Sintering, microstructure and microwave dielectric properties of Ca3Co2SiV2O12 through inequivalent substitution of Ca2+ with La3+

Novel garnet-structured compounds Ca 3- x La 2 x /3 Co 2 SiV 2 O 12 (0.01 ≤  x  ≤ 0.07) were fabricated via the traditional solid state reaction route. Single phase solid solution ceramics with high density were achieved when x  = 0.01−0.07, indicating that La 3+ inequivalent substitution for Ca 2+ could significantly stabilize crystal structure and improve microstructure. The La 3+ ions favored occupying the Ca 2+ site, which was confirmed by the X-ray diffraction (XRD) refinement method. Raman spectra revealed that the large differences in ionic radius and ionic polarizability between Ca 2+ and La 3+ strengthened the local distortion of crystal structure. The variation in dielectric constant ( ε r ) was the outcome of the combined action of intrinsic factor (molar volume ionic polarizability) and extrinsic factor (relative density). Oxygen vacancies played a significant role in regulating quality factor ( Q  ×  f ) value as an extrinsic factor. The Full Width at Half Maximum (FWHM) of A 1g , as an intrinsic factor, was also related to the Q  ×  f value. Increasing A-site bond valence was responsible for the reducing temperature coefficient of resonant frequency ( τ f ) due to the increase of lattice stress in ceramics. When x  = 0.07, the Ca 3- x La 2 x /3 Co 2 SiV 2 O 12 ceramics exhibited excellent microwave dielectric properties at 1160 °C for 6 h: ε r  = 11.50, Q  ×  f  = 43,200 GHz, τ f  = −37.78 ppm/°C.