Effect of Mn/Mg co-substitution on the phase composition, microstructure, and microwave dielectric properties of SrAl 2-x (Mn 0.5 Mg 0.5 ) x Si 2 O 8 ceramics

SrAl _2−x (Mn _0.5 Mg _0.5 ) _x Si ₂ O ₈ (x = 0–0.06) microwave dielectric ceramics were synthesized via a solid-state reaction route. The sintering behavior, phase composition, microstructure, and microwave dielectric properties were systematically investigated by X-ray diffraction (XRD) with Rietveld refinement and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), together with microwave dielectric measurements. XRD–Rietveld refinement confirms that a single-phase monoclinic SrAl ₂ Si ₂ O ₈ solid solution is maintained for x ≤ 0.02, whereas a Sr ₂ MgSi ₂ O ₇ secondary phase appears for x ≥ 0.03 and increases with substitution, indicating the solubility limit of Mn/Mg co-substitution. The introduction of Mn/Mg effectively lowers the optimum densification temperature from 1500°C (x = 0) to 1350°C (x = 0.06). The composition x = 0.02 sintered at 1400°C for 5 h exhibits the highest relative density (98.7%) and achieves ε _r  = 7.112, Q×f  = 43569 GHz, and τ _f  = − 37.8 ppm/°C. When x > 0.02, secondary-phase precipitation and microstructural inhomogeneity (as evidenced by SEM/EDS) reduce densification and lead to degraded dielectric performance, with Q×f decreasing to 28800 GHz at x = 0.06. These results clarify the solid-solution window and provide a feasible compositional strategy for lowering firing temperature while maintaining low-permittivity and low-loss performance in SrAl ₂ Si ₂ O ₈ -based microwave dielectrics.