Self-assembled light lanthanide oxalate architecture with controlled morphology, characterization, growing mechanism and optical property

Article abstract of DOI:10.1016/j.materresbull.2011.06.029

Flower-like Sm₂(C₂O₄)₃· 10H₂O had been synthesized by a facile complex agent assisted precipitation method. The flower-like Sm₂(C₂O ₄)₃·10H₂O was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, thermogravimetry- differential thermal analysis and photoluminescence. The possible growth mechanism of the flower-like Sm₂(C₂O₄) ₃·10H₂O was proposed. To extend this method, other Ln₂(C₂O₄)₃·nH₂O (Ln = Gd, Dy, Lu, Y) with different morphologies also had been prepared by adjusting different rare earth precursors. Further studies revealed that besides the reaction conditions and the additive amount of complex agents, the morphologies of the as-synthesised lanthanide oxalates were also determined by the rare earth ions. The Sm₂(C₂O₄) ₃·10H₂O and Sm₂O₃ samples exhibited different photoluminescence spectra, which was relevant to Sm ³⁺ energy level structure of 4f electrons. The method may be applied in the synthesis of other lanthanide compounds, and the work could explore the potential optical materials.

Full text of DOI:10.1016/j.materresbull.2011.06.029

Materials Research Bulletin 46 (2011) 1546–1552
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Self-assembled light lanthanide oxalate architecture with controlled morphology,
characterization, growing mechanism and optical property
*
Hongmei He, Youjin Zhang , Wei Zhu, Ao Zheng
Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Flower-like Sm2(C2O4)3ꢀ10H2O had been synthesized by a facile complex agent assisted precipitation
method. The flower-like Sm2(C2O4)3ꢀ10H2O was characterized by X-ray diffraction, X-ray photoelectron
spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy,
thermogravimetry–differential thermal analysis and photoluminescence. The possible growth mecha-
nism of the flower-like Sm2(C2O4)3ꢀ10H2O was proposed. To extend this method, other Ln2(C2O4)3ꢀnH2O
(Ln = Gd, Dy, Lu, Y) with different morphologies also had been prepared by adjusting different rare earth
precursors. Further studies revealed that besides the reaction conditions and the additive amount of
complex agents, the morphologies of the as-synthesised lanthanide oxalates were also determined by
the rare earth ions. The Sm2(C2O4)3ꢀ10H2O and Sm2O3 samples exhibited different photoluminescence
spectra, which was relevant to Sm3+ energy level structure of 4f electrons. The method may be applied in
the synthesis of other lanthanide compounds, and the work could explore the potential optical materials.
ß 2011 Elsevier Ltd. All rights reserved.
Received 27 April 2011
Received in revised form 2 June 2011
Accepted 16 June 2011
Available online 23 June 2011
Keywords:
A. Inorganic compounds
B. Chemical synthesis
C. Electron microscopy
C. X-ray diffraction
D. Optical properties
1. Introduction
the molar ratio of Na3Cit to Sm3+ had been carried out to investigate
their influence on the flower-like Sm2(C2O4)3ꢀ10H2O micro-parti-
In current years, the precise design and synthesis of lanthanide
compound micro/nanostructure materials with well-defined
morphologies and tunable sizes remain the research hot, because
the chemical and physical properties of functional lanthanide
compounds are fundamentally related to their size, morphology,
and dimensionality [1–5]. Particularly, as an important lanthanide
compounds, lanthanide oxalates were widely used in rare earth
purification industry [6], or as the precursor to prepare lanthanide
oxides [7]. There has been an increasing number of reports on the
precise design and synthesis of rare earth oxalates including
dysprosium gadolinium oxalate [8], neodymium praseodymium
oxalate [9], lanthanide calcium oxalate [10] and lanthanide metal-
organic frameworks [11] and Ln2(C2O4)3ꢀ10H2O (Ln = La, Ce, Pr, Nd,
Sm, Eu, Gd, Dy, Y and Yb) by various synthesis methods, such as
hydrothermal synthesis without complex agent, sol–gel method,
emulsion liquid membrane (ELM, water-in-oil- in-water emulsion)
system, etc. [12–18].
cles. In addition, more and more attentions have been focused on the
study of Sm3+ activated inorganic materials for application in
luminescent materials because of its electron transitions within the
4f shell. However, to the best of our knowledge, no studies havebeen
conducted on the luminescence performance of pure
Sm2(C2O4)3ꢀ10H2O crystal. Thus the photoluminescence properties
of the pure Sm2(C2O4)3ꢀ10H2O and Sm2O3 obtained by calcining
Sm2(C2O4)3ꢀ10H2O crystal, were investigated in the paper.
