## 3 Results and Discussion  
### Temperature and Structure  
The microstructure and elemental composition of the NaNbO\({}_{3}\):Pr\({}^{3+}\) samples were analyzed using SEM and energy-dispersive X-ray spectroscopy (EDS). Figure 1 shows that the size of the NaNbO\({}_{3}\):0.5 Pr\({}^{3+}\) grains gradually increased from 0.5 to 5 \(\upmu\)m (Figure 1a-d) with temperature increases from 1000 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C. The powder sample is not dense, and the particle size is not uniform, but the particle size generally grows with an increase in temperature. Owing to the increase in temperature, the liquid-phase content increases, resulting in particle agglomeration. Another reason is that the increase in temperature provides more energy for the solid-phase reaction, and the chemical driving force is significant, which promotes the reaction. From Figure 1e,f, it can be observed that the powder has melted and is no longer granular. There are some annular holes on the surface, which may be caused by the bubble rupture when the gas inside the material is discharged during melting at 1200 \({}^{\circ}\)C. There is no obvious grain boundary in the microscopic area of the scanned image. It can be observed from the EDS spectra in Figure 2 that there are uniformly distributed elements, such as Nb and Pr, in the observation area, except the local \"bubbles\". Furthermore, there are circular bubbles and holes with a diameter of more than 5 \(\upmu\)m in the intercepted area owing to the melting of the sample. It is confirmed that to reduce the inhomogeneity of the sample, the reaction temperature should be maintained at <1200 \({}^{\circ}\)C. Compared with the convenience of taking and obtaining samples at 1150 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C, the optimal NaNbO\({}_{3}\):0.5% Pr\({}^{3+}\) preparation temperature is 1150 \({}^{\circ}\)C. The microstructure and elemental composition of the LiNbO\({}_{3}\):Pr\({}^{3+}\) samples were analyzed using SEM and energy-dispersive X-ray spectroscopy (EDS) in Figure 3. Similarly, we analyzed the structure of LiNbO\({}_{3}\):Pr\({}^{3+}\) materials at different temperatures from Figure 3. It can be observed from Figure 3 that the shape of the LiNbO\({}_{3}\):0.5 Pr\({}^{3+}\) is irregular spherical or blocky, and the size of its grains gradually increases (Figure 3a-d) with the temperature increase from 1000 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C. Figure 3e, shows, similar to NaNbO\({}_{3}\), \"annular holes and bubbles\" are also generated on the surface of the 1200 \({}^{\circ}\)C LiNbO\({}_{3}\):0.5 Pr\({}^{3+}\) sample. In addition to the uneven distribution of elements caused by high temperature (1200 \({}^{\circ}\)C), the sample is not easy to remove and can become stuck to the crucible, which is inconvenient for subsequent operation. We can draw a conclusion, from this study, that the reaction temperature should be maintained at 1150 \({}^{\circ}\)C.  
Figure 4 shows the XRD pattern of the NaNbO\({}_{3}\):Pr\({}^{3+}\) and LiNbO\({}_{3}\):Pr\({}^{3+}\) samples sintered at different temperatures. As shown in Figure 4a, the lattice diffraction peaks of the samples correspond well with the standard diffraction peaks of NaNbO\({}_{3}\) (No. 33-1270) in the PDF database, indicating that the formed crystal phases are NaNbO\({}_{3}\) crystal phases belonging to the orthorhombic system and Pbma space group [28,30]. As shown in Figure 4b, the lattice diffraction peaks of the samples correspond well to the PDF cards (No. 20-0631) of the crystal form of LiNbO\({}_{3}\), and the formed LiNbO\({}_{3}\) crystal phases belong to the hexagonal crystal system and R3c space group [12]. With the increase in temperature, the crystallinity of NaNbO\({}_{3}\):0.5%Pr\({}^{3+}\) and LiNbO\({}_{3}\):0.5%Pr\({}^{3+}\) is gradually enhanced. No impurity phase was produced under different preparation conditions. Figure 3: Microstructures of the fabricated LiNbO\({}_{3}\):0.5% Pr\({}^{3+}\) sintered at different temperatures by scanning electron microscopy (SEM).