Photoluminescent properties of Eu3+, Tb3+ activated M3Ln(PO4)3 (M = Sr, Ca; Ln = Y, La, Gd) phosphors derived from hybrid precursors

Article abstract of DOI:10.1016/j.jallcom.2006.03.093

Ternary orthophosphates M₃Ln(PO₄)₃ doped with Eu³⁺, Tb³⁺ were prepared via an in situ chemical co-precipitation technology, and the assembly process of hybrid precursors were as follows: using rare-earth coordination polymers with salicylic acid as precursors and composing with the polyvinyl alcohol (PVA) as dispersing media. Their microstructure and micromorphology have been analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The emission spectra exhibited strong luminescence of ⁵D₀ → ⁷F₂ at 609 nm, indicating that the Eu³⁺ located in a noncentrosymmetric position in Eu-doped M₃Ln (PO₄)₃ matrix. Besides this, the values of red to orange emission intensities for Eu³⁺ in Sr₃Gd (PO₄)₃ strongly depend on the doping concentration.

Full text of DOI:10.1016/j.jallcom.2006.03.093

Journal of Alloys and Compounds 429 (2007) 255–259
Photoluminescent properties of Eu3+, Tb3+ activated M3Ln(PO4)3
(M = Sr, Ca; Ln = Y, La, Gd) phosphors derived from hybrid precursors
Xiuzhen Xiao, Shuai Xu, Bing Yan∗
Department of Chemistry, Tongji University, Shanghai 200092, China
Received 5 February 2006; received in revised form 27 March 2006; accepted 29 March 2006
Available online 12 June 2006
Abstract
Ternary orthophosphates M3Ln(PO4)3 doped with Eu3+, Tb3+ were prepared via an in situ chemical co-precipitation technology, and the assembly
process of hybrid precursors were as follows: using rare-earth coordination polymers with salicylic acid as precursors and composing with the
polyvinyl alcohol (PVA) as dispersing media. Their microstructure and micromorphology have been analyzed by X-ray powder diffraction (XRD)
7
and scanning electron microscopy (SEM). The emission spectra exhibited strong luminescence of 5D0 → F2 at 609 nm, indicating that the Eu3+
located in a noncentrosymmetric position in Eu-doped M3Ln (PO4)3 matrix. Besides this, the values of red to orange emission intensities for Eu3+
in Sr3Gd (PO4)3 strongly depend on the doping concentration.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Phosphors; Chemical synthesis; Luminescence
1. Introduction
luminescent materials have been fabricated by Liang et al. and
their photoluminescent spectra under VUV–UV excitation have
been discussed in details [26]. But the traditional synthesis meth-
ods for luminescent materials doped with rare-earth ions were
solid-state reactions at temperatures above 1300 ◦C or simple
wet-chemical technologies such as chemical co-precipitation,
sol–gel and colloidal reactions, etc [27,28]. Recently, a new
sol–gel technology based on hydrolysis and polycondensation
process is the most popular, which has been verified to be
very useful in the synthesis of nonagglomerated nanoparticles
[29–33].
Potential and effective technological applications, mainly
related to optical and optoelectronic domains (scintillators,
lasers, etc.), have significantly revived interest in new mate-
rials crystallizing with eulytite structure [1–6]. The struc-
tural type of eulytite (also called eulytine or agricolite) was
first recognized in mineral Bi4(SiO4)3. Since the discovery
of the structure of Bi4(SiO4)3, a large number of compounds
isostructural with this mineral have been synthesized over
the past 20 years [7–20]. The examples of these compounds
are: Bi4(GeO4)3, Pb3Bi(XO4)3 (X = P, As, V), Pb3M(GeO4)3
(M = M2+), M3Ln(PO4)3 (M = Ca, Sr, Ba, Pb; Ln = lanthanide),
Pb3M(PO4)3 (M = V, Cr, Fe) and Pb3.5M0.5(PO4)3 (M = (Ti,
Th) have been reported [1,11,12,21–23]. As far as spectro-
scopic properties are concerned, attention has been brought
to eulytite structural phosphates doped with rare-earth ions.
M3La(PO4)3:Ce3+ (M = Sr, Ba) phosphor has been reported
by Hoogendop et al. [24]. The powder laser material,
Ba3Y(PO4)3:Nd3+ has been synthesized by Znamierowska et al.
[25]. Besides this, Ce3+, Pr3+ and Tb3+ activated Sr3Ln(PO4)3
In this paper, we put forward a novel modified synthesis tech-
nology through in situ chemical co-precipitation reactions, and
obtained M3Ln(PO4)3:Eu3+, Tb3+ (M = Sr, Ca; Ln = Y, La, Gd)
crystalline phosphors. We select rare-earth coordination poly-
mers with ortho-hydroxylbenzoic acids as the precursors of
rare-earth species for their infinite chain-like polymeric struc-
ture similar to organic polymer template. The multicomponent
hybrid polymeric precursors were assembled with other func-
3−
tional component, NH4H2PO4 as source components for PO4
and carbonates were used as precipitation components. Besides
this, organic polymer, i.e., polyethyl glycol (PEG) was used as
dispersing media. Urea was used to modify the decomposition
behavior and ammonia for the pH value. Then through in situ
co-precipitation process, multicomponent hybrid polymeric pre-
∗
Corresponding author. Tel.: +86 216 5984 663; fax: +86 216 5982 287.
E-mail address: byan@tongji.edu.cn (B. Yan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.03.093

