Structure refinement, defects evolution and calcium doping of Sr₂CeO₄ microwave dielectric ceramics with high quality factor

Sr_2-2xCa_2xCeO₄ (x = 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8) ceramics were synthesized through cold isostatic pressing and solid-state reaction. The microstructure, defects, microwave dielectric properties, and the effect of Ca²⁺ doping of Sr₂CeO₄ ceramics were systematically investigated. As the sintering temperature increased, the densities of Sr₂CeO₄ ceramics rose, the content of oxygen vacancies increased, and Ce⁴⁺ reduction would be enhanced. In addition, the Sr₂CeO₄ structure had poor compatibility with Ca²⁺. The major phase could be kept unchanged only when x ≤ 0.1. The reason was that the doping of Ca²⁺ intensified the distortion of the CeO₆ octahedron and induced the structural transformation of the common edges (Sr₂CeO₄) to the common angles (SrCeO₃). With the increase of dopant, the densities of Sr_2-2xCa_2xCeO₄ ceramics increased significantly, while the content of oxygen vacancies also increased. The microwave dielectric properties were mainly influenced by the density, structural symmetry, defects, and the second phase SrCeO₃. The dielectric permittivity (ε_r) of 13.4–15, the quality factor (Qf) of 118,580–52,170 GHz, and the temperature coefficient of resonant frequency (τ_f) of −58.3 ∼ −47.5 ppm/°C were obtained for Sr_2-2xCa_2xCeO₄ ceramics When x ≤ 0.1. This work has provided a foundation for further research on cerate microwave dielectric ceramics.