Structure, bond features, and microwave dielectric properties of Li2(Mg1−xCax)2(MoO4)3 ceramics for ULTCC application

Ultra-low temperature sintered ceramics, Li₂(Mg_1−xCa_x)₂(MoO₄)₃ (x = 0.025–0.100), were synthesized using the solid-phase method. XRD analysis revealed that adding Ca²⁺ increased the second-phase CaMoO₄ content. Particularly, Li₂(Mg_0.975Ca_0.025)₂(MoO₄)₃ ceramics exhibited excellent properties: ε_r = 9.31, Q × f = 55,400 GHz, and τ_f = −38.9 ppm/°C. Incorporating Ca²⁺ significantly improved the performance of ceramics. The microwave dielectric properties of Li₂(Mg_0.9Ca_0.1)₂(MoO₄)₃ ceramics sintered at 650°C showed enhancements: ε_r of 9.55, Q × f of 63,500 GHz, and τ_f of −10.8 ppm/°C. The rise in ε_r can be attributed to polarizability, while a higher packing fraction led to a larger Q × f value, and the reduced distortion of the [Li/Mg/CaO₆] octahedra enhanced the τ_f value. Moreover, the increased of CaMoO₄ also positively influenced the ceramic's properties. Furthermore, in accordance with the principles of complex chemical bonding, the elevation of ionicity, lattice energy, and bond energy of the Mo–O bond contributed to the enhancement of ε_r, Q × f, and τ_f, respectively. Raman spectroscopy showed a positive correlation between dielectric loss and full width at half maximum of the Raman peak. Good chemical compatibility between silver and Li₂(Mg_0.9Ca_0.1)₂(MoO₄)₃ ceramic was demonstrated through our co-firing experiment.