Electric State and Chemical Bonding of (Mg_4−xMn_x)Nb₂O₉ Microwave Dielectric Ceramics

Solid solutions of (Mg_4−xMn_x)Nb₂O₉ (MMN), which have a corundum-type structure, showed a single phase over the entire composition range, and lattice parameters of MMN linearly increased with increased composition x. From the calculation of cation–oxygen bonds in NbO₆ and MO₆ (M = Mg and Mn) octahedra, it was found that the covalency of the Nb–O bond was decreased by the Mn substitution for Mg; the molecular orbital calculation in (NbM₁₂O₄₅)⁻⁶¹ cluster models also revealed that the covalency of the Nb–O bond in the (NbMn₁₂O₄₅)⁻⁶¹ cluster model is smaller than that of the Nb–O bond in the (NbMg₁₂O₄₅)⁻⁶¹ cluster model. This result is attributed to the differences in the density of state, which arise from the overlap population of Mg-3d, Mn-3d, Nb-5s, and O-2p orbitals. The dielectric constant and Q⋅f values varied from 11 to 16 and from 210,000 to 50,000 GHz, respectively. Thus, it is considered that these variations in the microwave dielectric properties may be related to the covalency of the cation–oxygen bonds.