Correlation between crystal structure and microwave dielectric properties of two garnet-type ceramics in rare-earth-free gallates

Two garnet-type rare-earth-free ceramics, Ca3M2SiGa2O12 (M = Sn and Zr), were prepared through a solid-state reaction method. The relationship between crystal structure and microwave dielectric properties was investigated. The larger deviation of εr from εtheo in Ca3Zr2SiGa2O12 could be ascribed to the rattling Zr4+. The increase in packing fraction and the decrease in FWHM enhance the Q × f value by substituting Zr4+ with Sn4+. The smaller oxygen bond valence in Ca3Zr2SiGa2O12 indicates a smaller τf value. Good microwave dielectric properties are obtained with εr = 9.14 ± 0.02, Q × f = 106,800 ± 1700 GHz and τf = -45.8 ± 1.8 ppm/°C for Ca3Sn2SiGa2O12 and εr = 11.98 ± 0.03, Q × f = 84,200 ± 1500 GHz, and τf = -32.8 ± 1.4 ppm/°C for Ca3Zr2SiGa2O12. Furthermore, near-zero τf values of +5.7 ± 1.9 ppm/°C and +4.5 ± 1.6 ppm/°C appear in 0.95Ca3Sn2SiGa2O12-0.05CaTiO3 and 0.96Ca3Zr2SiGa2O12-0.04CaTiO3, respectively.