## 3 Results and Discussion
The Commission internationale de l'eclairage (CIE) coordinates were calculated for both single-doped and co-doped samples, and the lum  
Figure 3: (**a**) Excitation and emission spectra of Tb\({}^{3+}\)-doped samples; (**b**) emission intensity as a function of dopant concentration; inset: emission spectra of samples with different Tb\({}^{3+}\) concentrations. Figure 2: (**a**) Excitation and emission spectra of Eu\({}^{3+}\)-doped samples; (**b**) emission intensity as a function of dopant concentration; inset: emission spectra of samples with different Eu\({}^{3+}\) concentrations. the red (Eu\({}^{3+}\)), green (Tb\({}^{3+}\)) or yellow (co-doped Eu\({}^{3+}\) and Tb\({}^{3+}\)) regions of the chromaticity scale (Figure 4b). The color purity was calculated based on the Equation (7) [36]:  
\[CP=\frac{\sqrt{\left(x-x_{i}\right)^{2}+\left(y-y_{i}\right)^{2}}}{\sqrt{\left(x _{d}-x_{i}\right)^{2}+\left(y_{d}-y_{i}\right)^{2}}} \tag{7}\]  
where \((x,y)\) is the CIE coordinates of the sample, \((x_{i},y_{i})\) = (0.3333, 0.3333) is the coordinates of white light, and \((x_{d},y_{d})\) is the coordinates of the dominant wavelength of the emission. The color purity was calculated to be equal to 100% for Eu\({}^{3+}\)-doped glass and 80% for Tb\({}^{3+}\)-doped glass. The Judd-Ofelt parameters were calculated for the Eu\({}^{3+}\)-doped sample (Table 1). The \(\Omega_{\lambda}\) parameters follow the \(\Omega_{2}>\Omega_{6}>\Omega_{4}\) relation, which has been reported for other germanate glass systems [19; 37]. Based on the Judd-Ofelt theory, the radiative luminescence decay time of the \({}^{5}\)D\({}_{0}\) excited state was calculated to be equal to 1.6 ms, which, compared to experimentally derived mean decay time, yields quantum efficiency equal to 86%. The measured external quantum yield (Figure 5a) of the Eu\({}^{3+}\)-doped glass has an expected maximum at 395 nm excitation and is equal to 23.0 \(\pm\) 2.5%. The external quantum yield for a Tb\({}^{3+}\)-doped sample is lower, with a maximum at 375 nm excitation and equal 6.9 \(\pm\) 2.5%. \begin{table}
\begin{tabular}{c c c c c c} \hline \(\Omega_{2}\) (10\({}^{-20}\) cm\({}^{2}\)) & \(\Omega_{4}\) (10\({}^{-20}\) cm\({}^{2}\)) & \(\Omega_{6}\) (10\({}^{-20}\) cm\({}^{2}\)) & \(\tau_{r}\) (ms) & \(\tau_{exp}\) (ms) & \(\eta\) (\%) \\ \hline
6.7 & 4.3 & 6.4 & 1.6 & 1.4 & 86 \\ \hline \end{tabular}
\end{table} 
Table 1: The Judd–Ofelt parameters (\(\Omega_{\lambda}\)), radiative (\(\tau_{r}\)) and experimental (\(\tau_{exp}\)) decay time and internal quantum efficiency (\(\eta\)) for the Eu\({}^{3+}\)-doped sample. The thermal quenching curves (Figure 5b) reveal the discrepancy between the Eu\({}^{3+}\) and Tb\({}^{3+}\) luminescence in function of temperature. The Eu\({}^{3+}\) emission is more stable, as the activation energy \(\Delta E_{act}\) derived from the fitting Equation (8) [38] is equal to 1510 \(\pm\) 90 cm\({}^{-1}\), which is twice as high as for Tb\({}^{3+}\) (810 \(\pm\) 60 cm\({}^{-1}\)). The difference in activation energy stems from the different energetic structures of the excited states in Eu\({}^{3+}\) and Tb\({}^{3+}\), as well as the different locations of these levels with respect to the conduction band of the host material [39], which causes different nonradiative relaxation mechanisms. \[I=\frac{I_{0}}{1+\frac{I_{g}-\Delta E_{act}/kT}{I_{v}}} \tag{8}\]  
where \(I\) is the intensity, \(I_{0}\) is the initial intensity for the lowest temperature, \(\Delta E_{act}\) is the activation energy for thermal quenching, \(T\) is the temperature, and \(k\) is the Boltzmann constant. The expression \(\frac{I_{g}}{I_{v}}\) can be omitted as a constant value. Due to the different activation energies and thermal quenching profiles of the co-dopants, it is possible to use the co-doped material as a temperature sensor. To estimate and compare its performance, the emission spectra of the co-doped samples were measured in the temperature range of \(-\)193-200 \({}^{\circ}\)C (Figure 6a).