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

A series of LiIn 1- x Al x O 2 ( 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 LiInO 2 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 LiAlO 2 were detected. In the LiIn 1- x Al x O 2 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 Al 3+ substitution for In 3+ . Bond valence theory analysis demonstrates that the Al 3+ entered the In 3+ site exhibits a strength rattling effect, which is beneficial to the increase of ε r . While Al 3+ substitution for In 3+ 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 .85 Al0 .15 O 2 ceramic sintered at 970°C.