Microwave Dielectric Properties of Corundum-Structured (Mg4-xMx)(Nb2-yAy)O9 (M=Mn, Co, and Zn, A=Ta and Sb) Ceramics

The (Mg_{4 -x}M_x)(Nb_2-yA_y)O₉ (M=Mn, Co, and Zn, A=Ta and Sb) compound with corundum structure were prepared and the microwave dielectric properties, crystal structure and covalency of cation-oxygen bonds of the compounds were investigated in this study. The global instability index (GII) of (Mg_4-xM_x)Nb₂O₉ compounds was higher than that of Mg₄(Nb_2-yA_y)O₉ compounds and the GIl values of (Mg_4-xM_x)Nb₂O₉ compounds increased with increasing the composition x. From the calculation of cation-oxygen bonds, it was found covalency of M-O bond in the (Mg_4-xM_x)Nb₂O₉ compounds decreased, depending on the composition. The dielectric constant of the compounds ranged from 11 to 16 by the M substitutions for Mg, whereas the quality factor (Q f) drastically decreased form 210000 to 5000 GHz. On the other hand, the A substitution for Nb enhanced the increase in the Q f value of the compounds; the highest value was 347000 GHz.