Crystal structural refinement of corundum-structured A_4M_2O_9 (A = Co and Mg, M=Nb and Ta) microwave dielectric ceramics by high-temperature X-ray powder diffraction

摘要

The microwave dielectric properties and crystal structure of corundum-structured Mg_4Nb_2O_9 (MN), Co_4Nb_2O_9 (CN) and Mg_4Ta_2O_9 (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 (tau_f) varied from -70 to - 10ppm/deg 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 Co-O bond is lower than that of Mg-O 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 (epsilon_r) and the quality factor (Q X f) of the ceramics, these values ranged from 10 to 16 and from 210,000 to 5000 GHz, depending on the composition x.