Phase composition, crystal structures and enhanced microwave dielectric properties of Mg²⁺/Zr⁴⁺ co-doped Al₂Mo₃O₁₂ ceramics

In this work, the substitution of Al³⁺ by Zr/Mg in Al₂Mo₃O₁₂ was designed to enhance the quality factor while simultaneously adjusting the thermal stability of the resonant frequency. The Al_2–2x(Zr_0.5Mg_0.5)_2xMo₃O₁₂ (AMZM) (0.1 ≤ x ≤ 0.5) ceramics were synthesized using a simple solid-state method, resulting in all components exhibiting single-phase crystallinity and a dense structure. A phase transition temperature below room temperature was observed when x ≥ 0.4. Furthermore, the quality factor of the synthesized samples, compared to pure-phase Al₂Mo₃O₁₂ (ε_r = 6.79, Q×f = 28471 GHz, τ_f = −48.77 ppm/℃), increased by nearly threefold. A detailed investigation of the crystal structure of the samples was conducted through Rietveld refinement. The composition with x = 0.3 (monoclinic phase) exhibited optimal dielectric performance after sintering at 800 ℃ for 6 h: ε_r = 7.84, Q×f = 86511 GHz, τ_f = −42.08 ppm/℃. By forming a composite ceramic with (Li_1/2Sm_1/2)MoO₄, the negative τ_f value was further adjusted to near zero.