Characterization and thermal behavior of amorphous rare earth phosphates

Article abstract of DOI:10.1016/j.jallcom.2003.11.069

Amorphous cerium and terbium pyrophosphates were synthesized by precipitation method at 353K. The characterization of these amorphous phosphates was carried out by using X-ray powder diffraction (XRD), thermal gravimetric and differential thermal analysis (TG-DTA), Fourier-transformed infrared spectroscopy (FT-IR), and Raman spectroscopy, especially focusing on the thermal behavior dependence on the calcination temperatures. The amorphous state of the as-prepared cerium phosphate was stable up to 723K, and it crystallized at 773K to form a mixture of cerium pyrophosphate (CeP₂O₇) and cerium orthophosphate (CePO₄). Heating at higher temperatures accelerates the phase change from pyrophosphate to orthophosphate accompanying the formation of metaphosphate (CeP₃O₉), and above 1173K, only a single phase of CePO₄ was obtained. In the case of terbium phosphate, crystallization of amorphous phase occurred at 973K to produce a mixture of TbPO₄ and Tb₂P₄O₁₃. The Tb₂P₄O₁₃ phase decomposed into TbP ₃O₉ and TbPO₄ at 1673K, and these compounds remained even after the calcination at 1873K.

Full text of DOI:10.1016/j.jallcom.2003.11.069

Journal of Alloys and Compounds 374 (2004) 84–88
Characterization and thermal behavior of amorphous
rare earth phosphates
Hidekazu Hiraia, Toshiyuki Masuia, Nobuhito Imanakaa,∗, Gin-ya Adachib
a
Department of Applied Chemistry, Faculty of Engineering and Handai Frontier Research Center, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Juri Institute for Environmental Science and Chemistry, College of Analytical Chemistry, 2-1-8 Temma, Kita-ku, Osaka 530-0043, Japan
b
Abstract
Amorphousceriumandterbiumpyrophosphatesweresynthesizedbyprecipitationmethodat353 K. Thecharacterizationoftheseamorphous
phosphates was carried out by using X-ray powder diffraction (XRD), thermal gravimetric and differential thermal analysis (TG–DTA),
Fourier-transformed infrared spectroscopy (FT-IR), and Raman spectroscopy, especially focusing on the thermal behavior dependence on the
calcination temperatures. The amorphous state of the as-prepared cerium phosphate was stable up to 723 K, and it crystallized at 773 K to
form a mixture of cerium pyrophosphate (CeP2O7) and cerium orthophosphate (CePO4). Heating at higher temperatures accelerates the phase
change from pyrophosphate to orthophosphate accompanying the formation of metaphosphate (CeP3O9), and above 1173 K, only a single
phase of CePO4 was obtained. In the case of terbium phosphate, crystallization of amorphous phase occurred at 973 K to produce a mixture
of TbPO4 and Tb2P4O13. The Tb2P4O13 phase decomposed into TbP3O9 and TbPO4 at 1673 K, and these compounds remained even after the
calcination at 1873 K.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Rare earth phosphates; Amorphous; X-ray diffraction; Thermal analysis; FT-IR; Raman
1. Introduction
powder diffraction (XRD), thermal gravimetric and dif-
ferential thermal analysis (TG–DTA), Fourier-transformed
infrared spectroscopy (FT-IR), and Raman spectroscopy.
In recent years, much attention has been paid to the rare
earth phosphates because of their potential applications for
optical materials, including laser [1], phosphors [2], and
more recently, anti-UV materials [3]. Although the latter
materials utilized their amorphous features for ultraviolet
shielding, there were few basic and systematic studies on the
thermal behavior of amorphous rare earth phosphates [4,5].
In the series of rare earth elements, cerium and terbium
can form their tetravalent ions as well as trivalent ones, and
it is very interesting to identify the most stable phases at
the lowest temperature which causes crystallization in these
phosphates. In addition, characterization of the thermal be-
havior of these amorphous phosphates, especially on the
structural changes with heating, is very important for several
applications.
