3 O 4 secondary phase (also with the space group Fm-3 m ) was detected, accounting for approximately 3% of the total. X-ray photoelectron spectroscopy (XPS) characterization confirms that Co 2+ and Co 3+ coexist in CoO after sintering in air, exhibiting a mixed valence state. Scanning electron microscopy (SEM) results revealed that the sintering temperature significantly influenced the grain morphology and densification process. It was found that the density of the ceramics played a crucial role in determining both the ε r and Q  ×  f values. Furthermore, based on the relationship τ f  = - α L — τ ε /2, the correlation between the τ f value, the cationic bond energy, and its composition dependence was discussed. Based on the analysis of P–V-L theory, the ionicity of Li–O bonds and the lattice energy of Mg-O bonds exert a dominant effect on ε r and Q  ×  f , respectively. This study holds potential reference value in the field of 5G/6G communications and lays a solid foundation for future developments in this area.