Olivine-type A₂BO₄ ceramics possess a low permittivity (ε_r) and low dielectric loss, contributing to a high quality factor (Q×f). However, the large negative temperature coefficient of the resonant frequency (τ_f) restricts their practical applications. In this work, we prepared a series of CaYGa_1−xAl_xO₄ (x = 0–1) ceramics with low ε_r values via solid-state sintering. By controlling the evolution of orthorhombic olivine to the tetragonal perovskite-like structure (K₂NiF₄) and the ordered regulation of the A-site, an ultralow loss (tanδ = 1.38×10⁻⁴, Q×f = 125,530 GHz, and f = 17.3 GHz for x = 0) and a nearly zero temperature coefficient (τ_f = −0.5 ppm/°C for x = 0.9) were achieved. The significant changes in the dielectric properties (ε_r = 8.3–16.2, Q×f = 125,530–50,660 GHz, and τ_f = −50.9 to 2.9 ppm/°C) of these ceramics are primarily influenced by the characteristics of the second phase, ion polarization, ion order and disorder, and the chemical bonds resulting from structural evolution. Furthermore, a cylindrical dielectric resonator antenna (CDRA) was designed using an ultralow-loss CaYGaO₄ ceramic, achieving a high gain (5.36–6.15 dBi) and efficiency (> 90%) in the bandwidth region (5.065–5.747 GHz), thereby enhancing the efficiency and quality of 5G communications. This work advances the development of control strategies for high-performance dielectric ceramics and dielectric resonator antennas in high-frequency communications.

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 ℃). 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 ℃⁻¹) to abnormally positive (+162.15 × 10⁻⁶ ℃⁻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 ℃⁻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.

Zircon ceramics have potential applications in next-generation wireless communication because of their low permittivity and adjustable temperature coefficient at microwave frequencies. However, the vast challenge of realizing ultralow dielectric loss still exists. Here, we propose a high-entropy strategy to enhance the bonding of the A-site dodecahedron in zircon and design (Nd_0.2Eu_0.2Y_0.2Ho_0.2Yb_0.2)VO₄ ceramics with a high quality factor (high Q × f, that is, low dielectric loss). The (Nd_0.2Eu_0.2Y_0.2Ho_0.2Yb_0.2)VO₄ high-entropy ceramics, which belong to the tetragonal zircon structure with the I4₁/amd space group, exhibit a low relative permittivity (ε_r = 11.55), a negative temperature coefficient of resonant frequency (τ_f = −37.3 ppm/°C), and a high Q × f of 76,400 GHz (at 12.31 GHz). The high Q × f value can be attributed to the high chemical bond strength and structural stability. Furthermore, the relationship between the crystal structure and the microwave dielectric properties of (Nd_0.2Eu_0.2Y_0.2Ho_0.2Yb_0.2)VO₄ high-entropy ceramics was analyzed through high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, far-infrared reflection spectroscopy, and chemical bond theory. This work provides an effective avenue for designing microwave dielectric materials with low loss to meet the demands of passive components.