S.A.M. Lima et al. / Journal of Alloys and Compounds 344 (2002) 280–284
283
causes the broad line transitions observed in the spectrum.
ratio between the intensities of the 5D0 →7F0 and
5D0 →7F2 transitions. The ZnO:Eu 0.1 at% has a high R02
value indicating large J-mixing effect. In this case sug-
gesting the presence of strong ligand field characteristic of
symmetry sites.
5
In addition, the wide D0→7F0 transition band of ZnO:Eu
0.1 at% observed at liquid nitrogen temperature, Fig. 5,
indicates europium ions are in a close chemical environ-
ment as observed in glasses which present an inhomoge-
neous broadening. Under the same measurement condi-
tions, the ZnO:Eu 3 at% presents this transition band
splitted indicating that Eu31 ions occupy at least two
defined symmetry sites, Fig. 6.
4. Conclusions
The experimental intensity parameters V2 and V4 were
determined from the emission spectra obtained at room
Emission spectra characteristic of europium is observed
on the green–yellow band luminescence attributed to ZnO.
The diffusion of europium ion into the zinc oxide lattice
depends on several parameters, e.g., preparation method,
thermal treatment, doping ion content. In this work the
Pechini method of preparing doped zinc oxide has fun-
damental importance because it allows a homogeneous
distribution on zinc oxide precursor. A high intense
emission of europium ion on zinc oxide was observed, in
particular, due to the presence of Zn1 into zinc oxide
lattice produced by thermal treatment. The Eu31 in the
structure allows the stabilization of Zn1 by a charge
compensation mechanism. The energy transfer from ZnO
to europium ion is not observed probably due to the
difference of energy levels.
5
5
temperature by using the D0→7F2 e D0→7F4 transition,
respectively, and by expressing the emission intensity I
[10] in terms of the integrated area under the emission
curves. In Eq. (1) "v is the transition energy, N is the
population of the emitting level (5D0). The coefficient of
spontaneous emission, A, is given by the expression (2).
I 5 "vArad
N
(1)
(2)
4e2v3
]
7
2
A
rad 5
x
O
V
F iU (l)i5D
K
L
3"c3
l
j
0
l
where x 5 no(n20 1 2)2 /9 is a Lorentz local field correc-
tion, no being the index of refraction of the medium
5
considered (1.5). The magnetic dipole allowed D0 →7F1
transition was taken as reference. The values of reduced
7
2
matrix elements are
F iU (2)i5D
5 0.0032 and
K
L
2
0
Acknowledgements
7
2
F iU (4)i5D
5 0.0023. The values of experimental
K
L
4
0
Financial support by FAPESP is gratefully acknowl-
edged. S.A.M.L. and F.A.S thank FAPESP for scholar-
ships.
intensity parameters in Table 1 can be related to changes in
the chemical environment around the rare earth ion. The
high V2 value obtained for zinc oxide doped with 3 at% of
europium is comparable to the value of crystalline
Gd2O3:Eu 3 at% obtained by Pires et al. [14]. This high
value might be interpreted as a consequence of the
hypersensitive behavior of the 5D0 →7F2 transition [10],
which is more intense when there is no inversion symme-
try and there is a mix of opposite-parity wavefunctions into
the 4f wavefunctions [15]. The V2 value obtained for zinc
oxide doped with 0.1 at% of europium is equivalent to
europium ions present in ZnO containing glasses [16] that
suggest a similar environment. The R02 parameter is the
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Table 1
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spectra obtained at room temperature
Sample
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V2
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ZnO:Eu 0.1 at%
ZnO:Eu 3 at%
Eu2O3
0.0230
0.0120
0.0120
0.0006
–
10.93
15.88
18.14
20.33
7.40
5.54
5.03
5.03
6.92
2.30
Gd2O3:Eu 3 at% [14]
Eu31-doped
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