Influence of Er³⁺ ions addition on thermal and optical properties of phosphate–germanate system

Abstract

Zinc phosphate–germanate samples doped with Er³⁺ ions and co-doped with silver nanoparticles in the 59.7KPO₃–0.3Ag(NPs)–20ZnO–(20 − x)GeO₂–xEr₂O₃ system where x ≤ 5 mol% were prepared by melting–quenching technique. The mentioned system was studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and ultraviolet–visible (UV–Vis) spectroscopy, too. XRD analysis shows the presence of amorphous state for low contents of erbium and the presence of the ErPO₄ (tetragonal, body-centered lattice) crystalline phase beside the amorphous phase for high contents of erbium (x ≥ 3 mol%). DTA investigation permitted the identification of some thermal parameters such as glass transition temperature, crystallization temperature and melting temperature. From these data, other two important parameters were calculated: the fragility index and the activation energy of glass transition. In our case, the obtained data reveal a good thermal stability for the matrix of the studied system. The increase in the content of erbium ions leads to more fragile glasses and glass ceramic samples. The DTA results also show that the samples are obtained from KS liquids. The UV–Vis spectroscopy shows seven f–f electronic transitions for the studied system, due to the presence of erbium ions. From the obtained UV–Vis data, the bonding parameter that shows an ionic character of the bonds from the glass ceramic network was calculated. Also, the values of optical band gap energy ($E_{\text{g}}^{\text{opt}}$) show that when the erbium ions content increases, the $E_{\text{g}}^{\text{opt}}$ values decrease due probably to the amount of non-bridging oxygen atoms from the network.