Effect of substituting Ti sites with (MgxNb0.8-0.4x)4+ on the microwave dielectric properties of Mg2TiO4 ceramics

The microwave dielectric properties of Mg2Ti0.95(MgxNb0.8-0.4x)0.05O4 (0 ≤ x ≤ 1) ceramics were investigated based on the structural characteristics obtained by Rietveld refinement. Specimens with 0 ≤ x ≤ 0.75 showed a single-phase inverse spinel structure with cubic symmetry. The average ionic radius increased with increase in x because of the larger ionic radius of Mg2+ (0.72 Å) than Nb5+ (0.64 Å). However, the lattice parameter decreased because of an increase in the bond valence of the Ti site, which in turn led to an increase in the packing fraction of the unit cell. The quality factor (Qf) of the specimens increased with increase in x up to x = 0.75 and then decreased because of the inclusion of secondary phases, such as MgTiO3 and Mg-rich spinels. For single-phase specimens, the Qf value depended on the packing fraction of the unit cell. Despite the higher ionic polarizability of Nb5+ (3.97 Å) than Mg2+ (1.32 Å), the dielectric constants remained constant due to the increase of relative density of the specimens with x. The temperature coefficients of the resonant frequency of all the specimens were similar.