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Structures and microwave dielectric properties of Ba2−2xSr2xSmSbO6 (x = 0–1) ceramics were studied in this article. The phase transitions were analyzed using X‐ray powder diffraction, transmission electron microscopy, and Raman spectroscopy. All structures are proved to be ordered double perovskites, and a series of composition‐induced phase transitions are confirmed in the Ba2−2xSr2xSmSbO6 system via the sequence: Fm‐3m → I2/m → P21/n. Dielectric properties of the ceramics were measured at microwave frequencies (9–11 GHz). With the increase of x, the dielectric constant (εr) varies gradually from 20.7 to 16.0, the quality factor (Q × f) decreases first from 86 500 GHz to a minimum value at x = 0.4 and then increases slightly to 93 000 GHz, and the temperature coefficient of resonant frequency (τf) decreases from +25 to −50 ppm/°C. The relationships between composition, structure, and microwave dielectric properties in Ba2−2xSr2xSmSbO6 were discussed from the phase transition consideration.

Perovskite ceramics of Ba[Ti1−x(Co0.5W0.5)x]O3 (x = 0.40–0.90) were synthesized by a conventional solid‐state reaction method. Crystal phase, microstructure, and microwave dielectric property of the ceramics were investigated. A phase transition from simple cubic (x = 0.40–0.55) to hexagonal and then to cubic double pervoskite (x = 0.75–0.90) was detected. A single hexagonal perovskite structure (P63/mmc) was obtained at 1425°C and stabilized at room temperature only at x = 0.61. For ceramics x = 0.55–0.60 and x = 0.62–0.75, they are mixtures that consisted of cubic and hexagonal perovskites. With increasing x, Qxf increased from 2800 to 20 700 GHz, whereas dielectric constant (εr) decreased from 65 to 18, and temperature coefficient of resonant frequency (τƒ) decreased from 44 to −17 ppm/°C. Hexagonal perovskite (εr = 18, Qxf = 7700 GHz, τƒ = −14 ppm/°C) had decreased εr, τƒ and increased Qxf values, compared with the cubic perovskite phases. The phase formation and the relationship between structures and microwave dielectric properties were discussed.

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