Low loss (Ba_1−xSr_x)(Co_1/3Nb_2/3)O₃ solid solution: phase evolution, microstructure and microwave dielectric properties

(Ba_1−xSr_x)(Co_1/3Nb_2/3)O₃ (0 ≤ x ≤ 0.5) microwave dielectric ceramics had been prepared using the solid-state reaction method. The dense microstructures with small grains were obtained. All Sr²⁺ substituted samples exhibited the perovskite phase. The structure transition from cubic to hexagonal occurred with the increase of x, which led to higher degree of 1:2 ordered structure in the B site. The improved Q × f value was correlated monotonously with the ordering parameters S. The dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) could be effectively tuned by tailoring x. A good combination of microwave dielectric properties was obtained for (Ba_0.95Sr_0.05)(Co_1/3Nb_2/3)O₃ sintered at 1380 °C: ε_r = 33.3, Q × f = 87,120 GHz, τ_f = 4 ppm/°C.