2. Experimental procedure
2.1. Preparation of amorphous cerium phosphate
Cerium(IV) sulfate tetrahydrate was dissolved in deioniz-
ed water and the concentration was adjusted to 1.0 mol dm−3
.
A 0.1 mol dm−3 of sodium pyrophosphate aqueous solu-
tion was used as a precipitant. These solutions were mixed
and the precipitate was agitated with magnetic stirrer for
30 min at 353 K. The particles obtained were separated by
centrifuging, washed with deionized water five times, and
dried at 353 K for 24 h. Then the powders were calcined in
air at 473, 723, 773, 823, 873, 923, 973, 1023, 1173, and
In this study, amorphous cerium and terbium phosphates
were prepared by precipitation method, respectively, to
investigate their thermal behavior in details using X-ray
1373 K for 5 h at a heating rate of 100 K h−1
.
2.2. Preparation of amorphous terbium phosphate
∗
Terbium(III) sulfate octahydrate was dissolved in
1.0 mol dm−3 sulfuric acid and the concentration of Tb3+
Corresponding author.
E-mail address: imanaka@chem.eng.osaka-u.ac.jp (N. Imanaka).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.11.069

H. Hirai et al. / Journal of Alloys and Compounds 374 (2004) 84–88
85
ion was adjusted to 1.0 mol dm−3. The solution was dropped
20
15
10
5
10.6
10.4
10.2
10
into a 0.1 mol dm−3 of sodium pyrophosphate aqueous so-
lution, and then the final pH value was adjusted to 5 with
a 3.0 mol dm−3 of ammonium hydroxide solution. After
stirring for 30 min at 353 K, the particles were separated by
centrifuging, washed with deionized water and acetone, and
dried at room temperature for 24 h. The powders obtained
were calcined in air at 873, 973, 1073, 1373, 1423, 1473,
881 K
900
1146 K
0
9.8
500
700
1100
1673, and 1873 K for 5 h at a heating rate of 100 K h−1
.
Temperature / K
Fig. 2. TG–DTA spectra of cerium phosphate dried at 473 K.
2.3. Characterization of the amorphous cerium and
terbium phosphates
the crystalline cerium pyrophosphate (CeP2O7) and cerium
orthophosphate (CePO4) was formed. Structural change of
the cerium pyrophosphate into cerium orthophosphate was
promoted by heating at higher temperatures, accompanying
with the formation of cerium metaphosphate (CeP3O9). Fi-
nally, a single phase of CePO4 was obtained at 1373 K.
TG–DTA spectra of the amorphous CeP2O7 dehydrated
at 473 K are shown in Fig. 2. Both the weight loss and an
exothermic peak were observed at 881 K. This exothermic
weight loss corresponded to the change from CeP2O7 to
both CeP3O9 and CePO4 with the evolution of oxygen gas
[4]. Another exothermic broad peak in the DTA curve and
the corresponding weight loss were also observed at 1146 K,
which were attributed to the change from CeP3O9 to CePO4
[4].
The compositions of the samples were determined by an
X-ray fluorescent spectrometer (Rigaku ZEX-100e). The
XRD data were obtained on an X-ray diffractmeter (Rigaku
Multiflex) with Cu K␣ radiation. FT-IR (JASCO FTIR-430)
and Raman spectra (Kaiser Optical Systems, HoloProbe)
were recorded for both samples. TG–DTA analysis was
carried out at a heating rate of 10 K min−1 (Shimadzu
DTG-60H).
3. Results and discussion
3.1. Thermal behavior of the amorphous
cerium phosphate
The FT-IR spectra of the as-prepared and calcined cerium
phosphate samples are illustrated in Fig. 3. In the FT-IR
spectra of amorphous cerium phosphate [5], four distinct
The XRD patterns of the as-prepared and calcined cerium
phosphate samples were summarized in Fig. 1. The sam-
ples heated at low temperatures below 723 K were amor-
phous. After the calcination at 753 K, the crystallization of
the amorphous cerium phosphate occurred and a mixture of
bands were observed at 1070, 925, 622, and 522 cm−1
.
