Phase composition, crystal structure, and microwave dielectric properties of LiIn_1-xAl_xO₂ ceramics

A series of LiIn1-xAlxO2 (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 LiIn1-xAlxO2 ceramics, the & tau;(f) was closely related to the & epsilon;(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 & epsilon;(r). While Al3+ substitution for In3+ simultaneously lowers the average ionic polarizability, resulting in a decrease in & epsilon;(r). A near-zero & tau;(f) (0.74 ppm/& DEG;C) combined with & epsilon;(r) approximately 12.83, Q x f = 58 200 GHz, was obtained in LiIn0(.85)Al0(.15)O(2) ceramic sintered at 970 & DEG;C.