Enhanced effects of vanadium ions non-stoichiometry on the microstructure and microwave properties of Ca5Mn4V6+xO24 ceramics

Ca5Mn4V6+xO24 (−0.15 ≤ x ≤ 0.05) microwave ceramics were synthesized by the solid-state reaction. The influence of vanadium ions non-stoichiometry on the crystal structure, microstructure, and microwave properties was investigated. XRD showed that the major crystalline phase was Ca5Mn4V6O24, while a few V-rich secondary phase appeared at x = 0.05. Rietveld refinement indicated that V non-stoichiometry cannot devastate the crystal structure, due to the slight change of the lattice parameters and the unit cell volume. The relationship between microstructure and microwave properties was further studied. The variation of εr was strongly correlated with the distortion polarization obtained from Raman peak intensity and the relative density. Q × f value enhanced with decreasing the FWHM of Raman peak, which was caused by the V ionic deficiency leading to the high cation ordering degree. A slight deviation of vanadium ions from stoichiometry reduced the oxygen dodecahedron distortion (Δ) and the oxygen bond valence (VO), resulting in the increase of τf value. Ca5Mn4V5.9O24 ceramic exhibited optimum microwave dielectric properties: εr = 11.5, Q × f = 39,141 GHz, and τf = −74.9 ppm/°C.