Structure, Raman spectra and properties of two low-ε microwave dielectric ceramics Ca3B2Ge3O12 (B = Al, Ga)

Abstract Ca3B2Ge3O12 (B = Al, Ga) cubic garnet ceramics with space group Ia-3d were synthesized by solid-state reaction. Ca3Al2Ge3O12 ceramic sintered at 1340 °C showed the optimal microwave dielectric performances with er = 8.97, Q × f = 114,871 GHz and τf = −54.6 ppm/°C. The trace amount of secondary phase Ca2Ga2GeO7 was observed by XRD in Ca3Ga2Ge3O12 ceramic. The pure phase Ca3Ga2Ge3O12 could be obtained at 1320 °C by adding an excess of 3.3 mol% GeO2, which exhibited er ~9.79, Q × f ~129,356 GHz and τf ~ −50.19 ppm/°C. The differences in microwave dielectric properties of Ca3B2Ge3O12 (B = Al, Ga) ceramics were discussed by Raman spectra, ionic polarizability, packing fraction and bond valence theories. Additionally, CaTiO3 was used to adjust the largely negative τf of Ca3B2Ge3O12 (B = Al, Ga) ceramics to near zero, and optimized performances of er = 10.18, Q × f = 59,350 GHz and τf = +2.53 ppm/°C were obtained for the composite 0.88Ca3Al2Ge3O12-0.12CaTiO3 ceramics at 1360 °C for 8 h, with er = 11.23, Q × f = 64,542 GHz and τf = −3.26 ppm/°C for 0.9Ca3Ga2Ge3O12-0.1CaTiO3 ceramics at 1320 °C.