Phase composition,crystal structure,and microwave dielectric properties of LiIn1-xAlxO2 ceramics

A series of LiIn_1-xAl_xO₂ (x =0.05, 0.10, 0.15, 0.20, 0.25) microwave dielectric ceramics with low permittivity were synthesized via a solid-state reaction method. XRD, Raman spectra, and SEM analysis reveal that a single LiInO₂ tetragonal structure phase could be obtained at the x < 0.10, and with the x increased further to 0.15–0.25, the diffraction peaks of the secondary phase LiAlO₂ were detected. In the LiIn_1-xAl_xO₂ ceramics, the τ_f was closely related to the ε_r, and the relative density, microstructure, and microwave dielectric properties were effectively improved by the Al³⁺ substitution for In³⁺. Bond valence theory analysis demonstrates that the Al³⁺ entered the In³⁺ site exhibits a strength rattling effect, which is beneficial to the increase of ε_r. While Al³⁺ substitution for In³⁺ simultaneously lowers the average ionic polarizability, resulting in a decrease in ε_r. A near-zero τ_f (0.74 ppm/°C) combined with ε_r approximately 12.83, Q × f = 58 200 GHz, was obtained in LiIn0_.85Al0_.15O₂ ceramic sintered at 970°C.