Spontaneous reduction of Eu3q ion in Al co ... - Davidson Physics

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A. Biswas et al.rMaterials Letters 39 1999 227–231
excitation peak at 378 nm corresponding to the Eu 2q
ions Ž Fig. 6b ..
The emission spectra, excitation spectra and UV–
Vis absorption spectra all point to the fact that the
Eu 3q ions have been reduced to Eu 2y in the presence
of Al 3q during the densification of the gel
around 10008C. The Eu-doped gel heat treated at
10008C shows a broad emission with a peak at 354
nm Ž Fig. 7 .. This emission is associated with oxygen
Ž
y
associated hole centers O states.
and was reported
earlier w16 x. Since Eu ions are in the trivalent state,
there must be charge compensation within the silica
matrix from the nonbridging oxygen atoms. The
absence of a sufficient number of nonbridging oxygen
atoms to coordinate the isolated rare earth ions
in a rigid silica network may cause oxygen associated
hole centers at high temperature. The electron
emitted from the defect centers may react with Eu 3q
in the silica matrix to form Eu 2q in the presence of
alumina. Defect center induced reduction of Eu 3q
ions has already been reported in xerogels at room
temperature w16 x.
The spontaneous reduction of Eu 3q ions in Al
co-doped glasses can be explained by the observaw
x
3q
tion of Nogami et al. 11 who found that the Sm
ions can only be reduced by flowing H 2–N2
gas
mixture in the presence of Al 2O 3. They attributed
this to a phenomenon classified as microscopic basicity
in which the electron donating ability of oxygen
surrounding the rare earth ions is affected by the
3q
Al ions as proposed by Duffy and Ingram w22 x.
Fig. 7. Emission spectra of Eu 3q -doped gel heated to 10008C.
The lowered basicity in Al 3q co-doped gel together
with the low reduction potential of Eu 3q Eu 2q
Ž y0.32 E8, V. w23x
may account for the spontaneous
reduction of Eu 3q by the electron ejected from the
defect center at higher temperatures w17 x.
4. Conclusion
Eu-doped silica glasses with and without alumina
have been prepared by impregnating porous silica
monoliths with ethanolic solutions of EuŽ NO .
3 3P
5H O and AlŽ NO .
2 3 3P9H 2O and subsequent densification.
In the silica matrix, Eu maintains its trivalent
state throughout the densification process. In the
Eu–Al co-doped silica, the Eu ion is in a trivalent
state only up to around 10008C. In the presence of
Al 2O 3, it spontaneously reduces to its divalent state
during densification above 11008C.
Acknowledgements
Fig. 6. Excitation spectra of the glasses densified above and doped
Ž. Ž.
with a Eu and b Eu–Al.
This work was supported in part by the Directorate
of Chemistry and Life Sciences of the Air
Force Office of Scientific Research through contract
number F496209610124 and in part by the MURI
program through the University of Southern California
primary contract number 000507.