Tuning εr and τf by the combined effects of rattling RE3+ and compressed Ca2+ at the A-site in microwave dielectric ceramics CaREAlO4 (RE = Eu, Ho, Er, Yb)

The well-densified CaREAlO4 (RE = Eu, Ho, Er, Yb) microwave dielectric ceramics with the K2NiF4 structure are fabricated at 1360 - 1500 °C. Crystal structures were investigated via XRD and HRTEM, and no phase transition occurred. The ceramics exhibit low εr of 17.9 - 19.3 and relatively high Q×f values of 25,110 - 48,310 GHz, and tunable τf values ranging from -1.1 ppm/°C to +18.4 ppm/°C and then to -20.4 ppm/°C. The variations in εr and τf were ascribed to the mutual effects of rattling RE3+ and compressed Ca2+ characterized by bond valence theory and temperature coefficient of ion polarizability (ταm). The Q×f values of CaREAlO4 ceramics positively correlate with structure stability and lattice energy, the main intrinsic factors affecting Q×f.