Crystal structure and dielectric properties of germanate melilites Ba2MGe2O7 (M = Mg and Zn) with low permittivity

Two melilite-structured ceramics, Ba2MgGe2O7 and Ba2ZnGe2O7, were fabricated using the simple solid-state sintering route and their microwave dielectric properties were reported for the first time. Rietveld refinements revealed that both ceramics crystallized in a tetragonal system with a space group P-421m. In contrast to Ba2MgGe2O7 that was single phase in the whole sintering range, Ba2ZnGe2O7 exhibited a second phase BaZnGeO4 which was confirmed by SEM and Raman analysis. The sintering temperature could optimize the relative density and dielectric properties. Dense Ba2MgGe2O7 and Ba2ZnGe2O7 ceramics could be obtained at 1280 °C and 1180 °C with low relative permittivity(εr) of 7.76 and 9.0, high quality factor (Q × f) of 20,700 and 13,950 GHz, and negative temperature coefficient of resonant frequency (τf) of −55 and −75 ppm/°C, respectively. Their differences in microwave dielectric properties were analyzed based on ionic polarizability and packing fraction. The thermal stability of Ba2MgGe2O7 was tuned through the solid solution formation with TiO2 and the τf value was adjusted successfully.