Lattice evolution, order transformation, and microwave dielectric properties of the Zn_1−xLi_2xTiO₃ (0 ≤ x ≤ 1) system ceramics

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.