2. Experimental
2.1. Materials
All reagents (samarium oxide, gadolinium oxide, dysprosium
oxide, lutetium oxide, yttrium oxide, oxalic acid, trisodium citrate
(Na3Cit), ethylene diamine tetraacetic acid (EDTA)) used were of
analytical purity, obtained from Shanghai Chemical Reagent Ltd.
Co. of China and used without further purification.
Herein, a simple and mild complex agent assisted precipitation
method at room temperature was proposed for the synthesis of
Ln2(C2O4)3ꢀnH2O (Ln = Sm, Gd, Dy, Lu, Y) micro-particles, in which
trisodium citrate (Na3Cit) was chosen as the complex agent. A series
of experiments for the reaction temperature, the reaction time and
2.2. Synthesis of Sm2(C2O4)3ꢀ10H2O
LnCl3 (Ln = Sm, Gd, Dy, Lu and Y) aqueous solution was obtained
by dissolving Ln2O3 (99.99%) (Ln = Sm, Gd, Dy, Lu and Y) in 1 mol/L
HCl solution and dried by heating to produce LnCl3 crystal. In a
typical procedure, 0.5 mmol Sm2O3 was dissolved in 5.0 mL of
1 mol/L HCl and dried by heating. The reactant was dissolved in
* Corresponding author. Tel.: +86 551 3492145; fax: +86 551 3492083.
E-mail address: zyj@ustc.edu.cn (Y. Zhang).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.06.029

H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
1547
30 mL of deionized water and then 1.2 mmol Na3Cit was added to
the solution with stirring. The mixed aqueous solution was stirred
at room temperature for 15 min to form samarium citrate
complexes, then 1.5 mmol H2C2O4ꢀ2H2O was introduced to the
samarium citrate complexes. After agitation for 10 min, the as-
obtained mixture was transferred into a 50-mL Teflon liner and
some distilled water was added up to 80% of the total volume. The
autoclave was sealed for 24 h at room temperature. The obtained
precipitate was collected after being washed several times with
deionized water and dried at 60 8C in air. The product was finally
obtained. A similar process was applied to prepare Ln(C2O4)3ꢀnH2O
(Ln = Gd, Dy, Lu and Y) samples by using a proper amount of
Gd(Cl)3, Dy(Cl)3, Lu(Cl)3, and Y(Cl)3 solutions instead of Sm(Cl)3 at
the initial stage as described above. Samples with other parameters
were prepared in a similar way.
2.3. Synthesis of Sm2O3
Sm2O3 was calcined from Sm2(C2O4)3ꢀ10H2O at 850 8C for 6 h.
2.4. Characterization
Fig. 1. XRD patterns of the Ln2(C2O4)3ꢀnH2O obtained at room temperature for 24 h
with the molar ratio of Na3Cit to Ln3+ of 1.2:1: (a) Lu; (b) Dy; (c) Y; (d) Gd; (e) Sm.
Powder X-ray diffraction (XRD) was carried out with a Japan
Rigaku D/max rA X-ray diffractometer equipped with graphite-
(Fig. 2b and c) [19,20]. No characteristic peaks of other metals are
detected. The O 1s spectrum of the sample demonstrates a O2ꢁ
peak at around 530.31 eV [21] (Fig. 2d).
monochromatized
high-intensity
Cu
K
a
radiation
range
(
l
= 0.15478 nm). The scanning rate was 0.058 sꢁ1 in the 2
u
To confirm the formation of the Sm2(C2O4)3ꢀ10H2O crystal
obtained at room temperature for 24 h with the molar ratio of
Na3Cit to Sm3+ of 1.2:1, FTIR spectroscopy was performed in the
wave number range from 200 to 4000 cmꢁ1, as illustrated in Fig. 3.
