## 3 Results and Discussion
### Photo- and Thermochromic Behavior of Na\({}^{+}\)-Ag\({}^{+}\) Ion-Exchanged Br-Doped Glasses
After coloration by the UV pulsed laser, samples were kept at room temperature to study bleaching kinetics. Only partial bleaching of the samples was observed, while a number of color centers were stable at room temperature. For the samples ion-exchanged for 0.25, 1, and 9 h, the absorbance increment reached 20-30% of the peak value after 72 h of relaxation. At the same time, the sample ion-exchanged for 21 h possessed a 45% decrease in absorbance. The observed differences in coloration and bleaching kinetics of the ion-exchanged samples with AgBr nanocrystals may be explained by the different thickness of the glass layer doped with the nanocrystals and by size effect. According to [PERSON], the mean size of silver halide nanocrystals in photochromic glasses is 8-18 nm because smaller particles exhibit low coloration efficiency [47]. Thus, the low saturation level in the samples ion-exchanged for 15 min and 1 h can be explained by small AgBr nanocrystals with a mean size of 5 and 6 nm, respectively, calculated according to the Scherrer equation. Partial bleaching of the studied samples resulted from the absence of polyvalent ions, for example,  
Figure 10: Absorbance spectra of the PTR-Br glass sample ion-exchanged for 9 h and heat-treated at 500 \({}^{\circ}\)C for 9 h after irradiation with UV pulses for different times. Figure 9: **(a)** Absorbance spectra of the ion-exchanged and heat-treated at 500 \({}^{\circ}\)C for 9 h PTR-Br glass samples before and after UV irradiation with a mercury lamp. **(b)** Photo of the samples before and **(c)** after the UV irradiation. copper, that participate in redox reactions occurring in conventional photochromic glasses during coloration and bleaching [45; 47]. It should be noted that full relaxation of color centers induced by a UV mercury lamp and UV pulsed laser was observed after heating PTR-Br glass samples above 150 \({}^{\circ}\)C for 10-15 min. Silver bromide is an indirect semiconductor with direct and indirect band gap energies E\({}_{\text{g,dir}}\) = 4.276 eV and E\({}_{\text{g,ind}}\) = 2.713 eV for bulk crystals at cryogenic temperatures [48]. The energy bandgap of a semiconductor depends on temperature and decreases with its increase. In situ measurements of absorbance spectra revealed that the UV absorption edge of silver bromide nanocrystals formed in PTR-Br glass shifted to a greater wavelength with heating the sample (Figure 12). The effect was manifested as colored PTR-Br glass samples due to increasing absorption in the blue range (Figure 12, inset). After cooling the sample to room temperature and removing SNIF formed during the heating, the spectrum returned to the initial state. Figure 13a shows the dependence of the location of the UV absorption edge on the temperature of the samples ion-exchanged for different time. The experimental data can be fitted using a straight line for all samples. Moreover, for a wide temperature range between 300 and 500 \({}^{\circ}\)C, absorbance at a fixed wavelength linearly depends on the temperature for samples ion-exchanged for 1 and 9 h (Figure 13b). Thus, the thermochromic properties of PTR-Br glass samples containing AgBr nanocrystals can be used as temperature sensors or an eye-readable overheating indicator. It should be noted that chlorine-doped PTR glass samples with AgCl nanocrystals formed with the Na+-Ag+ IE were also studied. However, the photo- and thermochromic properties of PTR glass samples with AgCl nanocrystals was poorer compared to their bromine-doped counterparts.