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X. Xiao et al. / Journal of Alloys and Compounds 429 (2007) 255–259
cursors were constructed to afford the M3Ln(PO4)3:Eu3+, Tb3+
with different doping concentration, whose luminescent proper-
ties were discussed in details.
2. Experimental
2.1. Synthesis of M3Ln(PO4)3:Eu3+, Tb3+
The initiative materials Y2O3, Gd2O3, La2O3H, Eu2O3 and Tb4O7 were
firstly dissolved into concentrated nitric acids. Then the preparation of the pre-
cursors was described in details in the following: superfluous salicylic acid
(6.0 mmol) was dissolved into 95% ethanol and its pH value was then adjusted to
7.0–8.0 with ammonia solution. Then the rare-earth nitrates solutions (Y(NO3)3,
Gd(NO3)3, Eu(NO3)3 and Tb(NO3)3) were added and mixed homogenously.
After being stirred for half hours, the urea and PEG were added into the above
solutions with appropriate ratio to RE. After an hour, the solutions were heated
to 70 ◦C and the ph value was adjusted to about 7.5. At the same time, the
NH4H2PO4 and MCO3 powders were added into the solutions. After this,
the solutions were stirred until they became homogenous systems. After being
deposited for several days they were filtered and the precursors were achieved.
Finally the samples were heat-treated at the temperature 1000 ◦C for about 4 h.
Fig. 1. The selected XRD patterns of the Sr3La(PO4)3:Tb3+ (A),
Sr3Y(PO4)3:Tb3+ (B), Ca3Y(PO4)3:Tb3+ (C) and Sr3Gd(PO4)3:Tb3+ crystalline
powders.
2.2. Physical measurements
the scanning electron microscope (SEM) we obtain the morphol-
ogy and the M3Ln(PO4)3:Eu3+, Tb3+ phosphor particle size is
about 0.5 ␮m which takes agreement with the data from the
XRD estimation (Figs. 2 and 3). It can be seen that both of them
mainly consist of solid microcrystalline structures. In addition,
there exist some conglomeration phenomena in the SEM micro-
graph which may result from the higher temperature of thermal
decomposition. Rare-earth complexes with aromatic carboxylic
acid (ortho-hydroxylbenzoic acid) show the polymeric structure
[34] and are used as the precursors to prepare the luminescent
species. The organic polymer, polyethylene glycol (PEG) was
combined both as dispersing medium and fuel to form the poly-
meric hybrid precursor template and show higher temperature
of thermolysis process. These rare-earth coordination polymers
can compose with organic polymers (PEG) to obtain an interpen-
etrating polymeric network structures for they show the similar
infinite polymeric structure [35]. Therefore, the particle sizes of
rare-earth oxysalts M3Ln(PO4)3:Eu3+, Tb3+ can be controlled
The particle sizes were characterized by means of X-ray diffraction
(XRD, Bruke, D8-Advance, 40 kV and 20 mA, Cu K␣). The morphology and
microstructure were characterized with scanning electronic microscope (SEM,
Philps XL-30). Excitation and emission spectra at room temperature were
determined with Perkin-Elmer LS-55 model fluorophotometer (excitation slit
width = 10 nm, emission width = 5 nm).
3. Results and discussion
Fig. 1 shows the XRD pattern of prepared samples checked
by X-ray powder diffraction using Cu K␣ radiation. The XRD
data indicated that the samples were single cubic phases and
belong to the large family of the eulytite type compounds, which
was in good agreement with those in JCPDS cards 44–0319
[Ca3Y(PO4)3], 44–0320 [Sr3Y(PO4)3], 80–1597 [Sr3La(PO4)3]
and 29–1301 [Sr3Gd(PO4)3], respectively. The representative
structureofSr3La(PO4)3 isdescribedincubicI−43d(Td )space
6
˚
group with lattice parameters a = 10.185 A and Z = 4 [8]. Using
Fig. 2. SEM micromorphologies of Sr3La(PO4)3:Tb3+ (A), Sr3Y(PO4)3:Tb3+ (B).

