Combustion synthesis and luminescent properties of Eu3+ and Dy3+
co-doped amorphous SrAl2O4
Bao-gai Zhai1, 2, Qing-lan Ma3 and Yuan Ming Huang1, a
1
School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China
2
College of Physics and Electronic Information, Yunnan Normal University, Yunnan 650094, China
3
Department of Physics, Wuhan University, Wuhan 430072, China
a
dongshanisland@126.com
Keywords: Amorphous strontium aluminate; Trivalent europium ions; Trivalent dysprosium ions;
Photoluminescence
Abstract. Trivalent Eu and Dy ions co-doped amorphous strontium aluminate oxide
(SrAl2O4:Eu3+Dy3+) were synthesized through solution combustion route. The phase of the
synthesized compound was examined with the X-ray diffractometry. The photoluminescence
spectra of the amorphous SrAl2O4:Eu3+Dy3+ has shown that the sharp blue emission of Dy3+ at 483
nm, the sharp yellow emission of Dy3+ at 570 nm and the sharp red emission of Eu3+ at 615 nm can
be integrated into the broad blue background emission of the amorphous host. The results indicate
that the amorphous SrAl2O4 host can provide good environment to tune the emission color for the
SrAl2O4:Eu3+Dy3+.
Introduction
The divalent europium and trivalent dysprosium ions co-activated strontium aluminate oxides
(SrAl2O4:Eu2+,Dy3+) are promising luminescent materials due to their high brightness and persistent
phosphorescence [1-4]. It is reported that the crystallographic phases of strontium aluminate oxides
can be described as amorphous SrAl2O4, hexagonal SrAl2O4, monoclinic SrAl4O7 and orthorhombic
Sr4Al14O25 [2]. Up to date, strontium aluminate oxides in crystalline phase were exclusive host
materials to prepare strontium-aluminate-oxide based phosphors. As a contrast, amorphous SrAl2O4
is scarcely employed to be the host for the rare-earth doped persistent phosphors. Compared to the
crystalline phases, the amorphous matrix of the strontium aluminate oxides makes it easier for the
trivalent rare-earth ions to substitute the Sr2+ ions. Therefore, it is significant to conduct a
systematic investigation on the optical properties of Eu3+ and Dy3+ amorphous SrAl2O4. In this
paper, we report on the synthesis and optical properties Eu3+ and Dy3+ co-activated amorphous
SrAl2O4.
Experiment
All chemicals were in analytical grade and were used as received. Strontium aluminate oxide was
prepared from the mixture of strontium nitride (20 mmol, 4.263 g), aluminum nitrate nonahydrate
(40 mmol, 15.095 g), urea (240 mmol, 14.472 g), boric acid (8 mmol, 0.498 g), nitric acid (5 ml)
and water (150 ml). The boric acid in the mixture was used as a flux. Co-activators of 0.5 mmol
(0.174 g) of europium oxide (Eu2O3) and 0.6 mmol (0.225 g) of dysprosium oxide (Dy2O3) were
dissolved into the mixture at 50oC. The designed composition of the phosphors was
SrAl2O4:0.05Eu3+,0.06Dy3+. About 20 ml of the solutions were transferred into an alumina crucible.
The phosphors were prepared by conventional combustion method in alumina crucibles [5-6]. The
phosphor was fired at 600oC in an alumina crucible without the cover of an alumina cap in order to
release the reducing gases as soon as they were generated. The resulted phosphor was shortened to
be SrAl2O4:Eu3+,Dy3+.
The X-ray diffraction characterization of the thin films was carried out on an X-ray
diffractometer (D/max 2500 PC, Rigaku Corportaion, Japan). The operating voltage was 40 kV and
the current was 100 mA. A copper target was utilized to generate the K radiation (λ = 1.5405 Å).
The photoluminescence spectra of the phosphors were recorded with a spectrophotometer (Aipha
Technologies, China). The excitation wavelength for the photoluminescence measurement was the
325 nm emission line from a helium-cadmium laser (Kimmon Koha Co. Ltd., China).
Results and discussions
The phase of the synthesized compound was characterized with X-ray diffraction. Figure 1(a)
shows the X-ray diffraction pattern of the synthesized SrAl2O4:Eu3+,Dy3+. The X-ray diffraction
pattern of the synthesized compound is characteristic of a series of sharp peaks and two broad
bands. The maxima of two broad bands in Fig. 1(a) are located at about 30 and 44 o, respectively. It
is found that all the peaks could be indexed to the Sr(NO3)2. Figure 1(b) depicts the X-ray
diffraction reference pattern of Sr(NO3)2 (JCPDS file no.25-0746). Comparison of the results in Fig.
