## 3 Results and Discussion
### Persistent Luminescence Properties  
The emission and excitation spectra and long persistent luminescence decay curves of NaNbO\({}_{3}\):0.75% Pr\({}^{3+}\) are shown in Figure 5. From Figure 5a, it can be observed that the maximum excitation and emission peaks of the sample are at 329 and 612 nm, respectively. The luminescent properties of the materials are typically related to two factors: the crystal field provided by the matrix material and the electron transition and energy transfer of the doped ions. That is, owing to the deviation from stoichiometry and the lack of Na atoms, VNa\({}^{-}\) is universal, and excess Nb\({}^{5+}\) will enter the Na site as NbNa\({}^{4+}\) to achieve charge compensation [13; 31] owing to the \({}^{1}\)D\({}_{2}\rightarrow\)\({}^{3}\)H\({}_{4}\) energy level transition of the Pr\({}^{3+}\) ions. Therefore, red light can be observed by irradiating the sample with a 300-350 nm UV lamp. Figure 5b illustrates the luminescence intensities of the samples prepared at different sintering temperatures, which is excited at 329 nm. It can be observed that as the sintering temperature increases, the LPL intensity of the sample increases. This may be observed because the higher the temperature, the better the degree of crystallization becomes. Furthermore, the crystal field of the matrix material is affected, and the electron transition of the doped ions is improved. To investigate the luminescence of the samples, the LPL properties of the samples are shown in Figure 5c, which displays the LPL decay curves of NaNbO\({}_{3}\):Pr\({}^{3+}\) (\(x\) = 0.25, 0.50, 0.75, 1.00, 1.50) (NaNbO\({}_{3}\):Pr\({}^{3+}\)) obtained by monitoring at 612 nm for 1200 s after the 329 nm UV excitation was stopped for 10 min. It can be observed that when \(x\) = 0.25, 0.50, 0.75, 1.00, and 1.50, the afterglow intensity of the material is not significantly different. When \(x\) = 0.75, the afterglow of the material is optimal. When \(x\) is \(<\)0.75, the LPL intensity increases with increasing \(x\). When \(x\) is \(>\)0.75, the LPL intensity decreases with increasing \(x\). Therefore, it is inferred that the optimal doping amount of Pr\({}^{3+}\) in NaNbO\({}_{3}\) is 0.75 mol\(\%\). This also provides a good reference and the available process conditions for the preparation of this NaNbO\({}_{3}\):Pr\({}^{3+}\) material. The emission and excitation spectra and long persistent luminescence decay curves of LiNbO\({}_{3}\):Pr\({}^{3+}\) are shown in Figure 5d-f. From Figure 5d, it can be observed that the maximum excitation and emission peaks of the sample are at 365 and 620 nm, respectively. There is a broad peak at 300-440 nm in the excitation spectrum, corresponding to the absorption of the LiNbO\({}_{3}\) host lattice, and the corresponding energy transfer is light source \(\rightarrow\) LiNbO\({}_{3}\) host lattice \(\rightarrow\) Pr\({}^{3+}\) luminescence center. The emission spectrum indicates that the sample emitted a bright-red light with high purity under excitation of 365 nm. The strongest emission peak of the emission spectrum is located at 620 nm, which corresponds to the energy level transition from \({}^{1}\)D\({}_{2}\) to the ground state \({}^{3}\)H\({}_{4}\) of Pr\({}^{3+}\). In addition, a secondary peak generated by the energy level splitting at 635 nm can also be observed. Otherwise, comparing NaNbO\({}_{3}\):Pr\({}^{3+}\) and LiNbO\({}_{3}\):Pr\({}^{3+}\), it can be seen that the highest positions of their excitation peaks and emission peaks are basically similar. This means that they have similar energy levels, electron transitions, and release pathways.