The main band at 1070 cm−1 was due to the asymmetric
CeP2O7 CeP3O9 CePO4
(k)
(j)
(i)
(h)
(g)
(f)
(e)
(d)
(c)
(b)
(a)
10
20
30
2
40
50
60
70
q
/ degree
Fig. 1. XRD patterns of cerium phosphate calcined at various temperatures:
as-prepared (a), calcined at 473 K (b), 723 K (c), 773 K (d), 823 K (e),
873 K (f), 923 K (g), 973 K (h), 1023 K (i), 1173 K (j), and 1373 K (k).
Fig. 3. FT-IR spectra of cerium phosphate calcined at various temperatures:
as-prepared (a), calcined at 473 K (b), 723 K (c), 773 K (d), 823 K (e),
873 K (f), 923 K (g), 973 K (h), 1023 K (i), 1173 K (j), and 1373 K (k).

86
H. Hirai et al. / Journal of Alloys and Compounds 374 (2004) 84–88
oxygen in P–O–P unit of pyrophosphate [11]. The 971 cm−1
band was attributed to symmetric stretching of PO4, and
the bands between 300 and 500 cm−1 were due to the de-
formation of PO3 group of orthophosphate. After the calci-
nations above 823 K, the peaks of pyrophosphate gradually
disappeared and new bands attributed to metaphosphate and
orthophosphate were produced. The peaks of former were
observed at 1265, 1178, and 694 cm−1 and those of the latter
were at 1074, 1056, 1028, 995, and 971 cm−1, respectively
[12,13]. The bands at 1265 and 1178 cm−1 were assigned to
symmetric and asymmetric stretching of PO2 group, and the
694 cm−1 peak was corresponded to symmetric motion of
the bridging oxygen in P–O–P unit of metaphosphate. After
calcination at 1373 K, the only peaks corresponding to or-
thophosphate were observed. The band at 971 cm−1 was due
to the symmetric stretching of the PO4 unit and those at 995
and 1028 cm−1 were attributed to the asymmetric stretching
of the PO4 group. The bands at 1056 and 1074 cm−1 were
from the splitting of the asymmetric stretching mode of the
PO4 group [13].
stretching of PO3 terminal groups from pyrophosphate. The
925 cm−1 band could be assigned to symmetric stretch-
ing vibrations of PO3 groups, while two bands at 622 and
522 cm−1 were regarded as deformation modes of PO2 and
PO3 groups [6]. As the calcination temperature increased
from 723 to 773 K, the band observed at 925 cm−1 shifted
to 950 cm−1, and a new small band appeared at 993 cm−1
.
The above shift corresponded to the crystallization of py-
rophosphate and the additional peak was attributed to the
formation of orthophosphate. In the range of the calcination
temperatures between 823 and 1023 K, the patterns both
of metaphosphate and orthophosphate were recognized in
the FT-IR spectra. The bands from metaphosphate were
observed at 1270, 1158, 954, 774, and 470 cm−1 [6,7] and
those from orthophosphate were assigned at 1093, 1058,
993, 622, 570, and 532 cm−1 [8,9], respectively. The sam-
ple calcined at 1173 K almost indicated the only pattern of
orthophosphate.
Fig. 4 shows Raman spectra of the as-prepared and the
calcined cerium phosphate samples. In the Raman spectra
of the amorphous materials, four bands were observed at
1007, 627, 578, and 498 cm−1. The principal broad band at
1007 cm−1 was assigned to the symmetric stretching of PO3
terminal groups from pyrophosphate [10]. The bands at 627
and 498 cm−1 corresponded to the asymmetric and symmet-
ric deformation modes of PO4, and the one at 578 cm−1 was
attributed to the deformation mode of PO3 group. Calcina-
tions at higher temperatures above 773 K promoted the main
peak shift from 1007 to 1037 cm−1, and at 823 K, several
new peaks appeared. The 940 and 780 cm−1 peaks were due
to the asymmetric and symmetric motions of the bridging
3.2. Thermal behavior of the amorphous
terbium phosphate
Fig. 5 shows the XRD patterns of the as-prepared and
the calcined terbium phosphate samples. In contrast to the
results of cerium phosphate, crystallization of amorphous
terbium phosphate occurred at higher 973 K to produce a
mixture of crystalline terbium orthophosphate (TbPO4) and
tetraphosphate (Tb2P4O13). The tetraphosphate phase de-
composed at 1673 K into a mixture of TbP3O9 and TbPO4.
