## 3 Results and Discussion
After adding the various amounts of LiF, the majority diffraction peaks could still be indexed well to the ZnZrNb\({}_{2}\)O\({}_{8}\) diffraction profile with JCPDS card #48-0324. However, an unidentified phase, denoted by asterisks, emerged consistently within the range of 32\({}^{\circ}\) to 35\({}^{\circ}\) in all specimens containing LiF additions. Notably, this unknown phase persisted even after sintering at approximately 1100 \({}^{\circ}\)C. Furthermore, its peak intensity augmented as the \(y\) values increased, suggesting a direct correlation with the addition of LiF. Additionally, the intensity of the (011) peak was enhanced with increasing LiF content, indicating that the diffraction peak of a substance associated with LiF also manifests at this specific position. Moreover, an impurity phase, highlighted by a solid circle, was detected in the specimen with \(y=10\), while another impurity phase, denoted by a solid square, appeared in the specimen with \(y=50\). Given their absence in other X-ray diffraction (XRD) patterns, these impurities are likely attributed to other  
Figure 3: Relative densities and microwave performance of Cu\({}_{x}\)Zn\({}_{1-x}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) (\(0.0\leq x\leq 0.4\)) ceramics. elements rather than LiF. Furthermore, the diffraction peak intensity of the specimen with \(y=50\) exhibited a marked decrease, indicating a significant increase in the formation of the liquid phase within the material. This observation is supported by the substantial reduction in relative density observed in subsequent stages, corroborating the conclusion regarding the increased liquid phase formation. To further investigate the variations in crystal structure and the unknown phases upon LiF addition, Rietveld refinements were conducted. The corresponding XRD refinement patterns are shown in Figure 5, where (a)-(f) stand for the representative results for samples with \(y=0\), \(10\), \(20\), \(30\), \(40\), and \(50\), respectively. The refined structural parameters for Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics with \(0\) mol% and \(40\) mol% LiF are selectively listed in Table S1 of the Supplementary Materials. As revealed by the results, the (Zn, Zr, Ti, Cu)Nb\({}_{2}\)O\({}_{8}\) phase dominates in all the specimens. However, during the LiF doping process, the formation of the (Li, Cu)NbO\({}_{3}\) secondary phase was observed in all the specimens, indicating that the unknown phase marked by the asterisks in Figure 4 was confirmed to be (Li, Cu)NbO\({}_{3}\). Besides the (Li, Cu)NbO\({}_{3}\) phase, small amounts of ZnO impurities with \(0.78\) wt.% and \(0.37\) wt.% were also recognized in the specimen with \(y=10\) (it should correspond to the peak marked by a solid circle in Figure 4) and \(y=20\), respectively. Due to its relatively low content, ZnO was not visible in the XRD pattern for \(y=20\). As the amount of LiF doping increased, the proportion of the (Li, Cu)NbO\({}_{3}\) phase showed a positive correlation with the doping level \(y\) (this phenomenon can also be clearly demonstrated by the increased intensity of the (\(011\)) peak, as well as the peaks denoted with asterisks), while the content of the (Zn, Cu, Ti, Zr)Nb\({}_{2}\)O\({}_{8}\) phase decreased correspondingly. Additionally, CuTiNb\({}_{2}\)O\({}_{8}\) emerged and increased progressively as \(y\) varied from \(20\) to \(50\) (the impurity phase denoted by a solid square in Figure 4 just corresponds to it), probably because the LiF with a lower melting point further promoted the formation of Cu-related liquid phase. This can be proved by the specimen with \(y=50\), where a small amount of CuNbO\({}_{3}\) was observed, indicating that Cu\({}^{2+}\) was reduced to Cu\({}^{+}\) possibly due to the reduction in divalent Cu ions under oxygen-deficient conditions during sintering at \(950\)\({}^{\circ}\)C. To further clarify this phenomenon, XPS spectra of the specimen with \(y=50\) were measured and presented in Figure S2.