X. Xiao et al. / Journal of Alloys and Compounds 429 (2007) 255–259
257
Fig. 3. SEM micromorphologies of Sr3Gd(PO4)3:Tb3+ (A) and Ca3Y(PO4)3:Tb3+ (B).
that of 7F1 and 5D0 of Eu3+, a Gd3+ in 6GJ state can excite Eu3+
into 5D0 state by resonance energy transfer, which results in the
energy transfer of Gd3+ to Eu3+ [36,37]. The excitation spectra
of Sr3Ln(PO4)3:Tb3+ activated powders in Fig. 5, presents three
strong bands centered at 210, 243, 256 nm and a week band in
the region 300–400 nm. The band at 210 nm was ascribed to the
host absorption. The ground states (4f8) of Tb3+ are 7FJ config-
urations, when one electron is promoted to 5d shell, it can give
rise to two 4f75d1 excitation states; the high-spin states with
and determined by the structures of hybrid precursors. Then this
preparationtechnologyconnectstheassemblyofhybridmaterial
with synthesis of micrometer material, which can be expected to
be a candidate for the synthesis of other luminescent materials
based on rare-earth oxides.
All the excitation spectra of M3Ln(PO4)3:Eu3+ show the sim-
ilar features, which exhibit no apparent excitation bands in long
wavelengthultravioletregionof300–400 nm. Therepresentative
excitation spectrum for Sr3Ln(PO4)3:Eu3+ annealed at 1000 ◦C
is shown in Fig. 4, which is taken at an emission wavelength
of 613 nm. The charge transfer state (CTS) band which is cor-
responded to an electron transfer from an oxygen 2p orbital to
an empty 4f orbital of europium ion, is observed in the region
of 230–280 nm. And the host absorption band (HAB) can be
seen around 210 nm. It also can be seen that the intensities of
absorption bands in Sr3Ln(PO4)3:Eu3+ phosphors are ordered
as follows: Gd3+ > Y3+ > La3+. The result can be interpreted by
the energy transfer between Gd3+ and Eu3+. It was well known
that the energy level difference of 6GJ and 6PJ of Gd3+ is close to
7
9DJ configurations or low-spin states with DJ configurations
[26]. It is obvious that 9DJ states will be lower in energy accord-
7
7
ing to Hund’s rule. And the transitions between FJ and DJ
are spin-allowed, while 7FJ–9DJ transitions are spin-forbidden.
Therefore, the band at 243 nm would seem to be attributed to the
7D level of the 4f–5d transition of Tb3+ due to s spin-allowed
4f8–4f7 d1 transitions, with higher energy; the band at 256 nm
5
couldbeassignedtothe9Dlevelofthe4f7 d1 configuration, with
5
lower energy. Besides this, there also appeared a week band in
Fig. 4. Excitation spectra of Sr3Ln(PO4)3:Eu3+
.
Fig. 5. Excitation spectra of Sr3Ln(PO4)3:Tb3+
.

