Modified Pechini synthesis of Na3Ce(PO4) 2and thermochemistry of its phase transition

Article abstract of DOI:10.1007/s10973-008-9129-5

Effect of the synthesis conditions of Pechini technique on crystallinity and purity of Na₃Ce(PO₄)₂compound was investigated. Nano-sized cerium-sodium phosphate obtained when EDTA was used as an additional chelating agent f

Full text of DOI:10.1007/s10973-008-9129-5

Journal of Thermal Analysis and Calorimetry, Vol. 93 (2008) 3, 689–692
MODIFIED PECHINI SYNTHESIS OF Na3Ce(PO4)2 AND THERMO-
CHEMISTRY OF ITS PHASE TRANSITION
Aleksandra Matraszek* and Irena SzczygieÓ
WrocÓaw University of Economics, Department of Inorganic Chemistry, ul. Komandorska 118/120, 53-345 WrocÓaw, Poland
Effect of the synthesis conditions of Pechini technique on crystallinity and purity of Na3Ce(PO4)2 compound was investigated.
Nano-sized cerium–sodium phosphate obtained when EDTA was used as an additional chelating agent for Ln3+. The total
enthalpy change of Na3Ce(PO4)2 phase transition was determined as 14.2 ±0.7 kJ mol–1 for sample synthesized by conventional
solid–solid reaction. The phase transition process was confirmed to occur at 1060°C or in temperature range 920–1060°C depend-
ing on thermal treatment of powders.
Keywords: binary phosphate, Pechini synthesis, phase transition
Introduction
process and investigated the influence of synthesis con-
ditions on the phase purity and crystallinity of
Na3Ce(PO4)2 compound. Ethylenediamine tetraacetic
acid (EDTA) as an additional chelating agent for Ce3+
has been proposed to decrease crystallite size. The phase
purity of obtained powders was examined by means of
XRD and FTIR methods. The IR spectroscopy was em-
ployed as complementary method to check XRD results.
Previous investigations on the system
Na3PO4–CePO4 showed, that parent phosphates at
molar ratio 1:1 form a compound with the formula
Several application fields of rare-earth phosphates
caused an increasing interest in research on physical and
chemical properties of compounds with the formula
MI3Ln(PO4 )2 (where MI denotes an alkali metal) [1–5].
Their chemical nature and particle size of synthesized
phosphates is highly dependent on the processing
method. The compounds are usually obtained at rela-
tively high temperatures by a solid state reaction [6] or
by growing from a flux of molten phosphates [3].
The hydrothermal crystallization has been proposed by
Xu [7], but there have been no up-to-date reports on
synthesis of nanometric powders of binary phosphates.
Actually, sol–gel methods are widely used to obtain
nano-sized crystals. The conventional sol–gel process
involves use of toxic and very expensive alkoxide pre-
cursors. An alternative approach is preparation of pow-
ders by the Pechini method [8], where inorganic salts
are used as precursors and citric acid acts as chelating
agent for metal-ion preventing phase separation before
the expected crystallization occurs. These chelates can
undergo polyesterification when heated in ethylene gly-
col to form a polymeric resin throughout which cations
are uniformly distributed. Further annealing of dried gel
to 800°C causes the forming of final product and remov-
ing of organic residues.
Na3Ce(PO4)2
[6].
The
low
temperature
b-Na3Ce(PO4)2 phase was described to have ortho-
rhombic structure [6, 7], that changes to glaserite-type
when heated [11, 12]. It was mentioned that a poly-
morphic transition of the compound occurs at 1060°C
or in temperature range of 920–1060°C, depending on
the thermal treatment of the synthesized samples
which was used. The investigated phosphate is stable
up to 1550°C and at this point it melts incongruently
depositing solid CePO4 and a liquid Na3PO4-rich [6].
The data on the thermochemistry of structural trans-
formations of binary phosphates is known only for
potassium-lanthanide compounds with the formula
K3Ln(PO4)2 (Ln=Ce–Sc) [3]. In presented paper the
enthalpy of structural change of Na3Ce(PO4)2 com-
pound was determined to complete the thermo-
chemical data on double orthophosphates.
The self-combustion synthesis of powders by the
auto-ignition process of citric acid is also known to
produce fine powders with nanometric crystallites
[9, 10]. The combustion process is an oxidation-re-
duction reaction in which NO–3 ions act as oxidants
for carboxyl groups (reductants).
Experimental
Samples of Na3Ce(PO4)2 composition were prepared us-
ing the stoichiometric amount of Ce(NO3)3·6H2O
(98.5%, POCh Gliwice), Na3PO4·12H2O (98%, POCh
In the presented paper we synthesized samples by
the Pechini method supported by an auto-combustion
*
Author for correspondence: aleksandra.matraszek@ue.wroc.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

