Low-permittivity and low dielectric loss in co-fired Na₂O–Bi₂O₃–Ln₂O₃–MoO₃ (Ln = Sm_0.3Y_0.7) microwave dielectric ceramics

This study examines the microwave dielectric properties of the Na₂O–Bi₂O₃–Ln₂O₃–MoO₃ (Ln = Sm and Y) system, synthesized via a solid-state process at ultra-low sintering temperatures ranging from 600°C to 660°C. The X-ray diffraction and Rietveld refinement results suggested that a tetragonal sheelite phase was found in the xNa₅Bi(MoO₄)₄–(1−x)Na₅Ln(MoO₄)₄ (0.05 ≤ x ≤ 0.20) ceramics. Scanning electron microscopy (SEM), backscattered electron image (BSE) and energy dispersive spectrometer (EDS) results indicated that the complex substitution of Bi³⁺ produced a significant effect on the microstructural morphology. Besides, the substitution of Bi³⁺ for (Sm_0.3Y_0.7)³⁺ effectively reduced the sintering temperature to 630°C. The variation of theoretical dielectric polarizabilities (α_theo.), the polarizability density (α_theo./V_m), the observed dielectric constant (ε_r), theoretical dielectric constant (ε_theo.), packing fraction (PF) values, and bond valence (V_i) as a function of x value were investigated, which was helpful for understanding microwave dielectric properties. The ceramic composition 0.1Na₅Bi(MoO₄)₄–0.9Na₅Ln(MoO₄)₄ sintered at 630°C for 2 h demonstrated the best microwave dielectric properties, exhibiting a ε_r ∼ 9.3, Q×f value of about 31,200 GHz (at 11 GHz) and a temperature coefficient (τ_f) of −41.3 ppm/°C. Both XRD and EDS results indicated that the 0.1Na₅Bi(MoO₄)₄–0.9Na₅Ln(MoO₄)₄ ceramic was chemically compatible with Ag at its sintering temperature (630°C). The xNa₅Bi(MoO₄)₄–(1−x)Na₅Sm_0.3Y_0.7(MoO₄)₄ (0.05 ≤ x ≤ 0.20) ceramics in the Na₂O–Bi₂O₃–Ln₂O₃–MoO₃ (Ln = Sm and Y) system are promising candidates for 5G and its successor 6G mobile communication technology.