## 3 Results and Discussion
The survey spectrum depicted in Figure S2a clearly exhibits peaks corresponding to Li 1s, Zr 3d, Nb 3d, Ti 2p, O 1s, F 1s, Cu 2p, and Zn 2p, providing additional evidence for the presence of Li and F, despite their non-detection by the EDS, as noted in the subsequent section. Furthermore, the peak-fitting curves of both the Cu 2p\({}_{3/2}\) and Cu 2p\({}_{1/2}\) peaks shown in Figure S2b can be split into two peaks by Gaussian-Lorentzian curve fitting, respectively, indicating the co-existence of Cu\({}^{+}\) and Cu\({}^{2+}\) in this specimen [27]. In addition to XRD, Raman scattering is a powerful tool for investigating lattice vibration, which significantly impacts both the polarization and structural attributes of ceramics. Furthermore, the intrinsic loss in microwave dielectric ceramics is predominantly determined by the anharmonicity and damping of the lattice phonon [28]. Consequently, Raman spectroscopy was employed to conduct a more in-depth exploration of the crystal structures and vibrational modes. Figure 6 shows the corresponding Raman spectra of \(y\) mol% LiF-added Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics (\(0\leq y\leq 50\)). As indicated by the XRD patterns, all the ceramic specimens with different amounts of added LiF mainly have a monoclinic structure, as exhibited by ZnZrNb\({}_{2}\)O\({}_{8}\). According to group theory calculations, ZnZrNb\({}_{2}\)O\({}_{8}\) ceramics possess a monoclinic columbite structure with space group P2/c (No. 013) and point group C2h (2/m), encompassing 39 vibrational modes (\(\Gamma=9A_{\rm g}+8A_{\rm u}\) + \(12B_{\rm g}+10B_{\rm u}\)), including Raman-active modes (\(9A_{\rm g}+12B_{\rm g}\)) and infrared-active modes (\(8A_{\rm u}+10B_{\rm u}\)) [13; 14]. However, the actual number of observed Raman peaks is fewer than theoretically predicted, which may be attributed to the broadening effect or overlap of weak vibrational peaks [14]. Based on previous reports [14; 29; 30; 31], the high-intensity modes at around 845 cm\({}^{-1}\) and 880 cm\({}^{-1}\) correspond to symmetric Nb-O bond vibrations that can be found due to a high covalent bond, while the asymmetric Nb-O bond vibrations appear at 770 cm\({}^{-1}\). For the lower Raman shift, the modes at 635 cm\({}^{-1}\) and 674 cm\({}^{-1}\) are related to the asymmetric vibrations of the Nb-O-Nb bond, and other vibrations correspond to symmetric Nb-O-Nb vibrations. Two modes at 355 cm\({}^{-1}\) and 466 cm\({}^{-1}\) are assigned to O-Zr-O bending, whereas the 186 cm\({}^{-1}\) and 408 cm\({}^{-1}\) modes originate from the stretching vibration of the Zr-O bond. Notably, the incorporation of LiF significantly affects the intensity of each vibrational peak, which may be attributed to changes in the relative concentration within the material system, thereby modulating the vibrational characteristics. Figure 7 shows the SEM images of the thermally etched interior surface of \(y\) mol\(\%\) LiF-added Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics (10 \(\leq y\leq\) 50, the SEM image with \(y=\) 0 is shown in Figure 2c). In all the images, there is a notable absence of any visible holes, conclusively suggesting that densification sintering was successfully achieved for all the specimens under their respective conditions. Moreover, the morphology of the specimens underwent significant changes after doping with LiF. The morphology of the grains in the specimen with \(y=\) 10 is uneven (Figure 7a), with an average grain size of approximately (6.0 \(\pm\) 1.4) \(\upmu\)m. Some precipitated grains can be found on the matrix. At \(y=\) 20 (Figure 7b), the average grain size decreased to approximately (4.8 \(\pm\) 1.2) \(\upmu\)m, accompanied by blurred grain boundaries. Additionally, the surface of the matrix ceramic exhibited a notable presence of precipitated grains.