Improvement of _f in HfTiO₄ microwave dielectric ceramics with Zr- and Sn-substitution

(Hf1-xZrx)TiO4 and (Hf1-xSnx)TiO4 ceramics were prepared through a standard solid-state reaction route, and the microwave dielectric characteristics were determined together with the microstructures. The single-phase solid solution in space group Pbcn was obtained in (Hf1-xZrx)TiO4, while the two-phase structure with (HfTiO4(s.s.) major phase in space group Pbcn and Sn0.3Ti0.7O2 secondary phase in space group P4(2)/mnm) was determined in (Hf1-xSnx)TiO4. The temperature coefficient of resonant frequency tau(f) could be significantly improved to near-zero in both systems, and the best combination of microwave dielectric characteristic was achieved at x = 0.5 in (Hf1-xZrx)TiO4: epsilon(r) = 39.0, Qf = 43,150 GHz at 5.3 GHz and tau(f) = 3 ppm/degrees C, and at x = 0.225 in (Hf1-xZrx)TiO4: epsilon(r) = 37.2, Qf = 52,600 GHz at 5.5 GHz and tau(f) = -6 ppm/degrees C. In the solid solutions, the obvious improvement of tau(f) could be deeply linked with the degree of covalency and the restoring forces of structure. While, the intermediate near-zero tau(f) is achieved by mixing the phases with opposite tau(f) in multiphase materials. The present work provided an effective way to modify the temperature coefficient of resonant frequency in microwave dielectric ceramics.