A strong absorption band at around 805 cmꢁ1 can be attributed to
the absorption of Sm–O [22] and a weak band at 486 cmꢁ1
represents the CO2 wagging mode [9], respectively. There is water
of crystallization in the crystal, which is evidenced by the intense
band at around 3375 cmꢁ1 [23]. The absorption bands located at
about 3375 and 1321 cmꢁ1 can be corresponded to the bending
mode of water and asymmetric stretch of CO2 [12,24].
from 108 to 708. The X-ray photoelectron spectroscopy (XPS)
analysis was performed on an ESCALAB MK II X-ray photoelectron
spectrometer, using Mg Kr radiation as the excitation source. The
Fourier transform infrared (FTIR) spectroscopic study was per-
formed on a MAGNA-IR 750 FTIR spectrometer. The field-emission
scanning electron microscopy (FESEM) images were obtained on a
JEOL-6300F field-emission scanning electron microscope with an
accelerating voltage of 15 kV. The thermogravimetry–differential
thermal analysis (TG–DTA) was conducted on a Rigaku Standard
Model thermal analyzer (in air atmosphere, flow rate:
90 mL minꢁ1; heat rate: 10 8C minꢁ1). The photoluminescence
(PL) was carried out using a Jobin Yvon Fluorolog-3-TAU steady-
state/lifetime spectrofluorometer.
The morphology of Sm2(C2O4)3ꢀ10H2O crystal was investigated
by FESEM. The higher magnification FESEM images illustrated the
flower-like Sm2(C2O4)3ꢀ10H2O, with the diameter about 5
mm, was
obtained through the simple precipitation process at room
temperature (Fig. 4a and b). It could be clearly seen the flower-
like Sm2(C2O4)3ꢀ10H2O was self-assembled by many thick and
smooth flakes from Fig. 4a, which should be beneficial to reduce
the surface area and surface energy of the product.
3. Results and discussion
3.1. Characterization, morphology and the growth mechanism of
Sm2(C2O4)3ꢀ10H2O crystal
It is well-known that the crystal morphology has important
effect on its physical and chemical properties. The morphologi-
cal uniformity of the product is found to be highly correlative
with the reaction temperature, reaction time and the additive
amount of the complex agent [25]. To control the morphology of
Sm2(C2O4)3ꢀ10H2O architectures, the influence of reaction
temperature, reaction time and the molar ratio of Na3Cit to
Sm3+ on the morphologies of the products was investigated by
FESEM.
The overall crystallinity and purity of the Ln2(C2O4)3ꢀnH2O
samples obtained at room temperature for 24 h with the molar
ratio of Na3Cit to Ln3+ of 1.2:1 were characterized by X-ray powder
diffraction as shown in Fig. 1. All the XRD patterns of the as-
prepared products can be indexed to anorthic Lu2(C2O4)3ꢀ6H2O
(space
group:P1,
JCPDS
card
49-1244),
monoclinic
Dy2(C2O4)3ꢀ10H2O (space group:P21/c, JCPDS card 21-0315),
monoclinic Y2(C2O4)3ꢀ10H2O (space group:P21/c, JCPDS card 33-
1460), monoclinic Gd2(C2O4)3ꢀ10H2O (space group:P21/c, JCPDS
card 20-0411) and monoclinic Sm2(C2O4)3ꢀ10H2O (space
group:P21/c, JCPDS card 20-1021) from Fig. 1a–e, respectively.
No XRD peaks for single metal oxides Ln2O3 in the lanthanide
oxalates were detected, which indicated the pure lanthanide
oxalate crystals can be achieved by this method.
The Sm2(C2O4)3ꢀ10H2O crystal obtained at room temperature
for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1 was also
characterized by XPS for the evaluation of its composition on the
particle surface (Fig. 2). The binding energies obtained in the XPS
analysis are corrected for specimen charging by referencing the C
1s orbital to 284.17 eV. The Sm 4d5/2, 4p3/2 and 3d5/2 peaks
centered at 132.54 eV, 249.44 eV and 1083.21 eV, respectively
The FESEM images indicated the reaction time and reaction
temperature had a great role on the flower-like Sm2(C2O4)3ꢀ10H2O
(Fig. 5). The reaction for too short or too long a time was
unfavorable
for
the
formation
of
the
flower-like
Sm2(C2O4)3ꢀ10H2O. The flower-like Sm2(C2O4)3ꢀ10H2O was not
well developed when the reaction time was 12 h, indicating that
the self-assembling process was incomplete (Fig. 5a). The regular
flower-like Sm2(C2O4)3ꢀ10H2O architecture was obtained when the
reaction time was prolonged to 24 h (Fig. 5b). As the reaction time
further increased to 30 h (Fig. 5c) or the temperature increased to
60 8C (Fig. 5d), the flower-like architecture fell to pieces.
