## 3 Results and Discussion  
Phosphate glasses were prepared using standard melting method in air and with 0.5 mol % Er\({}_{2}\)O\({}_{3}\) and 1.5 mol % Yb\({}_{2}\)O\({}_{3}\). Different concentrations of ZnO (x) were added in the glass within the glass system (98-x) (0.50P\({}_{2}\)O\({}_{5}\)-0.40 SrO-0.10 Na\({}_{2}\)O) -0.5 Er\({}_{2}\)O\({}_{3}\)-1.5 Yb\({}_{2}\)O\({}_{3}\)-xZnO (in mol %). The thermal and physical properties of the investigated glasses are listed in Table 1.  
The glass density increases with an increase in x due to the partial replacement of P\({}_{2}\)O\({}_{5}\), Na\({}_{2}\)O and SrO by the heavier Zn atoms in glass network. The decrease in T\({}_{\rm g}\) with an increase in ZnO content (x) could indicate that Zn acts as a network modifier. Similar changes in the physical and thermal properties induced by ZnO were reported in [11]. Also shown in the table is \(\Delta\)T, the temperature difference between T\({}_{\rm x}\) and T\({}_{\rm g}\), which is an indicator of the glass resistance to crystallization. No real impact of the addition of ZnO can be noticed on the thermal stability of the glasses. In order to understand better the changes in the physical and thermal properties of the glasses induced by the addition of ZnO, the IR and Raman spectra of the glasses were measured and they are presented in Figure 1a,b, respectively. The IR spectra in Figure 1a are normalized to the main band at 880 cm\({}^{-1}\), thus all the discussed intensity changes are expressed relatively to the main band. The spectra exhibit a broad band between 650 and 800 cm\({}^{-1}\), three absorption bands located at around 885, 1085 and 1260 cm\({}^{-1}\), and a shoulder at 980 cm\({}^{-1}\). They are similar to the IR spectra reported in [5]. The broad band between 650 and 800 cm \begin{table}
\begin{tabular}{c c c c c c c} \hline Glass & \multirow{2}{*}{x} & \(\rho\) (g-cm\({}^{-3}\)) & T\({}_{\rm g}\) (\({}^{\circ}\)C) & T\({}_{\rm x}\) (\({}^{\circ}\)C) & T\({}_{\rm p}\) (\({}^{\circ}\)C) & \(\Delta\)T (\({}^{\circ}\)C) \\ Label & & \(\pm\)**0.02 g-cm\({}^{-3}\)** & \(\pm\)3 \({}^{\circ}\)C & \(\pm\)3 \({}^{\circ}\)C & \(\pm\)3 \({}^{\circ}\)C & \(\pm\)6 \({}^{\circ}\)C \\ \hline REF & 0 & 3.21 & 462 & 580 & 593 & 118 \\
2.5 Zn & 2.5 & 3.23 & 466 & 576 & 596 & 110 \\
5 Zn & 5 & 3.26 & 445 & 560 & 577 & 115 \\
10 Zn & 10 & 3.32 & 444 & 549 & 570 & 105 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical and thermal properties of the glasses. Figure 1: IR (**a**) and Raman (**b**) spectra of the investigated glasses. can be related to symmetric vibrational modes \(\
u_{\rm sym}\) (P\(-\)O\(-\)P) of Q\({}^{2}\) units [12]. The strong absorption band at 885 cm\({}^{-1}\) is due to asymmetric stretching vibrational modes \(\
u_{\rm as}\) (P\(-\)O\(-\)P) in Q\({}^{2}\) units [12; 13]. The shoulder appearing in the 930-1010 cm\({}^{-1}\) range can be associated to rings-type [14]. The shoulder centered at 980 cm\({}^{-1}\) and the band at 1085 cm\({}^{-1}\) are due to symmetric and asymmetric stretching vibrations of Q\({}^{1}\) units, respectively [15]. The band at 1085 cm\({}^{-1}\) can be attributed to an overlap between Q\({}^{1}\) units and Q\({}^{2}\) units in metaphosphate [15]. The shoulder at 1160 cm\({}^{-1}\) and the band at 1250 cm\({}^{-1}\) are mainly due to symmetric and asymmetric vibrations of PO\({}_{2}\)\({}^{-}\) in Q\({}^{2}\) units respectively [16]. As seen in [5], no bands can be seen at wavenumber larger than 1300 cm\({}^{-1}\), where the \(\upsilon\)(P=O) of Q\({}^{3}\) groups typically appear. The progressive addition of ZnO leads to a decrease in intensity of the bands at 1085 and 1250 cm\({}^{-1}\) and also of the shoulders in the 930-1010 cm\({}^{-1}\) range compared to the main band. A shift of all the bands towards smaller wavenumbers can also be observed, which is a clear sign of changes in the strength of the chemical bonds in the glass network.