1 demonstrates that the sharp diffraction peaks of the synthesized compound are originated from the
residual of the SrAl2O4 left after the reaction. The presence of the two broad bands in Fig. 1(a)
confirms that the phase of the resulted compound is the amorphous SrAl2O4.
Fig. 1. (a) X-ray diffraction pattern of the Fig. 2. Photoluminescence spectra of
synthesized SrAl2O4:Eu3+,Dy3+; (b) XRD amorphous SrAl2O4 (shaded curve) and three
reference pattern of Sr(NO3)2 (JCPDS file Eu3+, Dy3+ co-doped amorphous SrAl2O4.
no.25-0746).
Figure 2 depicts the typical photoluminescence spectra of amorphous SrAl2O4 (shaded curve)
and the Eu3+, Dy3+ co-doped amorphous SrAl2O4. The amorphous SrAl2O4 host gives of weak blue
emissions upon the ultraviolet excitation of our He-Cd laser. The broad photoluminescence
spectrum of the amorphous SrAl2O4 host is represented by the shaded curve in Fig. 2. The maximal
intensity of the photoluminescence spectrum is located at about 400 nm. The oxygen defects in the
amorphous SrAl2O4 host are believed to be the origin of the observed blue emission. After Eu3+ ions
are incorporated into the amorphous SrAl2O4 host, dopant Eu3+ ions are accommodated in
substitutional sites of Sr2+. The radius of host Sr2+ (0.118 nm) differ from Eu3+ (0.0947 nm). For
Eu3+ dopant, charge compensation would require that two Eu3+ ions are substituted for three Sr2+
ions. For Eu3+ dopant state, there could be two ways to maintain overall charge neutrality in the
lattice [6]. The first option of substitution is described by creating one Sr2+ vacancy for the
incorporation of two Eu3+ions. The balance in charge is described with Equation (1)
3
3Sr 2  2EuSr
 VSr 2
.
(1)
The second option is realized by introducing or creating one oxygen interstitial
following manner
3
3Sr 2  2 Eu Sr
 Sr 2  Oi2
.
(Oi2−)
defect in the
(2)
As the radius of substitutional Eu3+ is smaller than Sr2+ and the substitution demands the presence
of vacancy or interstitial in neighboring position for charge compensation, hence the strain in the
SrAl2O4 lattice will be more for Eu3+ substitution than for Eu2+ substitution. The rich oxygen
vacancies in the amorphous SrAl2O4 host provide abundant recombination centers for the blue
emission.
Instead of the strong green emissions from the SrAl2O4:Eu2+,Dy3+ nanocrystals, the
photoluminescence spectrum of the amorphous SrAl2O4:Eu3+,Dy3+ host is characteristic of the three
sharp emissions at 481, 574 and 614 nm, respectively. Trivalent Eu3+ and Dy3+ ions are ones of the
most important luminescent species. A variety of Eu3+ and Dy3+ co-doped host materials can give
off strong emissions in the visible spectral region. The energy level diagrams of Eu3+ and Dy3+ in
the crystal lattices are schematically shown in Fig. 3. Horizontal lines represent narrow energy
states of 4f levels. Shaded areas represent the broad charge transfer states for Eu3+. It is established
that the 4f electrons in the Eu3+ and Dy3+ ions are well shielded by the 5s25p6 outer shells, so the 4f
to 4f transitions in Eu3+ and Dy3+ are likely responsible for the recorded sharp emissions because
the crystal field experienced by 4f electrons is weak. In Dy3+ doped phosphors, the strongest
emissions are observed in the yellow and blue spectral ranges. As reported by Pavani et al., the
most intense yellow line is centered at 582 nm and can be assigned to the 4F9/2→6H13/2 transition,
while the blue emission peaked at 483 nm is due to its 4F9/2→6H15/2 transition [7]. Comparison of
the characteristic emissions of Dy3+ ions in the literature leads us to conclude that the blue
emissions observed at 481 nm (2.58 eV) and the yellow emissions at about 574 nm (2.16 eV) in Fig.
2 can be attributed to the characteristic transitions 4F9/2→6H15/2 and 4F9/2→6H13/2 of Dy3+ ions,
respectively. The two following lines at 676 and 762 nm associated with 4F9/2→6H11/2 and
4
F9/2→6F11/2 transitions of are Dy3+ ions are hardly perceptible in our case.
