Enhanced emission from Eu2+ ions in sol-gel derived Al2O3–SiO2 glasses

Eu2+ ions were incorporated in SiO2 and Al2O3–SiO2 glasses using a sol-gel process of Si(OC2H5)4, Al(OC4H9sec)3, and EuCl3 6H2O. Glasses incorporated with the Eu2+ ions showed the emission with peak at ∼22000 cm−1 due to 4f 6 5d→4f 7(8S7/2) transition of the Eu2+ ions. 1Al2O3⋅99SiO2 (mol %) glass had an intense emission, 250 times larger than that of 100SiO2 glass. The covalency of the Eu–O bonds is decreased by the Al3+ ions. We concluded that this weak covalency causes the offset of the excited 4f 6 5d level to be small, resulting in an increase in the probability of the radiative relaxation from the 4f 6 5d to 4f 7(8S7/2) level.