Fig. 4. Raman spectra of cerium phosphate calcined at various tempera-
tures: as-prepared (a), calcined at 473 K (b), 723 K (c), 773 K (d), 823 K
(e), 873 K (f), 923 K (g), 973 K (h), 1023 K (i), 1173 K (j), and 1373 K (k).
Fig. 5. XRD patterns of terbium phosphate calcined at various tempera-
tures: as-prepared (a), calcined at 873 K (b), 973 K (c), 1073 K (d), 1373 K
(e), 1423 K (f), 1473 K (g), 1673 K (h), and 1873 K (i).

H. Hirai et al. / Journal of Alloys and Compounds 374 (2004) 84–88
87
40
30
20
10
0
16
14
1643 K
12
10
600
1000
1400
Temperature / K
Fig. 6. TG–DTA spectra of terbium phosphate dried at 573 K.
Even after the calcination at 1873 K, the two-phase mixture
remained and no single phase was obtained.
TG–DTA spectra of the amorphous terbium phosphate
dried at 573 K are presented in Fig. 6. In the DTA curve,
an exothermic peak was observed at 1643 K, which was
corresponding to the decomposition of the tetraphosphate
described above. The accompanying weight loss observed
around 1600 K came from the volatilization of phosphorus
oxide produced by the decomposition of the tetraphosphate.
Fig. 7 depicts the FT-IR spectra of the as-prepared and the
calcined terbium phosphate samples. In the amorphous ter-
bium phosphate, several broad bands were observed. As the
calcination temperature increased, these broad bands sharp-
ened accompanying the shift to higher wave numbers. The
bands observed at 748 and 923 cm−1 in the amorphous sam-
ple were assigned to symmetric and asymmetric stretching
of P–O–P group, which shifted to 760 and 957 cm−1 after
the heating at 973 K. The band at 557 cm−1 was the defor-
mation mode of PO3 in pyrophosphate group. The increase
in the calcination temperature induced the splitting of this
band into several ones because of the deformation of the
PO2 and PO4 groups in tetra and orthophosphate, respec-
tively. Consequently, the peaks based on tetraphosphate were
Fig. 8. Raman spectra of terbium phosphate calcined at various tem-
peratures: as-prepared (a), calcined at 873 K (b), 973 K (c), 1073 K (d),
1373 K (e), 1423 K (f), 1473 K (g), 1673 K (h), and 1873 K (i).
observed at 1313, 1258, 1182, 1102, 1034, 958, 795, 757,
684, 636, 560, 502, and 461 cm−1 [14], and those from or-
thophosphate were assigned at 957, 634, and 524 cm−1 [9].
Above 1673 K, the tetraphosphate was disappeared and the
new bands from trimetaphosphate appeared at 1249, 1058,
and 1019 cm−1. These peaks corresponded to asymmetric
stretching of PO2, symmetric stretching of PO2, and asym-
metric stretching of P–O–P group, respectively [6,7].
