Influence of Ca²⁺ substitution for Ba²⁺ on the crystal structure and microwave dielectric properties of Ba_1−xCa_x(Mg_1/3Ta_2/3)O₃ ceramics

The Ba_1–xCa_x(Mg_1/3Ta_2/3)O₃ (BCMT, 0 ≤ x ≤ 0.05) microwave dielectric ceramics were synthesized by the conventional solid-state ceramic route. In this paper, the effects of Ca²⁺ substituting for Ba²⁺ on the phase structure, microstructure, and microwave dielectric properties of Ba_1–xCa_x(Mg_1/3Ta_2/3)O₃ ceramics were investigated. X-ray diffraction (XRD) exhibited that the Ba_1–xCa_x(Mg_1/3Ta_2/3)O₃ ceramics possessed the main phase Ba(Mg_1/3Ta_2/3)O₃ (BMT) when 0 ≤ x ≤ 0.05 and secondary phase Ba_0.5TaO₃ for x = 0.05. SEM micrographs showed that the substitution of Ca²⁺ for Ba²⁺ could increase the grain size. The apparent density was influenced by the concentration of liquid phases caused by the substitution of Ca²⁺ for Ba²⁺ ions. The microwave dielectric properties depended heavily on apparent density, B-site cation ordering degree, Ba-deficient, secondary phase and distortion of oxygen octahedral. Finally, Ba_1–xCa_x(Mg_1/3Ta_2/3)O₃ (x = 0.04) ceramics sintered at 1450 °C for 8 h showed superior microwave dielectric properties of ε_r = 24.1, Q × f = 165,800 GHz and τ_f = 0.12 ppm/℃.