Crystal structure and ultra-high quality factor of MgTi1-x(Co1/3Nb2/3)xO3 solid-solution microwave ceramics

Abstract

MgTi1-x(Co1/3Nb2/3)xO3 (x = 0–0.3) ceramics were prepared by a solid-state reaction method. The (Co,Mg)4Nb2O9 and MgTiO3 phases formed solid solutions, which presented an ilmenite-type with R-3 space group. In low (Co1/3Nb2/3)4+ ion content, the (Co,Mg)4Nb2O9 dissolves in MgTiO3 matrix, whereas the MgTiO3 dissolves in (Co,Mg)4Nb2O9 with high (Co1/3Nb2/3)4+ ion content. In the transition from MgTiO3 to (Co,Mg)4Nb2O9 matrix, MgTiO3 and (Co,Mg)4Nb2O9 phases can coexist. A secondary phase, MgTi2O5, existed in the x = 0 sample. The introduction of (Co1/3Nb2/3)4+ inhibited the formation of the second phase and promoted grain growth. The variation trend of the dielectric constant (εr) was the same as the theoretical dielectric constant (εtheo), indicating that the εr was dominated by the ion polarizability. The stable microstructure, which was caused by the high total lattice energy, enhanced the quality factor (Qf) value. And an ultrahigh Qf value (∼492 500 GHz) was obtained for MgTi0.96(Co1/3Nb2/3)0.04O3 ceramics sintered at 1250 °C. Moreover, the TiO6 octahedral distortion, which is caused by the ion substitution, contributes to the change in bond restoring force. The TiO6 octahedral distortion conduces to the fluctuations of the temperature coefficient of resonance frequency (τf) value.