Crystal structure and microwave dielectric properties of Ca3M2Ga2GeO12 (M = Ti, Sn, Zr) ceramics with low permittivity

Crystal structure refinement and Raman spectroscopy were performed on Ca 3 M 2 Ga 2 GeO 12 (M = Ti, Sn, Zr) garnet-structured ceramics to explore the relationships between crystal structure, phonon modes, and microwave dielectric properties . The Ca 3 Ti 2 Ga 2 GeO 12 , Ca 3 Sn 2 Ga 2 GeO 12 , and Ca 3 Zr 2 Ga 2 GeO 12 ceramics were obtained by sintering at 1325–1475 °C for 6 h, and good microwave dielectric properties were achieved: ε r  = 15.04, 9.53, and 11.89; Q × f  = 59,515 GHz, 49,355 GHz, and 45,587 GHz; and τ f  = −47.3 ppm/°C, −52.3 ppm/°C, and −43.0 ppm/°C, respectively. M 4+ (M = Ti, Sn, Zr) cations entirely occupied the octahedral site, leading to an average underbonding between cations and anions, a weak rattling effect, and measured ε r values deviating from theoretical values. The packing fraction and Raman FWHM (A 1g mode) correlated highly with the Q × f values. The negative τ f values were derived from the large positive temperature coefficient of permittivity ( τ ε ) and the temperature coefficient of ionic polarizability ( τ αm ).