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X. Xiao et al. / Journal of Alloys and Compounds 429 (2007) 255–259
Fig. 6. Emission spectra of Sr3Ln(PO4)3:Eu3+
.
Fig. 8. The value of red to orange emission intensities for Eu3+ with the its
concentration in Sr3Gd (PO4)3 matrix.
the region 300–400 nm resulting from the Tb3+ f–f transition.
At the same time, the order for the intensities of the absorption
bands is same to that of absorption band for Sr3Ln(PO4)3:Eu3+.
The emission spectra of all phosphors under the excitation
wavelength at 222 nm show the similar features. As shown
in Fig. 6, the emission spectra of all Eu3+ activated phos-
phors have been involved in the following emission lines:
5D0–7F1, 5D0–7F2, 5D0–7F3 and 5D0–7F4, which were situated
at 592, 609, 654 and 704 nm, respectively. For all compounds
peak positions had no significant change. The emission spec-
and Sr3Gd(PO4)3, the RED emission appears more intense in the
Sr3Gd(PO4)3 than in other two rare-earth orthoniobates, which
agreed to the excitation intensity of three powders. It seems to
be that Gd3+ plays an important role in this compound. Besides
this, it can be observed that the luminescent properties strongly
depend on the doping concentration in the host lattices in Fig. 7.
When the concentration of Eu3+ increases up to 5 mol%, the
intensity shows the strongest and slowly decreases at higher
europium concentrations. This is a typical property named con-
centration quenching of lanthanide-doped system due to mutual
7
tra exhibited strong luminescence of 5D0 → F2 at 609 nm. The
5
Eu3+–Eu3+ interactions. It was well known that the D0–7F2
7
5D0 → F2 transition was of electric dipole transition, indicating
transition is an electric dipole transition and is extremely sensi-
tive to chemical bonds in the vicinity of Eu3+ ions. The intensity
increases as the site symmetry decreases and becomes more
asymmetric. On the other hand, the 5D0–7F1 transition is a mag-
netic dipole transition, and its intensity varies with the crystal
field strength acting on Eu3+ ions. Therefore, the intensity ratio
of the 5D0–7F2 transition to 5D0–7F1 transition is widely used as
a measure of rare-earth site symmetry. Fig. 8 exhibits the change
of the values of red to orange emission intensities for Eu3+ in
Sr3Gd(PO4)3 as a function of the europium concentration. It can
be found that the value increases with the increasing of Eu3+ con-
centration from 0.5 to 5 mol%, indicating the site symmetry of
Eu3+ decreases. However, the site symmetry increases at higher
concentration.
that the Eu3+ located in a noncentrosymmetric position in Eu-
doped M3Ln(PO4)3 matrix. According to group theory selection
rules, the magnetic dipole and the electric dipole are permit-
ted and the electric dipole transition is stronger. As shown in
Fig. 7, among these three powders Sr3Y(PO4)3, Sr3La(PO4)3
As shown in Fig. 9, the emission spectra of Tb3+-doped
Sr3La(PO4)3, Sr3Y(PO4)3 and Sr3Gd(PO4)3 phosphors present
the four feature lines at 487, 546, 588 and 621 nm, corresponding
5
5
5
5
to the transition of D4–7F6, D4–7F5, D4–7F4 and D4–7F3,
respectively. All the emission spectra of M3Ln(PO4)3:Tb3+
wearsthesimilarfeatures. InFig. 9, wecanfindthatthephosphor
Sr3Gd(PO4)3 doped with Tb3+ can perform more intense green
fluorescence than other two phosphors, indicating that Gd3+ in
Sr3Gd(PO4)3:Tb3+ phosphor possibly plays a role in the energy
transfer process: Gd3+ can behave as a intermediate to absorb
3−
Fig. 7. The relative luminescent intensity of Eu3+ as a function of its content in
Sr3Gd(PO4)3 matrix.
the ultraviolet excitation energy from PO4 and then transfer
to enhance the emission of Tb3+.

X. Xiao et al. / Journal of Alloys and Compounds 429 (2007) 255–259
259
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Fig. 9. Emission spectra of Sr3Ln(PO4)3:Tb3+
.
4. Conclusions
In summary, ternary orthophosphates M3Ln(PO4)3 doped
with Eu3+, Tb3+ phosphors were achieved by a novel in situ
assembling hybrid precursor co-precipitation technology. Both
XRD and SEM results indicate that these phosphor particles
exhibit novel crystal morphology and the size of the particles is
about0.5 ␮m. ThecharacteristicemissionlinesofEu3+ andTb3+
have been detected and the strong luminescence of 5D0 → F2 at
7
609 nm indicates that the Eu3+ located in a noncentrosymmetric
position in Eu-doped M3Ln(PO4)3 matrix. Moreover, the value
of red to orange emission intensities for Eu3+ in Sr3Gd(PO4)3
host has been calculated.
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Acknowledgements
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The work was supported by the Science Fund of Shanghai
University for Excellent Youth Scientists and National Natural
Science Foundation of China (20301013).
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