## 3 Results and Discussion
### Structural Properties
The resulting peak positions and area under the peaks of the different oxygen sites from the curve fitting of O 1s core levels for specimens are displayed in Table 4. Typically, the 1s O peak of oxide glass consists of two components: bridging oxygen (BO) at a higher energy and nonbridging oxygen (NBO) at a lower energy [30,31]. In particular, it has been reported that the XPS spectra of a P2O5-V2O5-based glass system was deconvoluted into more than three curves [32,33]. In this work, the area of the BO peak at 532.55 eV shows 33.95 % and 32.46 % for Mg(PO3)2, and Al(PO3)3, respectively. The area of the BO peak indicating P-O-P bonds is not significantly distinguished by the type of Mg(PO3)2 and Al(PO3)3. On the other hand, the areas of BO peaks at 531 eV are 29.55 % and 40.24 % for Mg(PO3)2 and Al(PO3)3, respectively, as shown in Figure 7.  
XPS studies on vanadium phosphate-based glasses reported that the P-O-P bonds as a BO structure would have the strongest covalent bonds, since the Pauling electronegativity is in the order of P (2.19), V (1.63), Al (1.61), Mg (1.31), and Ba (0.89). Consecutively, the binding energy would be weakened in the order of P-O-V, P-O-Al, V-O-V, and V-O-Al bonds as intermediate BO structures, and P-O-Mg, V-O-Mg, P-O-Ba, and V-O-Ba bonds as NBO structures [32,33]. The area of BO peak at 531 eV by Al(PO3)3 is larger than that by Mg(PO3)2, which means that Al(PO3)3 preferentially formed intermediate BO, such as P-O-Al bonds in the current glass system. On the other hand, it is believed that the area of NBO peak from Mg(PO3)2 means the forming of major P-O-Mg and V-O-Mg bonds, as well as the V-O-Ba and P-O-Ba bonds in a 55V2O5-5 ZnO-22 BaO-3B2O3-Mg(PO3)2 (mol%) glass system. A similar effect by Al3+-ions reported that the relative fractions of P-O-P bonds (BO) and P-O-Na bonds (NBO) decrease, and the fraction of P-O-Al bonds (BO) increases \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **15 mol\% Al(PO3)3** & **15 mol\% Mg(PO3)2** \\ \hline  & **Area (\%)** & **Centroid (eV)** & **Area (\%)** & **Centroid (eV)** \\ \hline NBO & 27.3 & 530.09 & 36.49 & 529.97 \\ BO & 40.24 & 531.17 & 29.55 & 531.07 \\ BO & 32.46 & 532.55 & 33.95 & 532.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Peak positions and area from the curve fitting of O 1s core levels for 55V2O5-5 ZnO-22 BaO-3B2O3-15 Al(PO3)3 and 55V2O5-5 ZnO-22 BaO-3B2O3-15 Mg(PO3)2 glasses (mol%). with an increase of Al\({}_{2}\)O\({}_{3}\) content in an Al\({}_{2}\)O\({}_{3}\)-NaPO\({}_{3}\) glass system [34]. Moreover, Al\({}^{3+}\)-ions, when added to phosphate glass, have been known to increase the degree of structural polymerization via forming the crosslinked P-O-Al structure within the glass networks. The substitution of AlPO\({}_{4}\) for NaPO\({}_{3}\) consumes both P-O-P and P-O-Na\({}^{+}\) bonds to form P-O-Al bonds, which crosslink and shorten the P-O-P chains in the Al(PO\({}_{3}\))\({}_{3}\)-Na(PO\({}_{3}\)) system [35; 36]. On the other hand, it is believed that the area of NBO peak from Mg(PO\({}_{3}\))\({}_{2}\) means the forming of major P-O-Mg and V-O-Mg bonds, as well as the V-O-Ba and P-O-Ba bonds in 55V\({}_{2}\)O\({}_{5}\)-5 ZnO-22 BaO-3B\({}_{2}\)O\({}_{3}\)-Mg(PO\({}_{3}\))\({}_{2}\) (mol%). This trend can be described as a role of the alkaline earth within the glass. Generally, the incorporation of alkaline earth, such as MgO, CaO, SrO, and BaO, within the glass matrix results in depolymerization of glass structure. Indeed, it was reported that Mg\({}^{2+}\)-ions break the glass network and can take a position in the vanadate or phosphate chain within the MgO-V\({}_{2}\)O\({}_{5}\) and MgO-P\({}_{2}\)O\({}_{5}\) systems [26; 27]. Consequentially, the trivalent Al-ion within the glass is well-known as a network intermediate which plays a role in increasing thermal/chemical stability, melting temperature, and viscosity, and reducing crystallization tendency [37].