Phase, microstructure and microwave dielectric properties of Nb and Ga doped Ca_0.6La_0.267TiO₃ ceramics

Nb and Nb_0.5Ga_0.5 doped A-site deficient Ca_0.6La_0.267TiO₃ perovskites were prepared via a mixed oxide solid state sintering route. Analysis of X-ray diffraction patterns of Nb-doped Ca_0.6La_0.267TiO₃ samples sintered at 1300–1500 °C revealed the formation of single phase ceramics with orthorhombic symmetry. Ca_0.6La_0.267Ti_1−x (Nb_0.5Ga_0.5) _x O₃ compositions crystallized into single phase ceramics at x = 0.3; however, the formation of a secondary phase (Ca₂Nb₂O₇) was also observed at x > 0.3. The microstructure of Nb-doped samples comprised randomly oriented sharp-edged rectangular and/or square plate-like grains along with some voids while the microstructure of Nb/Ga-doped samples comprised relatively larger densely packed irregular-shaped grains with deformed edges. The substitution of Nb increased the relative permittivity (ε _r) from 114 to 136 but at the cost of both the quality factor (Q × f _o) and temperature coefficient of resonant frequency (τ _f). On the other hand, Nb and Ga co-doping was effective in minimizing τ _f accompanied by an increase in Q × f _o. The optimum microwave dielectric properties (i.e. ε _r = 41, Q × f _o = 16,052 GHz and τ _f = −13 ppm/ °C) were achieved for Ca_0.6La_0.267Ti_0.5(Nb_0.5Ga_0.5)_0.5O₃.