Glasses doped with well‐controlled Eu
3+ and Eu
2+ ions have attracted considerable interest due to the possibility of tuning the wavelength range of the emitted light from violet to red by using their
5D
0→
7F
j and 5
d–4
f electron transitions. Glasses were prepared to dope Eu
3+ ions in a Na
2O–Al
2O
3–SiO
2 system, and the changes in the valence state of Eu
3+ ions and the glass structure surrounding the Eu atoms during heating under H
2 atmosphere were investigated using fluorescence spectroscopy, X‐ray absorption fine‐structure spectroscopy, and
27Al magic‐angle spinning solid‐state nuclear magnetic resonance spectroscopy. The reduction behavior of Eu
3+ ions was dependent on the Al/Na molar ratio of the glass. For Al/Na < 1, the Al
3+ ions formed the AlO
4 network structure accompanied by the Na
+ ions as charge compensators; the Eu
3+ ions occupied the interstitial positions in the SiO
4 network structure and were not reduced even under heating in H
2 gas. On the other hand, in the glasses containing Al
2O
3 with the Al/Na ratio exceeding unity, the Eu
3+ ions commenced to be coordinated by the AlO
4 units in addition to the SiO
4 network structure. When heated in H
2 gas, H
2 gas molecules reacted with the AlO
4 units surrounding Eu
3+ ions to form AlO
6 units terminated with OH bonds, and reduced Eu
3+ ions to Eu
2+ via the extracted electrons.
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