# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results
### Temperature-Dependent PL  
Figure 8a,b illustrate the temperature dependence of the Cr\({}^{3+}\) emission observed in a ZGGO:Cr sample obtained upon excitation in the defect absorption band (325 nm) and intraionic \({}^{4}A_{2}\rightarrow{}^{4}T_{1}\) transition (442 nm). For both cases, a noticed thermally populated anti-Stokes emission is observed with increasing temperature. Additionally, a low-energy shift of 2.6-3.0 meV for the R- and N-lines peak position is seen occurring with increasing temperatures from 15 K to RT, accompanied by line broadening due to the contributions of strain and impurity phonon coupling. By using 325 nm of excitation, the overall intensity of the superposed Cr\({}^{3+}\)-emitting centres decreases in the studied temperature range, reaching a RT value of 45% of its original value at 15 K. On the contrary, upon intraionic excitation on the \({}^{4}A_{2}\rightarrow{}^{4}T_{1}\) absorption band (442 nm), the intensity of the overall emission decreases between 15 K and 90 K and then increases for higher temperatures, likely due to thermal population mechanisms. The overall intensity at RT corresponds to 95% of its original value at 15 K. This behaviour also supports the presence of multiple Cr\({}^{3+}\)-related optical centres, as well as provides information about the existence of different thermal de-excitation paths under the different wavelength excitation conditions employed. Moreover Figure 8c,d also show a distinct temperature behaviour for the integrated intensity of the R-lines when the excitation is performed either by 325 nm or 442 nm photons. In particular, and as shown in Figure 9, using 325 nm of excitation, the intensity of the R1-line decreases over the entire temperature range, while the R2-line intensity increases up to about 100 K and decreases thereafter likely due to further non-radiative processes. Under intraionic \({}^{4}A_{2}\rightarrow{}^{4}T_{1}\) excitation (442 nm), the intensity of the R1-line decreases appreciably up to 60 K and then evidences a slight increase up to RT. At this excitation, the intensity of the R2-line always increases in the temperature range assessed. The observed behaviour reveals the spectral complexity of the presence of several optically active Cr\({}^{3+}\) in the ZGGO host that emit in almost the same spectral region and are excited by close energies. Furthermore, the increase in the intensity of the R-lines indicates the existence of thermal populating processes between the different optical centres. The temperature behaviour of the R1- line intensity, depicted in Figure 9 when excited with 325 nm can be well accounted for by assuming that the non-radiative competitive processes involved in its temperature dependence are described by the classical Mott model [34; 35],  
\[\frac{I(T)}{I_{0}}=\frac{1}{1+C\exp\left[-E_{a}/k_{B}T\right]} \tag{1}\]  
where \(T\) is the absolute temperature, \(I_{0}\) the intensity at low temperature (15 K), \(C\) corresponds to the ratio of the electronic levels' effective degeneracies, \(k_{B}\) is the Boltzmann constant, and \(E_{a}\) represents the activation energy for the non-radiative processes. On the other hand, in the case of the R2-line excited with the same wavelength, the following model should be considered [34; 35],  
\[\frac{I(T)}{I_{0}}=\frac{1+C_{pop}\exp\left[-\frac{E_{a,pop}}{k_{B}T}\right]}{ 1+C_{depop}\exp\left[-E_{a\_depop}/k_{B}T\right]} \tag{2}\]  
which describes both the increase and decrease of the intensity with increasing temperature. \(E_{a\_pop}\) corresponds to the activation energy for the population of the R2 level and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\lambda_{exc}\) **(nm)** & **R2** & **R1** \\ \hline ZGO:Cr & 325 & 684.5 nm/1.8089 eV & 687.3 nm/1.8042 eV \\ ZGGO:Cr & 325 & 685.6 nm/1.8086 eV & 687.4 nm/1.8039 eV \\ ZGGO:Cr & 405 & 685.6 nm/1.8086 eV & 687.5 nm/1.8037 eV \\ ZGGO:Cr & 442 & 685.6 nm/1.8086 eV & 687.4 nm/1.8037 eV \\ ZGGO:Cr & 532 & 685.6 nm/1.8086 eV & 687.5 nm/1.8037 eV \\ \hline \hline \end{tabular}
\end{table}