Characterization of structure and chemical bond in high-Q microwave dielectric ceramics LiM₂GaTi₂O₈ (M = Mg, Zn)

LiM₂GaTi₂O₈ (M = Mg, Zn) ceramics with the Fd-3m space group were synthesized using the solid-state method. In comparison with Mg²⁺ that fully occupied the tetrahedral (A) site in LiMg₂GaTi₂O₈, LiZn₂GaTi₂O₈ was jointly occupied by Zn²⁺ and Li⁺ at the A site. Excellent microwave dielectric properties of Q×f = 133,400 ± 500 GHz, 101,800 ± 500 GHz, ε_r = 17.1 ± 0.2, 15.8 ± 0.2, and τ_f = −60.1 ± 3.0 ppm/°C, − 42.2 ± 3.0 ppm/°C for LiZn₂GaTi₂O₈ and LiMg₂GaTi₂O₈ were obtained, respectively. The large deviations (30.3% for LiMg₂GaTi₂O₈ and 19.6% for LiZn₂GaTi₂O₈) between the corrected ε_corr and theoretical ε_th were observed, which might be attributed to the underestimated Shannon's ionic polarizability of Ti⁴⁺ in Ti-containing spinels. Their intrinsic microwave dielectric properties were discussed based on bond valence, lattice energy (U), and B-site bond energy (E). Besides, their large negative τ_f values were compensated to near-zero by CaTiO₃.