Control of Oxidation State of Eu Ions in Na 2 O–Al 2 O 3 –SiO 2 Glasses

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 5 D 0 → 7 F j and 5 d –4 f electron transitions. Glasses were prepared to dope Eu 3+ ions in a Na 2 O–Al 2 O 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 27 Al 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 2 O 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.