## 3 Results and Discussion
### Photo- and Thermochromic Behavior of Na\({}^{+}\)-Ag\({}^{+}\) Ion-Exchanged Br-Doped Glasses  
In contrast to the PTR-M matrix glass with no dopants, Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged bromine-containing glass PTR-Br (see Table 2) revealed an additional long-wavelength shift of the UV edge of strong absorption after the heat treatment at 500 \({}^{\circ}\)C (Figure 8a). Silver bromide crystals are known to possess strong absorption in the UV and visible range, with absorption coefficients as high as 1000 cm\({}^{-1}\) at 400 nm, which may, thus, cause the long-wavelength shift [44]. Formation of AgBr nanocrystals in the ion-exchanged and heat-treated PTR-Br glass samples was confirmed by XRD analysis. XRD peaks related to silver bromide were clearly observed for all ion-exchanged and heat-treated samples (Figure 8b). Figure 6: SEM image of the ion-exchanged PTR-M glass sample heat-treated at 350 \({}^{\circ}\)C (**a**) and 450 \({}^{\circ}\)C (**b**–**d**; various magnifications) for 3 h.  
The mean diameter of AgBr nanocrystals calculated according to the well-known Scherrer equation increased from 5 to 10 nm upon extending the IE process from 15 min to 21 h.  
Silver bromide is known to decompose to silver and bromine under UV or visible light. \[\mathrm{AgBr}\xrightarrow{\mathrm{hy}}\mathrm{Ag}^{0}+\mathrm{Br}^{0}. \tag{3}\]  
The process is used in photographic films; after exposure and decomposition of AgBr, silver atoms aggregate further into colloidal particles and form a latent image, while the produced bromine volatilizes. In the case of a glass matrix, bromine atoms are unable to leave the matrix, which enables multiple cycles of recording and relaxation of color centers. This principle underlies photochromic glasses that reversibly changes transmission depending on the illumination [45]. In 1968, [PERSON] showed that photochromic glasses can be produced by Na+-Ag+ ion exchange of borosilicate glasses doped with copper and bromine/chlorine [46]. Elliptical silver nanoparticles are known to be color centers in conventional photochromic glasses [47]. The experimental absorbance spectra consist of two broad bands corresponding to longitudinal and transverse plasmonic modes of elliptical SNPs. In order to study the photosensitivity of ion-exchanged PTR-Br glass, some samples were exposed to a mercury UV lamp (\(\lambda_{\mathrm{max}}=365\) nm). However, only the sample ion-exchanged for 21 h showed a noticeable change in absorbance (Figure 9). Figure 8: (**a**) Absorbance spectra of PTR-Br glass samples before and after the IE and heat treatment at 500 \({}^{\circ}\)C for 3 h. (**b**) XRD patterns of PTR-Br glass samples ion-exchanged for different time and heat-treated at 500 \({}^{\circ}\)C for 9 h.  
Figure 7: Raman spectra of PTR-M glass sample and R6G measured at ion-exchanged and heat-treated at 350 \({}^{\circ}\)C for 3 h (**a**). Absorbance spectra of PTR-M glass with SNIF after several cycles of the heat treatment at 500 \({}^{\circ}\)C for 15 min and removing the film before the following heat treatment at the same parameters (**b**). To enhance the formation of color centers in PTR-Br glass samples with AgBr nanocrystals, the samples were irradiated with a pulsed third-harmonic YAG:Nd laser (\(\lambda\) = 355 nm, f = 10 Hz, pulse energy 20 mJ, pulse duration 7 ns). Intense laser UV light resulted in the growth of SNPs and, as a consequence, origin of a broad absorbance band in the visible range (Figure 10). Coloration and bleaching kinetics for the studied PTR-Br samples are shown in Figure 11. The amplitude of UV-induced absorbance increased with the Na\({}^{+}\)-Ag\({}^{+}\) IE time as the concentration of the embedded silver ions determined concentration and distribution of AgBr nanocrystals formed during the subsequent heat treatment. It should be noted that saturation of coloration was observed already after 100 s of UV irradiation for samples ion-exchanged for 0.25, 1, and 9 h, while the sample ion-exchanged for 21 h did not show saturation even after 1000 s of UV irradiation.