Crystal structure, bond characteristics and enhanced microwave dielectric properties of Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 ceramics

SrLaAlO4-based ceramics have attracted intense interest due to their high quality factor (Qf) at high frequency. However, the poor resonant frequency temperature stability (τf) limits their practical applications. Therefore, obtaining near-zero τf while guaranteeing high Qf values is challenging. (Zn0.5Ti0.5)3+ is utilized in Sr0.6Ca0.4LaAlO4 ceramics to improve the τf value effectively. The relationship between the structural features and microwave dielectric properties of Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 (x = 0–0.5) ceramics has been systematically investigated by XRD, Rietveld refinement, SEM, Raman spectroscopy, TEM and P–V-L theory. XRD and Raman spectroscopy reveal the effect of (Zn0.5Ti0.5)3+ substitution on the evolution of the crystal structure. According to the chemical bond theory, (Zn0.5Ti0.5)3+ substitution leads to the increase of the dielectric constant, which is related to the increase of the average bond ionicity, with the La–O bond contributing predominantly to the bond ionicity. The variation of the Qf values can be attributed to the densification of the ceramics, the lattice energy of the Al/Zn/Ti–O bond, and the FWHM. The τf value is strongly affected by the coefficient of thermal expansion of the lattice and the Al/Zn/Ti–O bond valence. It is noteworthy that excellent microwave dielectric properties are obtained for Sr0.6Ca0.4LaAl1-x(Zn0.5Ti0.5)xO4 ceramics at x = 0.4: εr = 21.62, Qf = 87,700 GHz (at 7.23 GHz), and τf = −1.81 ppm/°C. The above work provides deeper insights into establishing the relationship between the K2NiF4-type crystal structure and dielectric properties from the perspective of chemical bond.