Crystal structure and microwave dielectric properties of non-stoichiometric Li_2+<i>x</i>Mg₂Mo₃O₁₂ ceramics

Ceramics of Li2+xMg2Mo3O12 (LxMM, 0 <= x <= 0.05) were prepared via a conventional solid-state method at 625-650 degrees C. XRD and Raman spectroscopy were utilized to characterize the phase composition, crystal structure changes, and lattice vibrations of the ceramics. The results indicated that as the Li content increased, the damping coefficient of the lattice decreased, leading to a reduction in the intrinsic loss of the ceramics. For x <= 0.02, an increase in x resulted in an increase in the relative permittivity of the ceramic, while the theoretical permittivity decreased. Moreover, the quality factor progressively increased, and the temperature coefficient of the resonant frequency neared zero. This behavior is ascribed to the enlarged cell volume, diminished lattice strain, and decreased bond valence. However, when x >= 0.03, the emergence of a new Li2MoO4 phase led to a deterioration in the microwave dielectric properties of the ceramics. The optimal microwave dielectric properties were achieved at x = 0.02, with epsilon(r) = 9.17, Qxf = 70,536 GHz@10.08 GHz, and tau(f) = -51.2 ppm/degrees C.