Crystal structure, phase composition, dielectric properties, bond characteristics, and Raman spectra of novel CoTeMoO6- CoMoO4 composite ceramics for ultra low sintering temperatures

Abstract

Novel CoTeMoO₆-CoMoO₄ composite ceramics were synthesized employing the solid-state method at sintering temperatures within the range of 500–620 °C. The Rietveld refinement results indicate that the CoTeMoO₆ crystals possess an orthorhombic structure with the space group P2₁2₁2(18). Surprisingly, a secondary phase of CoMoO₄ with a monoclinic structure belonging to the space group P2/c(13) was formed during the synthesis process. SEM analysis demonstrated that the sintering temperature favored grain growth and densification of the samples. Theoretical density and theoretical dielectric constant were calculated using the rule of mixtures, Shannon's additive rule, and the Rietveld refinement results. Remarkably, CoTeMoO₆-CoMoO₄ composite ceramics exhibited optimal microwave dielectric properties at a sintering temperature of 580 °C (ε_r = 13.87 ± 0.06, Q×f = 44,110 ± 1,616 GHz, τ_f = -86 ± 0.8 ppm/°C). The underlying factors influencing the microwave dielectric properties were investigated using the P-V-L theory, revealing that the Mo-O bond played a pivotal role in determining the dielectric properties. Furthermore, the characteristic peaks observed in the Raman spectra were closely correlated with the vibrational properties of the Mo-O-Te bridges.