The morphologies of the products obtained with different
dosages of Na3Cit at room temperature for 24 h illuminated the

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H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
Fig. 2. XPS spectra of the as-synthesized Sm2(C2O4)3ꢀ10H2O: (a) survey spectrum; (b) Sm 4d and Sm 4p region; (c) Sm 3d region; (d) O 1s region.
molar ratio of Na3Cit to Sm3+ had crucial effect on the formation of
the flower-like Sm2(C2O4)3ꢀ10H2O. For a lack of Na3Cit (with the
molar ratio of Na3Cit to Sm3+ 0:1 or 0.6:1), the samples were
composed of some assembled micro-particles but not regular
flower-like structure (Fig. 6a and b). However, when the molar
ratio was increased to 1.8:1, all the as-prepared flower-like
Sm2(C2O4)3ꢀ10H2O crystal was inclined to fragmentation and the
flake became larger (Fig. 6d). Only when the molar ratio was 1.2:1,
could the uniform flower-like Sm2(C2O4)3ꢀ10H2O crystal be
observed clearly (Fig. 6c).
be fabricated at room temperature for 24 h with the molar ratio of
Na3Cit to Sm3+ of 1.2:1.
A
possible process for the self-assembled flower-like
Sm2(C2O4)3ꢀ10H2O is shown in Fig. 7. In the paper, it is found that
the Cit3ꢁ plays
key role in the formation of flower-like
a
Sm2(C2O4)3ꢀ10H2O. At first, the Cit3ꢁ is a strong chelating agent
with three carboxylate groups for metal ions [26]. In the solution,
Sm3+ ionsareprotectedbyformingSm (Cit)23ꢁ units(Eq. (1)), greatly
decreasing the free Sm3+ ion concentration. When oxalic acid
solution is added, Sm3+ would be released gradually from the
Sm(Cit)2 units and the free Sm3+ ions can react with the vicinal
3ꢁ
Onthebasisoftheaboveexperimentresults, itcan begeneralized
a conclusion that the uniform flower-like Sm2(C2O4)3ꢀ10H2O could
2ꢁ
C2O4 ions to form numerous tiny Sm2(C2O4)3 crystalline nuclei
(Eq. (2)). During the subsequent crystal growth stage, these tiny
nuclei grow to form particles. According to LaMer’s model, the
formation of such complexes could control the concentration of free
Sm3+ ions in the solution, and thus help to control the nucleation and
growth rate of the Sm2(C2O4)3 particles. The relatively slow
generation rate of particles is favorable for the subsequent growth
of hierarchical structures [27]. The kinetic control leads to the
formation of the flakes. When the reaction time was increased, these
initialflakes continued togrow ina self-assembled growthstyle, and
finally the flower-like structure was formed.
3ꢁ
Sm3þ þ 2Cit3ꢁ Ð SmðCitÞ2
(1)
(2)
Sm3þ þ C2O42ꢁ þ 10H2O Ð Sm2ðC2O4Þ3ꢀ10H2O
To further investigate the influence of complex agents on the
Sm2(C2O4)3ꢀ10H2O morphology, the products with different
complex agents were obtained. The FESEM image showed many
large flakes appeared in the product when EDTA is used as the
complex agent (Fig. 8b).
Fig. 3. The FTIR spectrum of Sm2(C2O4)3ꢀ10H2O sample.

H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
1549
Fig. 4. FESEM images of the as-synthesized product obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.
The different impacts of Na3Cit and EDTA on the product
morphology have some connection with their different chelating
constants and coordination modes with Sm3+. On the one hand, the
chelating constant for Cit3ꢁ is smaller than that for EDTA [26,28].
