Crystal structure and microwave dielectric properties of temperature stable (Co_xZn_1–x)TiNb₂O₈ ceramics

(Co_xZn_1–x)TiNb₂O₈ (x = 0.2–0.8) microwave dielectric ceramics were synthesized via the conventional solid-state reaction route, and the correlation of microwave dielectric properties on the crystal structure was discussed. Crystal structures of ceramic samples were systematically investigated by X-ray powder diffraction. Moreover, composition-induced phase transitions were confirmed via the following sequence: for x ≤ 0.2, single-phase orthorhombic ixiolite (ZnTiNb₂O₈) was formed, whereas for 0.3 ≤ x<0.8, ixiolite and rutile coexisted. When x ≥ 0.8, only single-phase rutile was detected. For the (Co_xZn_1–x)TiNb₂O₈ ceramics, the microwave dielectric properties were changed with the crystal structural transitions: the dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) increased upon increasing the Co²⁺, but the quality factor (Q) decreased. A near-zero τ_f = +1.6 ppm/°C was obtained in the Co_0.38Zn_0.62TiNb₂O₈ ceramics with ε_r = 40.7 and high Q × f = 16 790 GHz. These research outcomes are expected to have great significance for developing microwave dielectric ceramics in practical applications.