Rattling effect mechanism on the temperature stability of low-sintered Ca_1–x(Li_1/2Eu_1/2)_xWO₄ microwave dielectric ceramics for dielectric resonant antenna applications

The low dielectric constant (ε_r < 15) is the key to improving the signal transmission speed of microwave communication devices. However, the resonant frequency temperature coefficient (τ_f) of most low-ε_r microwave dielectric ceramics is usually negative. Aiming to modify the large negative τ_f of scheelite CaWO₄ and explore the underlying mechanism between the structure and microwave dielectric properties, a series of Ca_1–x(Li_1/2Eu_1/2)_xWO₄ (x = 0.1−1.0) (CLEWO_x) ceramics were prepared at low sintering temperatures (750−875 °C). The ε_r increased from 10.46 to 18.55, and the Q× f decreased from 39,032 GHz–7425 GHz, mainly due to the enhanced rattling effect of Li⁺. The τ_f rapidly increased from negative (−19.91 × 10⁻⁶ °C⁻¹) to abnormally positive (+162.15 × 10⁻⁶ °C⁻¹), influenced by the reduced temperature coefficient of ion polarizability (τ_αm) caused by the rattling Li ⁺ cation. The CLEWO_0.15 sample has good comprehensive performance (ε_r = 12.28, Q×f = 28,027 GHz, and τ_f = −0.5 × 10⁻⁶ °C⁻¹) and compatibility with the Ag electrode, showing the potential of LTCC applications. Additionally, a dielectric resonator antenna based on CLEWO_0.15 ceramic was designed with a bandwidth of 254 MHz at 4.504−4.758 GHz and a gain of 4.87 dBi at 4.62 GHz, indicating that CLEWO_0.15 may be a potential candidate for dielectric resonator antennas.