title: Reduction mechanism for Eu ions in Al2O3-containing glasses by heat treatment in H2 gas.
abstract: Superior functional glasses doped with rare-earth ions have been prepared by controlling the valence states of rare-earth ions. However, recent work has revealed unresolved questions about the controlling mechanism of rare-earth ions' valence states. To address these questions, oxide glasses with and without Al2O3 and doped with Eu(3+) ions were prepared by a melting process; then, the valence states of Eu(3+) ions were investigated during heating under a hydrogen environment. The Eu(3+) ions were reduced to Eu(2+) only in the glass containing Al(3+) ions; the reduction occurred in the center of the glass over a short heating period. It was discovered that the reduction of Eu(3+) ions concurrently occurred with the formation of OH bonds which were bound with Al(3+) ions. Considering this and the data for the H2 gas diffusion through the glass, we conclude that diffusing H2 gas molecules react with Al-O(-) bonds surrounding Eu(3+) ions to form AlOH bonds and reduce Eu(3+) ions to Eu(2+) via the extracted electrons. When H2 reacts with a glass structure, that hydrogen has transformed into -OH bonds and the hydrogen concentration in the glass decreases. In order to make up the lost hydrogen, more hydrogen molecules can enter into the glass, resulting in the fast reduction of Eu(3+) ions in the center of the glass.
authors.affiliation: Toyota Physical and Chemical Research Institute , Nagakute, Aichi 480-1192, Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Nogami