## 3 Results and Discussion
### Mechano-Luminescence Properties  
After determining the most suitable preparation conditions, we fabricated the corresponding small discs to observe their mechanical luminescence properties. Under UV light, we can observe that the sample emits red light, and the sample is placed approx  
Figure 5: The emission and excitation spectra and long persistent luminescence decay curves. (**a**) NaNbO\({}_{3}\):0.75% Pr\({}^{3+}\): photoluminescence emission spectrum under 329 nm laser excitation and excitation spectrum monitoring at 612 nm; (**b**) the long persistent luminescence decay curves of NaNbO\({}_{3}\):0.75% Pr\({}^{3+}\) with different sintering temperature, luminescence decay curves under a 329 nm excitation for 10 min; (**c**) the long persistent luminescence decay curves of NaNbO\({}_{3}\) with different Pr\({}^{3+}\) doping, luminescence decay curves under 329 nm excitation and excitation spectrum monitoring at 612 nm. (**d**) LiNbO\({}_{3}\):2.00% Pr\({}^{3+}\): photoluminescence emission spectrum under 365 nm laser excitation and excitation spectrum monitoring at 620 nm; (**e**) the long persistent luminescence decay curves of LiNbO\({}_{3}\):2.00% Pr\({}^{3+}\) with different sintering temperatures, luminescence decay curves under a 365 nm excitation for 10 min; (**f**) the long persistent luminescence decay curves under 365 nm excitation and excitation spectrum monitoring at 620 nm. The Y axis in (**b**,**c**,**e**,**f**) is in log scale. imately 4 cm after 10 min of excitation; the light source is then turned off, and the red afterglow can be observed. As shown in Figure 6, after 20 min of excitation under UV light, the disc edges are clamped using pliers. After the excitation light is removed, the disc demonstrates persistent luminescence. When the persistent luminescence weakens (stop excitation light for 1 min) and the grip strength of pliers' force grows (to 100 MPa), the edge of the sample suddenly becomes bright along specific textures and cracks and then darkens. This indicates that when the macroscopic sample is very thick and the luminescent center is sufficiently aggregated, an external force will affect the intensity of luminescence. Figure 6a demonstrates that the sample emits bright-red light under UV irradiation. The luminescence of the sample at the moment of turning off the UV lamp can be observed at this time in Figure 6b. Figure 6c shows the afterglow of the sample after closing the UV lamp for 1 min; it can be observed that the afterglow became weak. Figure 6d shows the change in the sample when the edge is clamped with a force of 100 MPa. When stress is applied, the edge luminescence intensity of the sample suddenly increases, and a bright-red light appears. Figure 6e,f show the luminescence intensities of the samples after 1 and 2 s, respectively. It can be observed that NaNbO\({}_{3}\):Pr\({}^{3+}\) exhibits mechanical luminescence, which the sample emits bright-red light that can be sustained with a force of 100 MPa. The long persistent luminescence lasts at least 2 s and can be strengthened by applying stress. This phenomenon is caused by the release of carriers trapped by stress [16]. This shows that NaNbO\({}_{3}\):Pr\({}^{3+}\) has a luminescence phenomenon of materials under mechanical stress, which provides the possibility for visual mechanical sensing. In Figure 7, LiNbO\({}_{3}\):Pr\({}^{3+}\) shows the characteristics of red ML luminescence. Figure 7a,b show that, when a force of 100 MPa is applied, bright-red luminescence appears and lasts 1 s and 2 s, respectively. This is, also, consistent with the view that 'ML originates from the triple coupling of piezoelectric, trap energy and Pr\({}^{3+}\) centers' [16]. From Figure 7 and previous Figures 5 and 6, we find that the two luminescent materials NaNbO\({}_{3}\):Pr\({}^{3+}\) and LiNbO\({}_{3}\):Pr\({}^{3+}\) exhibit similar red persistent luminescence and mechanical luminescence properties.