The Raman spectra of terbium phosphate calcined at var-
ious temperatures are illustrated in Fig. 8. In amorphous
state, a considerable broad band was observed. In the sam-
ples calcined between 973 and 1473 K, the bands of TbPO4
were observed at 1050, 1016, 997, 636, 576, and 485 cm−1
[13], as well as those of Tb2P4O13 at 1232, 1182, 804,
and 689 cm−1 [15]. The band at 997 cm−1 corresponded to
the symmetric stretching and those observed at 1016 and
1050 cm−1 were associated with the asymmetric stretch-
ing of the PO4 group. The peaks at 1182 and 1232 cm−1
were assimilated to the symmetric and asymmetric of PO2
and those at 689 and 804 cm−1 came from the symmet-
ric and asymmetric of P–O–P bridges. After the calcination
above 1673 K, the bands for stretching of P–O–P bridges
and PO2 group in trimetaphosphate were observed at 683,
1204 and 1235 cm−1, which means the structural change
from tetraphosphate to trimetaphosphate. The bands at 1235
and 1204 cm−1 were corresponding to the symmetric and
asymmetric stretching of PO2 group [7,13] and the 694 cm−1
band was due to the symmetric stretching of P–O–P group
in trimetaphosphate.
3.3. Comparison of cerium and terbium phosphate
Fig. 7. FT-IR spectra of terbium phosphate calcined at various tempera-
tures: as-prepared (a), calcined at 873 K (b), 973 K (c), 1073 K (d), 1373 K
(e), 1423 K (f), 1473 K (g), 1673 K (h), and 1873 K (i).
In comparison with the thermal behavior between
cerium and terbium phosphate, two clear differences were

88
H. Hirai et al. / Journal of Alloys and Compounds 374 (2004) 84–88
recognized. First, the crystallization temperature of amor-
phous terbium phosphate was higher than that of cerium
phosphate. According to the results of Raman spectra
of these amorphous phosphates, larger value of the P–O
stretching wave number was observed in terbium phosphate
(1053 cm−1) than in cerium phosphate (1007 cm−1). This
indicates that stronger P–O bond is formed in the amor-
phous terbium phosphate, and therefore higher temperature
was required to rearrange atoms for crystallization.
The second difference is that orthophosphate single
phase was obtained only in cerium phosphate, while in the
case of terbium phosphate, a mixture of orthophosphate
and trimetaphosphate was obtained after the high temper-
ature calcinations. Since the standard formation enthalpy
of cerium orthophosphate (−1954 kJ mol−1) is lower than
that of terbium (−1925 kJ mol−1), it is considered that the
production of orthophosphate single phase is easier in the
case of cerium than terbium [16].
negative standard formation enthalpy than that of terbium
phosphate.
Acknowledgements
The present work was partially supported by a Grant-in-
Aid for Scientific Research Nos. 13555241 and 15550172
from The Ministry of Education, Science, Sports, and Cul-
ture, Japan. The authors sincerely thank Dr. Kuniaki Murase
and Prof. Dr. Yasuhiro Awakura (Kyoto University) for their
assistance with the Raman spectra measurements. This work
was supported by the Industrial Technology Research Grant
Program in 02 (Project No. 02A27004c) from the New En-
ergy and Industrial Technology Development Organization
(NEDO) based on funds provided by the Ministry of Econ-
omy, Trade and Industry, (METI) Japan.
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Amorphous cerium phosphate maintained its structure
up to 723 K and it crystallized at 773 K to form a mix-
ture of cerium pyrophosphate and orthophosphate, which
means that these phases are the most stable crystalline
phases at low temperatures. Heating at higher temper-
atures promoted the phase change from pyrophosphate
to orthophosphate with the formation of metaphosphate,
and only a single phase of CePO4 was obtained at above
1173 K.
Crystallization of amorphous terbium phosphate occurred
at 973 K to form a mixture of terbium orthophosphate
and tetraphosphate. The tetraphosphate phase decomposed
at 1673 K to produce a mixture of TbP3O9 and TbPO4.
The mixture of these compounds was stable even after the
calcination at 1873 K, and therefore no single phase was
obtained.
It has been recognized that amorphous cerium phosphate
is easier to crystallize than terbium phosphate due to the
difference of P–O bond strength in these phosphates. In ad-
dition, cerium phosphate tends to form a single phase of
orthophosphate at high temperatures because of the more
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