Microwave/terahertz dielectric properties of high-Q willemite ceramics: Dual control through non-stoichiometric design and Ge⁴⁺ substitution

The advancement of communication technologies demands dielectric materials with superior performance characteristics, particularly low permittivity and minimal dielectric loss. This study investigates Ge⁴⁺-substituted willemite ceramics, including Zn₂Si_1–xGe_xO₄ (ZS-xGe, x = 0 and 0.1) and Zn_1.8Si_1–yGe_yO_3.8 (ZS-yGe, y = 0 to 0.3), synthesized via the conventional solid-state method. The non-stoichiometric design effectively suppresses the formation of ZnO. The intrinsic and extrinsic losses of the ZS-xGe ceramics are separated by a systematic comparative analysis of the dielectric losses in the terahertz band, and the extrinsic losses are fitted by the Drude term in the Lorentz-Drude dielectric response model. Consequently, ZS-yGe ceramics exhibit lower ε_r and significantly improved Q×f values across microwave to terahertz band. In ZnO-free ceramics, Ge⁴⁺ substitution enhances the ionic polarizability, the unit cell volume and the bond strain, increasing ε_r and Q×f values (decreasing intrinsic losses), and decreasing τ_f. The optimized Zn_1.8Si_0.9Ge_0.1O_3.8 ceramics demonstrate superior dielectric properties with ε_r = 6.66, Q×f = 225,500 GHz and τ_f = −60.0 × 10⁻⁶ °C⁻¹ at 12.45 GHz, and ε_r = 7.02, Q×f = 401,800 GHz at 1 THz. These novel ceramics are positioned as promising candidates for next-generation microwave and terahertz communication devices.