Phase Evolution, Crystal Structure, and Microwave Dielectric Properties of Water-Insoluble (1 - X)LaNbO₄-xLaVO₄ (0 ≤ x ≤ 0.9) Ceramics

In the present work, a series of low-temperature firing scheelite structured microwave dielectric in water-insoluble La₂O₃-Nb₂O₅-V₂O₅ system was prepared via the traditional solid-state reaction method. Backscattering electron diffraction, X-ray diffraction (XRD), energy-dispersive analysis, and Rietveld refinements were performed to study the phase evolution and crystal structure. In the full composition range of (1 - x)LaNbO₄-xLaVO₄ (0 ≤ x ≤ 0.9) ceramics, at least four typical phase regions including monoclinic fergusonite, tetragonal sheelite, B-site ordered sheelite, and composite of monoclinic LaVO₄ and tetragonal sheelite phases can be detected according to XRD analysis. The variations of relative dielectric constant ϵ_r, quality factor Q × f, and resonant frequency τ_f could be attributed to Nb/V-O bond ionicity, lattice energy, and the coefficient of thermal expansion. Infrared reflectivity spectra analysis revealed that ion polarization contributed mainly to the permittivity in microwave frequencies ranges. Furthermore, the 0.7LaNbO₄-0.3LaVO₄ ceramic sintered at 1160 °C possessed excellent microwave dielectric properties with an ϵ_r of ∼17.78, a Q × f of ∼75 940 GHz, and a τ_f of ca. -36.8 ppm/°C. This series of materials might be good candidate for microwave devices.