Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si, Ge) ceramics

Novel BaCa 2 M 3 O 9 ( M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa 2 M 3 O 9 ( M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa 2 Ge 3 O 9 ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa 2 M 3 O 9 ( M = Si, Ge) exhibited a triclinic structure with a P 1 ¯ $\bar 1$ space group and good microwave dielectric properties. The ε r , Q × f , and τ f values of BaCa 2 M 3 O 9 ( M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M –O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa 2 Ge 3 O 9 ceramic due to its high r c (Ge–O) and low intrinsic dielectric loss. The BaCa 2 Si 3 O 9 ceramic exhibited small | τ f | value (‒36.4 ppm/°C) due to its large E Si-O . Excellent microwave dielectric properties ( ε r = 8.31, Q × f = 61 800 GHz, and τ f = ‒58.7 ppm/°C) were obtained for the BaCa 2 Ge 3 O 9 ceramic.