Compositional modulation in ZnGa₂O₄ via Zn²⁺/Ge⁴⁺ co-doping to simultaneously lower sintering temperature and improve microwave dielectric properties

AB₂O₄-type spinels with low relative permittivity (ε_r) and high quality factor (Q × f) are crucial to high-speed signal propagation systems. In this work, Zn²⁺/Ge⁴⁺ co-doping to substitute Ga³⁺ in ZnGa₂O₄ was designed to lower the sintering temperature and adjust the thermal stability of resonance frequency simultaneously. Zn_1+xGa_2-2xGe_xO₄ (0.1 ≤ x ≤ 0.5) ceramics were synthesised by the conventional solid-state method. Zn²⁺/Ge⁴⁺ co-substitution induced minimal variation in the macroscopical spinel structure, which effectively lowered the sintering temperature from 1385 to 1250 ℃. All compositions crystallized in a normal spinel structure and exhibited dense microstructures and excellent microwave dielectric properties. The compositional dependent quality factor was related to the microstructural variation, being confirmed by Raman features. A composition with x = 0.3 shows the best dielectric properties with ε_r ≈ 10.09, Q × f ≈ 112,700 THz, and τ_f ≈ -75.6 ppm/℃. The negative τ_f value was further adjusted to be near-zero through the formation of composite ceramics with TiO₂.