Structure, bond characteristics and microwave dielectric properties of glass-free molybdate ceramics ReBaK(MoO4)3 (Re = Gd, Y)

In light of advancements in 5G/6G technologies, current research on microwave dielectric ceramics is focused on the development of materials characterized by low sintering temperatures and low permittivity, in order to satisfy the requirements of high-frequency low-temperature co-fired ceramic (LTCC) applications. Novel monoclinic structure GdBaK(MoO₄)₃ and YBaK(MoO₄)₃ ceramics, with C2/c space group, were synthesized using a conventional solid-state method. GdBaK(MoO₄)₃ and YBaK(MoO₄)₃ ceramics were sintered at 830 °C and 850 °C, exhibited ε_r ~ 8.54 and 8.77, τ_f ~ − 51.1 ppm/°C and − 45.8 ppm/°C, Q × f ~ 23, 240 GHz and 59, 277 GHz, respectively. The compression from Ba²⁺ and K⁺ cations was identified as the main reason for the higher theoretical ε_theo compared to the corrected ε_corr. The dielectric loss of both ceramics is mainly affected by the density and packing fraction. The full width at half maximum (FWHM) values of the vibrational peak at 932 cm⁻¹, corresponding to the symmetric stretching modes of MoO₄²⁻, exhibits a negative correlation with the Q × f values. Based on the P–V–L theory, the lattice energy and bond energy associated with the Mo–O bond played a substantial role in influencing the Q × f and τ_f. Moreover, YBaK(MoO₄)₃ showed good chemical compatibility with Ag, indicating its potential for LTCC applications.