In this paper, the structural, thermal, optical, and spectroscopic properties of Er 3+ -doped tellurite glasses with the composition 68.25TeO 2 -19.5ZnO-9.75X-2.5Er 2 O 3 (in mol%) with X = BaO, Na 2 O, and Bi 2 O 3 are reported. The glasses were prepared using the standard melt quenching method. The investigated glasses exhibit low phonon energy (∼745 cm −1 ) and low glass transition temperature varying between 300 and 350 • C depending on the glass composition. The Raman spectra show a regular tellurite structure with variations in the number of bridging and non-bridging oxygens depending on the glass composition, the Na 2 O and Bi 2 O 3 -containing glasses having the most and the least polymerized network, respectively. A thermal treatment of the glasses leads to the formation of crystals, the composition of which depends on the glass composition, as revealed by X-ray diffraction analysis and confirmed using scanning electron microscope-energydispersive spectroscopy. The precipitation of Er-containing crystals in the Na 2 O and BaO-containing glasses leads to an increase in the intensity of the upconversion emissions. Although the Er 3+ ions remain in the amorphous part of the Bi 2 O 3 -containing glass after heat treatment, it is the precipitation of Bi 3.2 Te 0.8 O 6.4 crystals in this glass, which is thought to decrease the distance between the Er 3+ ions leading to an increase in the intensity of the upconversion and mid-infrared emissions.
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