## 3 Results and Discussion  
### Infrared Absorption Spectrum  
The OH\({}^{-}\) absorption peak of pure LiNbO\({}_{3}\) crystal is 3482 cm\({}^{-1}\). V\({}_{\rm Li}^{-}\) possesses a negative charge and exhibits an attractive force toward H\({}^{+}\) ions, thereby resulting in the formation of a complex defect group Nb\({}_{\rm Li}^{4+}-3V_{\rm Li}^{-}-\rm OH_{\rm Li}^{-}\), corresponding to the absorption peak at 3482 cm\({}^{-1}\)[36; 37].  
The absorption peaks of samples Zn1, Zn3, and Zn5 in Figure 3 exhibit slight deviations from those observed in pure LiNbO\({}_{3}\) crystal, occurring at 3483 cm\({}^{-1}\), 3484 cm\({}^{-1}\), and 3486 cm\({}^{-1}\), respectively, while sample Zn7 displays two distinct OH\({}^{-}\) absorption peaks at 3502 cm\({}^{-1}\) and 3528 cm\({}^{-1}\). The shift in the absorption peak can be elucidated by the underlying mechanism, as follows: The [Li]/[Nb] ratio of the congruent LiNbO\({}_{3}\) crystals is 0.946, leading to the generation of intrinsic defects such as Li vacancy and anti-site Nb during the crystal growth process due to the absence of Li. The presence of a significant number of Li vacancies in the crystal leads to the incorporation of Nb into the Li site, resulting in the formation of anti-site Nb. In this study, the concentrations of doped Fe\({}^{3+}\) and Cu\({}^{2+}\) ions were fixed at a low level. When Fe\({}^{3+}\) and Cu\({}^{2+}\) ions enter the LiNbO\({}_{3}\) crystal, they first occupy Nb\({}_{\rm Li}^{4+}\) and form defects Fe\({}_{\rm Li}^{2+}\) and Cu\({}_{\rm Li}^{+}\), which are positively charged and therefore do not attract H\({}^{+}\). The doping of Fe\({}^{3+}\) and Cu\({}^{2+}\) ions has little impact on the absorption peak.  
Therefore, the primary factor influencing the absorption peak is the variable Zn\({}^{2+}\) ions, and it has been reported that the threshold concentration of Zn\({}^{2+}\) ions is approximately 6 mol%. Additionally, when Zn\({}^{2+}\) ions enter LiNbO\({}_{3}\) crystals, they exhibit a preference for occupying Nb\({}_{\rm Li}^{4+}\) to form Zn\({}_{\rm Li}^{+}\). Before the doping concentration of Zn\({}^{2+}\) reached the threshold, the main components in the crystal were still Nb\({}_{\rm Li}^{4+}\) and V\({}_{\rm Li}^{-}\), so the absorption peaks of samples Zn1, Zn3, and Zn5 were still Nb\({}_{\rm Li}^{4+}-3V_{\rm Li}^{-}-\rm{OH}_{\rm Li}^{-}\). The OH\({}^{-}\) absorption bands of samples Zn1, Zn3, and Zn5 were all located at about 3482 cm\({}^{-1}\). With the increase in the Zn\({}^{2+}\) ion concentration, the decrease in the concentrations of Nb\({}_{\rm Li}^{4+}\) and V\({}_{\rm Li}^{-}\) in the crystal had a certain effect on H\({}^{+}\), and the position of the absorption peak moved within a certain range.  
When the concentration of Zn\({}^{2+}\) ions exceeds the threshold concentration of 7 mol%, Zn\({}^{2+}\) ions continue to occupy the conventional Li sites owing to their ionic radius and polarization capabilities, while pushing Fe\({}^{3+}\) ions and Cu\({}^{2+}\) ions to the Nb site, resulting in defect groups of Fe\({}_{\rm Nb}^{2-}\) and Cu\({}_{\rm Nb}^{3-}\). Subsequently, some H\({}^{+}\) ions are attracted toward the vicinity of Fe\({}_{\rm Nb}^{2-}\), resulting in the formation of a defect group consisting of Fe\({}_{\rm Nb}^{2-}\) - OH\({}_{\rm Li}^{-}-\) Zn\({}_{\rm Li}^{+}\), leading to an absorption peak near 3502 cm\({}^{-1}\). The H\({}^{+}\) ion exhibits a higher affinity toward Cu\({}_{\rm Nb}^{3-}\) compared to V\({}_{\rm Li}^{-}\) and Fe\({}_{\rm Nb}^{2-}\), leading to the aggregation of H\({}^{+}\) ions near Cu\({}_{\rm Nb}^{3-}\) and the formation of a defect group of Cu\({}_{\rm Nb}^{3-}-\rm{OH}_{\rm Li}^{-}-2 Zn_{\rm Li}^{+}\), thereby resulting in an additional absorption peak at 3528 cm\({}^{-1}\). From the above analysis, we can draw the conclusion that when the doping concentration of Zn\({}^{2+}\) ions reaches 7 mol%, Fe\({}^{3+}\) and Cu\({}^{2+}\) ions occupy the Nb sites, and Zn\({}^{2+}\) ions continue to occupy the Li sites to complete the charge compensation.