Microstructure and microwave dielectric properties of Ca[Ti1−x(Mg1/3Nb2/3)x]O3 ceramics

Microwave dielectric ceramics of Ca[Ti1−x(Mg1/3Nb2/3)x]O3 (x = 0.40–0.70) were prepared by solid-state reaction method. The microwave dielectric properties, such as dielectric constants, Q × f values and τf (temperature coefficient of resonant frequency) were studied as a function of composition and sintering temperature. In all the studied composition range, the sintered ceramics have orthorhombic perovskite structure in which the lattice parameters increase with increasing x. The substitution of Mg and Nb for Ti promotes the grain growth. With x increasing from 0.40 to 0.70, the dielectric constant decreases from 62.97 to 41.56, Q × f value increases from 12,213 to 29,428 GHz, and τf decreases from 92.6 to −12.6 ppm/°C. A near-zero τf was achieved at x = 0.65 with dielectric constant of 44.24 and Q × f value of 28,340 GHz. The change of dielectric constant and τf with x is related to the variation of the tolerance factor, which influences the tilting of oxygen octahedra in perovskite structure.