The low Qf values (4487 GHz) of as

The low Qf values (4487 GHz) of as cold-sintered LiF ceramics are ascribed to the porous microstructure but they improve greatly to a maximum of 110,800 GHz for samples annealed at 800 ◦C, surpassing reported values via conventional sintering (73,880 GHz). These results indicate that CS and the subsequent annealing are highly beneficial in suppressing dielectric loss [12]. In the absence of phase transitions or impurities, the variation of Qf is mainly determined by extrinsic defects such as pores and grain boundaries [24]. Optimization of microstructure is therefore, critical for improving Qf. Moreover, as shown in Fig. 3(b), the variation of grain size with annealing temperature adopts a similar trend to that of Qf, which further confirms the positive effect of microstructural optimization through enhanced densification. In principle, a defect-free LiF single-crystal should exhibit close to intrinsic dielectric losses (~192,400 GHz), which can be regarded as the upper limit for LiF ceramics [13]. This suggests that further improvement of Qf in LiF ceramics is still possible via optimization of the microstructure.