The sharp peak at 614 nm in Fig. 2 can be assigned to the 5D0–7F2 transition of Eu3+ ions in the
amorphous SrAl2O4 host. It is established that radiative recombination can happen by transitions
from the excited 5D0 level to the ground 7FJ (J = 0-6) levels of the 4f6 configuration in Eu3+ ions [6].
In principle, parity selection rules forbid electric dipole transitions as the transitions are between the
states of same parity. That is why only magnetic dipole transitions can occur between ΔJ = ±1 states
for Eu3+ ions in perfect crystals. In other words, the magnetic dipole transition (5D0–7F1) with the
resulted emission at 591 nm should be relatively strong whereas electric dipole transitions
(5D0–7F2,3,4,5,6) are parity forbidden and should be very weak if Eu3+ occupies the inversion centre
site. Howerver, a small deviation from inversion symmetry in practice allows the ΔJ = ±2, ±4 forced
electric dipole transitions become significant. The relative intensity of the forced electric dipole
transition (5D0–7F2) in Eu3+ is linked to the local site symmetry of the Eu3+ luminescent centre in the
lattice. This is the reason why the strong 5D0–7F2 (614 nm) transition of Eu3+ is clearly seen in Fig.
2 but the emissions from the magnetic dipole transition (5D0–7F1) are hard to be seen. The emission
from the 5D0–7F4 (698 nm) transition was not detected in our synthesized compound. Hence the
results in Fig. 2 showed that amorphous SrAl2O4 host was prepared by the combustion synthesis
with successful incorporation of Eu3+ and Dy3+ ions.
Fig. 3. Schematic energy level diagram of
Dy3+ and Eu3+ in crystal lattices. Horizontal
lines represent narrow energy states of 4f
levels. Shaded areas represent broad charge
transfer states in the case of Eu3+.
Fig. 4. Chromaticity coordinates of the
amorphous SrAl2O4:Eu3+,Dy3+in the CIE 1931
XYZ color space. Data of typical green
phosphors SrAl2O4:Eu2+,Dy3+ and the amorphous
SrAl2O4 host (solid green dot near point A) were
supplied for comparison.
The results in Fig. 2 have demonstrated that sharp red, yellow and blue emissions of Eu3+ and
Dy3+ ions can be integrated into the blue luminescent background of the amorphous SrAl2O4 host.
The results indicate that the luminescent properties of the amorphous SrAl2O4:Eu3+,Dy3+ can be
tuned by tuning the relative molar percentages of Eu3+ and Dy3+ in the host. To quantitatively study
the change in the colors for the Eu3+ and Dy3+ co-doped amorphous host, we performed colorimetric
investigations on the phosphors by employing the method reported in our previous publications [8,
9]. The chromaticity coordinates x and y were obtained for the amorphous SrAl2O4:Eu3+,Dy3+ in the
CIE 1931 XYZ color space, and the results are shown in Fig. 4. The results in Fig. 4 shows that the
chromaticity coordinates of the amorphous SrAl2O4:Eu3+,Dy3+ can be effectively shifted from the
initial point A (0.172, 0.112) to its final point C (0.226, 0.207) as more and more Eu 3+ and Dy3+
ions are incorporated into the host. The chromaticity coordinates of typical green phosphors
SrAl2O4:Eu2+,Dy3+ were put into Fig. 4 for comparison. Furthermore, the solid green dot near point
A represents the chromaticity coordinates of the amorphous SrAl2O4 host itself. In principle, white
colored emissions at the point (0.333, 0.333) can be expected for the amorphous SrAl2O4:Eu3+,Dy3+
if the molar ratios of Eu3+ and Dy3+ ions in the host are finely adjusted.
Summary
Eu3+ and Dy3+ co-doped amorphous SrAl2O4 was synthesized through solution combustion route.
The photoluminescence spectra of the amorphous SrAl2O4:Eu3+Dy3+ has shown that the sharp blue
emissions of Dy3+ at 483 nm, the sharp yellow emissions of Dy3+ at 570 nm and the sharp red
emissions of Eu3+ at 615 nm can be integrated into the broad background emissions of the
amorphous host. These results indicate that the emission color of the Eu3+ and Dy3+ co-doped
amorphous SrAl2O4 can be effectively tuned by finely tuning the relative molar ratio of the dopants
in the amorphous SrAl2O4 host.
Acknowledgements
The authors are grateful to Changzhou University for the financial support under the grant no.
ZMF1002132.
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