The smaller the chelating constant, the faster the nucleation rate,
which would influence the growth rate of the Sm2(C2O4)3ꢀ10H2O
particles. On the other hand, the coordination modes between the
complex agents with Sm3+ are clearly different [29]. A citrate
molecule (COOHCH2–COHCOOH–CH2COOH) has four binding
sites, including three carboxylic groups and one hydroxyl group,
the theoretical value of 24.17% according to Eq. (3). The second
weight loss (28.725 wt%) is consistent with the theoretical value of
29.00%according toEq. (4). The processisfrom403 to850 8C and has
a strong endothermic peak at 421 8C in the DTA curve due to the
decomposition of Sm2(C2O4)3 (Eq. (4)), which can also be proved by
the XRD in Fig. 9b. The XRD pattern of Fig. 9b can be indexed to the
cubic Sm2O3 (space group:I231, JCPDS card 74-1989).
Sm2ðC2O4Þ3ꢀ10H2O ! Sm2ðC2O4Þ3 þ 10H2O
Sm2ðC2O4Þ3 ! Sm2O3 þ 3CO þ 3CO2
(3)
(4)
whereas
an
EDTA
molecule
((HOOCCH2)2–NCH2CH2N–
(CH2COOH)2) has six binding sites, including four carboxylic
groups and two single pairs of electrons on the nitrogen atom. The
difference might lead to the different morphology because of its
influence on growth orientation of the crystals.
3.2. Synthesis of Gd2(C2O4)3ꢀ10H2O, Dy2(C2O4)3ꢀ10H2O,
Lu2(C2O4)3ꢀ6H2O and Y2(C2O4)3ꢀ10H2O
The thermal behavior of the flower-like Sm2(C2O4)3ꢀ10H2O was
investigated with TG–DTA (Fig. 9a). TG curve shows that the total
weight loss is 53.115 wt% and can divide into two processes. The
first process from 40 to 403 8C belongs to the evaporation of the
crystal water. The weight loss of 24.39 wt% is in agreement with
Monoclinic
Gd2(C2O4)3ꢀ10H2O,
Dy2(C2O4)3ꢀ10H2O,
Y2(C2O4)3ꢀ10H2O and anorthic Lu2(C2O4)3ꢀ6H2O also had been
successfully synthesized by the above method (Fig. 10). It is well
known that rare earth elements are frequently divided into two
groups based on the atomic weight and chemical properties. The
Fig. 5. FESEM images of the samples obtained (a–c) at room temperature with the molar ratio of Na3Cit to Sm3+ of 1.2:1 for different times: (a) 12 h; (b) 24 h; (c) 36 h; (d) at
60 8C for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.

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H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
Fig. 6. FESEM images of the samples obtained at room temperature for 24 h with different molar ratio of Na3Cit to Sm3+: (a) 0:1; (b) 0.6:1; (c) 1.2:1; (d) 1.8:1.
‘‘light’’ rare earths consist of the elements with atomic number 57–
63, such as Sm. The ‘‘heavy’’ rare earths consist of Sc, Y and the
elements 64–71, such as Dy and Lu. Meanwhile, the element 64 Gd
possesses the similar chemical behavior with the ‘‘light’’ rare
earths because they are very close. It is interesting to find the self-
assembled architecture of the Gd2(C2O4)3ꢀ10H2O crystal is similar
to that of the Sm2(C2O4)3ꢀ10H2O (Fig. 10a) and only large irregular
nubby-like particles for Dy2(C2O4)3ꢀ10H2O, Lu2(C2O4)3ꢀ6H2O and
Y2(C2O4)3ꢀ10H2O are obtained (Fig. 10b–d), which might result
from the differences of their ionic radius. These results show that
besides the reaction conditions and the additive amount of
complex agent, the morphology evolution of lanthanide oxalates
is also determined by the rare earth ions.
3.3. Photoluminescence property
Many studies have been focused on the luminescent application
of Sm3+ activated inorganic materials such as YVO4:Sm3+ and
LuVO4:Sm3+ [26,30], but the papers on the intrinsic photolumi-
nescence property of pure Sm2(C2O4)3ꢀ10H2O material are not
Fig. 7. Schematic illustration for the possible formation mechanism of flower-like
reported up to now. In this work, the photoluminescence property
Sm2(C2O4)3ꢀ10H2O crystal.
of the as-prepared Sm2(C2O4)3ꢀ10H2O and Sm2O3 which was
obtained by calcining Sm2(C2O4)3ꢀ10H2O at 850 8C for 6 h was
Fig. 8. The FESEM images of the samples obtained room temperature for 24 h with the molar ratio of different complex agents to Sm3+ of 1.2:1: (a) Na3Cit; (b) EDTA.

