SrGa₁₂O₁₉: The first low-ε_r Ga-based microwave dielectric ceramic with anomalous positive τ_f

Dielectric ceramics with low permittivity (ε_r), high quality factor (Q×f), and near-zero resonant frequency (τ_f) in the microwave bands are key materials used in fifth/sixth-generation (5G/6G) telecommunication, whileτ_f of most low-ε_r microwave dielectric ceramics is relatively negative. In this work, the first low-ε_r Ga-based ceramic SrGa₁₂O₁₉ with an anomalous positive τ_f was reported, and the causes of the positive τ_f, intrinsic polarization, and loss mechanism were systematically studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that the SrGa₁₂O₁₉ ceramic formed a pure hexagonal magnetoplumbite structure with spinel blocks and rock-salt blocks stacked along the crystallographic c-axis. When sintered at 1430 °C, it possessed the optimal microwave dielectric properties of a low ε_r of 14.46, high Q×f of 64,705 GHz, and exceptional positive τ_f of +55.7 ppm/°C, along with a low linear thermal expansion coefficient (α_L) of 11.617 ppm/°C. The large positive deviation between ε_r and ε_r(C–M) of 45.31% resulted from the rattling effect of atoms in the rock-salt block. The unique positive τ_f (+55.7 ppm/°C) was governed by the rattling effect, resulting in a positive τ_αm (the temperature coefficient of ion polarizability) of 8.489 ppm/°C and a large negative temperature coefficient of permittivity (τ_ε) of −132.864 ppm/°C. Phillips–Vechten–Levine (P–V–L) chemical bond theory revealed greater contributions of the spinel block to bond ionicity (f_i, 52.95%), permittivity (ε, 55.15%), bond energy (E, 56.87%), and lattice energy (U, 74.88%) than those of the rock-salt block. The intrinsic dielectric properties were analyzed using infrared (IR) reflectivity spectra. The favorable performance of the SrGa₁₂O₁₉ ceramic indicated that it is a novel τ_f compensator. This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.