This study introduces a novel microwave dielectric ceramic, MgAl_5/4(Li_1/3Ti_2/3)_3/4O₄, tailored for modern communication technologies. MgAl_5/4(Li_1/3Ti_2/3)_3/4O₄ ceramics feature a composite spinel structure (Fd-3m space group) comprising MgAl₂O₄ and Li₄Ti₅O₁₂ type phases. By substituting Al³⁺ of MgAl₂O₄ ceramic with the composite ion (Li_1/3Ti_2/3)³⁺, differences in elemental diffusion induced by sintering temperature (1200–1280 ℃) significantly affect the microwave dielectric properties: a low ε_r (11.83) and enhanced microwave properties (Q×f = 79,381 GHz and τ_f = −28.5 × 10⁻⁶/℃) at 1240 ℃. With further optimization of the ceramics, a near-zero τ_f is realized in 0.93MgAl_5/4(Li_1/3Ti_2/3)_3/4O₄-0.07CaTiO₃ ceramics with excellent comprehensive performance (ε_r = 14.36, Q×f = 44,144 GHz). Building on this, a multi-band dielectric resonant antenna (DRA) was designed for applications in communication and aeronautical radio navigation, featuring a wide relative bandwidth of 39.37% (5.97–6.49 GHz and 7.19–9.83 GHz). This study presents an optimization strategy for obtaining microwave dielectric ceramics with low ε_r, high Q×f, excellent frequency-temperature stability, low sintering temperature, and low density.

Research on doping modification of ZnTiO₃ ceramics to enhance microwave dielectric properties has been hindered by poor performance, unclear structure-function mechanisms. To expand the applicability of ZnTiO₃ ceramics, this study explores Zn_1–xLi_2xTiO₃ (0 ≤ x ≤ 1) ceramics using a phase engineering strategy. Our findings reveal that the introduction of Li⁺ into the ZnTiO₃ system initiates a multiple phase transition, starting at x = 0.1. Initially, ilmenite ZnTiO₃ transforms into a cubic ordered spinel phase (space group P4332). Subsequently, a transition to a disordered spinel phase (space group Fd $\overline{3}$m) occurs at x = 0.5, culminating in the formation of a monoclinic rock salt-structured Li₂TiO₃ phase. Significantly, two sets of ceramics with near-zero temperature coefficients of resonance frequency (τ_f) were obtained at x = 0.1 and 0.75. Moreover, the quality factor (Q×f) demonstrated a 4.4-fold increase compared to that of ZnTiO₃ ceramics at x = 0.25 (105,013 GHz). Additionally, it was observed that the Ti⁴⁺ polarization in Zn_1−xLi_2xTiO₃ ceramics was underestimated by 11.3%–13.3%, causing the measured dielectric constant (ε_r) exceeding the theoretical dielectric constant (ε_th). The ionic polarizability of Ti⁴⁺ was adjusted to stabilize around 3.29 ų. Evaluation using multiple methods, including Phillips–van Vechten–Levine (P–V–L) theory, Raman vibrational mode analysis, bond valence, bond energy theory, and octahedral distortion, confirms that the Ti–O bonds within the octahedron predominantly affect ε_r, the increasing lattice energy (U) contributes to the enhancement of Q×f, and the strengthened Li–O bond energy effectively regulates τ_f.

The Ba₁₂ReNb₉O₃₆ (ReYb, Ce, Tm, Er, Y, Ho, Dy, Gd) ceramics are synthesized by solid-phase reaction method. The phase composition, crystal structure, microstructure, and microwave dielectric properties of the ceramics are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, Scanning electron microscopy and Raman spectrum. The optimal microwave dielectric properties (ε_r = 37.23, Q × f = 36600 GHz, and τ_f = 34 ppm/℃) are obtained for Ba₁₂YbNb₉O₃₆ ceramic sintered at 1420 ℃ for 6 h. In this system, the variation of dielectric constant is dominated by polarizability. The Q × f is mainly affected by internal strain/fluctuation of d-spacing. The variation of τ_f is related to the temperature coefficient of dielectric constant and the oxygen octahedron distortion. Furthermore, the reduction of Ce⁴⁺ ions, the relative density, linear thermal expansion coefficient, and the second phase are also important factors affecting microwave dielectric properties.