Raman spectra and microwave dielectric properties of Na5Tm1 − x(Y1/3Yb2/3)x(MoO4)4 ceramics with ultra-low sintering temperature

The Na₅Tm_{1 - x}(Y_1/3Yb_2/3)_x(MoO₄)₄ ceramics (x=0-0.18) were successfully synthesized via the solid-state reaction method. XRD analysis revealed that they possessed a single-phase tetragonal scheelite structure belonging to the space group I4₁/a(88). SEM analysis demonstrated that the samples sintered at 600 °C exhibited a dense microstructure, with a relative density as high as 94.9% and an average grain size ranging from 2.172 μm to 2.424 μm. The relative density serves as an external factor influencing ε_r and Q×f, while τ_f is related to bond valence. The ionicity of the Tm-O bond, and the lattice energy and bond energy of the Mo-O bond are the intrinsic factors determining ε_r, Q×f, and τ_f, respectively. Raman spectroscopy provided an intuitive characterization of the vibrational properties of the [MoO₄]^2- tetrahedra within the ceramics. Although the pure phase Na₅Tm(MoO₄)₄ ceramic exhibited optimal dielectric properties (ε_r=8.025, Q×f=18,967 GHz, and τ_f = -101 ppm/°C) at 625 °C, the Na₅Tm_0.91Y_0.03Yb_0.06(MoO₄)₄ ceramic sintered at 600 °C demonstrated superior dielectric properties (ε_r=7.82, Q×f=34,752 GHz, and τ_f = -71.9 ppm/°C).