H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
1551
Fig. 9. (a) TG–DTA curve of the flower-like Sm2(C2O4)3ꢀ10H2O; (b) XRD pattern of the Sm2O3.
Fig. 10. The FESEM images of the samples with the molar ratio of Na3Cit to Ln3+ (Ln = Gd, Dy, Lu and Y) of 1.2:1 for 24 h at room temperature: (a) Gd2(C2O4)3ꢀ10H2O; (b)
Dy2(C2O4)3ꢀ10H2O; (c) Lu2(C2O4)3ꢀ6H2O; (d) Y2(C2O4)3ꢀ10H2O.
Fig. 11. (a) Excitation spectrum for Sm2(C2O4)3ꢀ10H2O, monitoring the emission at 645 nm; (b) emission spectra for Sm2(C2O4)3ꢀ10H2O and Sm2O3 under direct excitation of
402 nm; (C) positions of the Sm2(C2O4)3ꢀ10H2O (A), Sm2O3 (B) in the chromaticity diagram.

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H. He et al. / Materials Research Bulletin 46 (2011) 1546–1552
studied. The excitation spectrum of the flower-like
Sm2(C2O4)3ꢀ10H2O sample consists of a strong absorption band
with a maximum at 402 nm and lots of weak bands monitoring
with 642 nm emission of Sm3+ (Fig. 11a). The room-temperature
emission spectrum of the flower-like Sm2(C2O4)3ꢀ10H2O exhibits
the typical red-orange emissions at 561, 596, 640 and 703 nm
under 402 nm excitation, which are ascribed to the transitions 4G5/
electrons. This work may present a way for the morphology-
controlled synthesis of other inorganic materials.
Acknowledgments
This research is supported by the Structure Research Laboratory
of CAS. We thank S.Q. Fu (University of Science and Technology of
China) for the helpful discussion.
6
6
6
4
6
2 ! H5/2
,
4G5/2 ! H7/2
,
4G5/2 ! H9/2 and G5/2 ! H11/2, respec-
tively (black line of Fig. 11b), while the Sm2O3 powder shows
obviously different PL spectral emission (red line of Fig. 11b). (For
interpretation of the references to colour in this figure legend, the
reader is referred to the web version of the article.) The emission
spectrum of Sm2O3 displays a broad emission band in the
wavelength range from 550 to 705 nm. It is well known that the
energy level structure of 4f electrons is the key factor controlling
the PL spectrum of rare earth ions, while the energy level structure
depends on the crystal field [31]. The different PL spectral emission
may be firstly explained by the different crystal field of the
monoclinic Sm2(C2O4)3ꢀ10H2O and cube Sm2O3. Secondly, the
crystal field, namely, the energy level structure of 4f electrons in
microcrystal specimen is also influenced by the defects and the
reconstruction of atomic arrangement on the surface. Besides
them, reabsorption effect has a certain influence on the emission,
which occurs as dips in the broad emission feature and makes PL
spectral emission of Sm2O3 more unconspicuous [32,33]. As
illustrated in Fig. 11c, the emission colour of the above samples
can be expressed by the CIE (Commission International de
I’Eclairage 1931 chromaticity) coordinates. The flower-like
Sm2(C2O4)3ꢀ10H2O and Sm2O3 emit red-orange and violet light
and their chromaticity coordinates are x = 0.4905, y = 0.3792 and
x = 0.3943, y = 0.2609, respectively.
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4. Conclusions
In summary, a convenient and facile complex agent assisted
precipitation method has been utilized for the preparation of
Ln(C2O4)3ꢀnH2O (Ln = Sm, Gd, Dy, Lu, Yb). The condition experi-
ment results show room temperature, 24 h and the molar ratio of
Na3Cit to Sm3+ of 1.2:1 are the preferable experiment parameters
to synthesize the flower-like Sm2(C2O4)3ꢀ10H2O. Further
researches show that besides the reaction conditions and the
additive amount of the complex agent, the morphology of the
lanthanide oxalates is also determined by the rare earth ions. The
optical properties of Sm2(C2O4)3ꢀ10H2O and Sm2O3 samples are
distinct, which is relevant to Sm3+ energy level structure of 4f
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