Crystal structural refinement of corundum-structured A4M2O9 (A = Co and Mg, M = Nb and Ta) microwave dielectric ceramics by high-temperature X-ray powder diffraction

The microwave dielectric properties and crystal structure of corundum-structured Mg4Nb2O9 (MN), Co4Nb2O9 (CN) and Mg4Ta2O9 (MT) compounds were investigated in this study. The crystalline phases of the ceramics were characterized by using the high-temperature X-ray powder diffraction and the crystal structures of the compounds were refined in terms of Rietveld analysis. The temperature coefficient of resonant frequency (τf) varied from −70 to −10 ppm/°C with increasing the composition x when Mg was substituted by Co. From the calculation of covalency of cation–oxygen bonds, it is found that the covalency of CoO bond is lower than that of MgO bond and the difference in the covalency of these cation–oxygen bonds influences on the temperature dependence of dielectric constant on the compounds. As for the dielectric constant (ɛr) and the quality factor (Q·f) of the ceramics, these values ranged from 10 to 16 and from 210,000 to 5000 GHz, depending on the composition x.