Crystal structure and microwave dielectric properties of Li-modified BaSi2O5 ceramics

Ba 1- x Li x Si 2 O 5- x /2 ( x  = 0–0.5) ceramics were prepared by solid-state reaction method. The occupation tendency of Li + was well explained based on First-principle calculation and unit cell volume changes. Li + non-equivalent substitution for Ba 2+ considerably reduced the sintering temperature to 975 °C, in comparison with 1225 °C for pure BaSi 2 O 5 . Single phase solid solution with orthorhombic structure was observed with x  = 0–0.1. Nevertheless, trace amounts of unexpected secondary phases Li 2 Si 2 O 5 and SiO 2 were detected for higher Li-containing samples. Notably, the Q×f significantly enhanced up to 58.16% from 16,134 GHz ( x  = 0) to 25,518 GHz ( x  = 0.01), which predominantly depended on the structural characteristics, such as packing fraction and bond covalence. Increase x from 0 to 0.5 led to the reduction of dielectric constant ε r from 7.11 to 5.95. The resonant frequency temperature coefficient τ f was directly dominated by oxygen bond valence ( V O ), rather than V Ba and V Si . Ba 0.99 Li 0.01 Si 2 O 4.995 ceramic sintered at 1050 °C for 4 h exhibited a high Q×f value of 25,518 GHz, and the ε r and τ f values were 6.94 and −29.46 ppm/°C, respectively.