Structure stability and microwave dielectric properties of double perovskite ceramics – Ba2Mg1−xCaxWO6 (0.0 ≤ x ≤ 0.15)

The structure stability of double perovskite ceramics – Ba2Mg1−xCaxWO6 (0.0 ≤ x ≤ 0.15) has been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small amount of Ca substitution for Mg increases the Mg/CaO bond strength, and hence the stability of the double perovskite. But it cannot completely suppress the decomposition of Ba2Mg1−xCaxWO6 at high temperature. Although space group Fm−3m is adopted for all compositions, nonrandom distribution of Ca2+ and Mg2+ on 4b-site within the short range scale is observed due to their large cation size difference. Small level doping of Ca (x ≤ 0.1) increases the dielectric permittivity monotonically, but does not affect the Q × f value greatly. As expected, the substitution of Ca tuned the temperature coefficient of resonant frequency (τf value) from negative to positive value. Excellent combined microwave dielectric properties with ɛr = 20.8, Q × f = 120,729 GHz, and τf = 0 ppm/°C could be obtained for x = 0.1 composition. However the Q × f value degrades considerably when the sample was stored under ambient conditions for a long time.