Influence of Re-ions with different ionic radius in Ba₁₂ReNb₉O₃₆ on crystal structure and microwave dielectric properties

The Ba₁₂ReNb₉O₃₆ (ReYb, Ce, Tm, Er, Y, Ho, Dy, Gd) ceramics are synthesized by solid-phase reaction method. The phase composition, crystal structure, microstructure, and microwave dielectric properties of the ceramics are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, Scanning electron microscopy and Raman spectrum. The optimal microwave dielectric properties (ε_r = 37.23, Q × f = 36600 GHz, and τ_f = 34 ppm/℃) are obtained for Ba₁₂YbNb₉O₃₆ ceramic sintered at 1420 ℃ for 6 h. In this system, the variation of dielectric constant is dominated by polarizability. The Q × f is mainly affected by internal strain/fluctuation of d-spacing. The variation of τ_f is related to the temperature coefficient of dielectric constant and the oxygen octahedron distortion. Furthermore, the reduction of Ce⁴⁺ ions, the relative density, linear thermal expansion coefficient, and the second phase are also important factors affecting microwave dielectric properties.