MATRASZEK, SZCZYGIE˜
Kl
Gliwice) and NH4H2PO4 (99.5%, POCh Gliwice). The
D=
molar ratios of citric acid (R1), ethylene glycol (R2) and
nitric acid to the cerium precursor (R3) used for obtain-
ing the samples are listed in Table 1. The cerium(III) ni-
trate, citric acid (99.5%, Alfa Aesesar) and ethylene gly-
col (pure, POCh Gliwice) were dissolved in the
concentrated nitric acid (65%, POCh Gliwice), distilled
water (sample C) or 20 mL of saturated EDTA (99.0%,
Fluka) solution (samples A and B) under stirring. Sam-
ples were heated at 70°C under mixing until a viscous
gel obtained. Then Na3PO4·12H2O and NH4H2PO4 were
added under stirring. The heating was continued until a
white voluminous foam obtained, then temperature was
raised up to 120°C to dry the samples for 20 h. The cal-
cination of dried samples was performed in furnace pre-
heated to 480°C for 20 h. During this step an auto-com-
bustion process of samples at about 450°C was
observed, which caused release of large volume of gases
in a few minute period and obtaining the light gray
foam-like gel. The as-obtained gels were ground in an
agate mortar and heated for 20 h at 780°C to remove or-
ganic residues.
Bcosq
where D is the mean crystallite size in the direction
perpendicular to the (hkl) plane of reflexes in nm, K is
a Scherrer constant, l=0.154 nm, B is the full-width at
half-maximum (FWHM) of diffraction peak at angle
q. In presented paper the (332) and (324) peaks were
chosen for calculations as the most suitable for crys-
tallite size determination, because they didn’t overlap
with other reflexes. The K=0.9 was used in calcula-
tions as proposed by Klug et al. [13]. The precision of
the crystallites size estimation (usually 10–20%) is
dependent on particles geometry and size distribution,
that could be evaluated by TEM.
Results and discussion
The XRD results of samples calcined at 780°C are
shown in Fig. 1. They confirm the presence of crystal-
line Na3Ce(PO4)2 phase, however secondary phases
such as CeO2 in the samples A, B and D, Na3PO4
phase in B–D samples and Na4P2O7 in sample B have
The thermal properties of Na3Ce(PO4)2 were in-
vestigated on samples synthesized from CePO4 by
method described in [6] and Na3PO4·12H2O. The par-
ent phosphates were mixed in molar ratios 1:1 and
1.5:1 in agate mortar. So obtained materials were pel-
letized and annealed at 1150°C for 20 h.
The synthesized light yellow powders were ana-
lyzed
by
XRD
using
CuKa
radiation
(Siemens D5000) and by thermal analysis in a fully
automated system of SetsysTM (TG-DSC 1500;
Setaram). From 15 to 20 mg of powdered samples
placed in Pt crucible were heated to 1200°C and
cooled to room temperature with heating/cooling rate
10°C min–1 using Ar as a purge gas. The temperature
and sensitivity calibration were carried out using the
phase transition and the melting points of K2SO4 (584
and 1069°C, respectively). A Fourier transform IR
spectrometer (Perkin-Elmer System 2000 FTIR) was
used to record IR spectra of the samples in KBr
pressed pellets in the range of wavenumbers
400–4000 cm–1. Crystallite size was estimated from
XRD measurements by Scherrer’s equation:
Fig. 1 X-ray diffractograms of samples A–D calcined at
780°C; – denotes CeO2, u – Na4P2O7 and ¡ – Na3PO4
*
secondary phases
Table 1 Initial composition of prepared samples, determined crystallite size of Na3Ce(PO4)2 phase and the phase composition
of samples after heating at 780°C
Sample
R1
R2
R3
Crystallite size/nm
Phase composition
A
B
C
D
20
20
2
12
46
2
–
12
–
38
47
53
50
Na3Ce(PO4)2, CeO2
Na3Ce(PO4)2, CeO2, Na3PO4, Na4P2O7
Na3Ce(PO4)2, Na3PO4
4
18
280
Na3Ce(PO4)2, CeO2, Na3PO4, pyrophosphate*
*determined by FTIR; R1=citric acid to Ce3+ molar ratio, R2=ethylene glycol to Ce3+ molar ratio, R3=HNO3 to Ce3+ molar ratio
690
J. Therm. Anal. Cal., 93, 2008

MODIFIED PECHINI SYNTHESIS OF Na3Ce(PO4)2
been observed. No secondary phases were detected in
orthophosphates synthesis was observed and ex-
plained in [16–18]. In acidic solutions, the HPO24– for
PO34– substitution is possible. By further heating of
the prepared material at temperatures higher than
500°C, the P2O74– anions are formed from HPO24– ac-
cording to the reaction: 2HPO24– ®P2O74– +H2O.
The XRD results confirm that the pyrophosphate
is present in sample B as the Na4P2O7 phase. Nature of
P2O47– anion in sample D is non-crystalline and it
could be not detected by XRD analysis.
Karpowich et al. [18] reported that pyrophosphates
could occur as a phosphate-rich amorphous phase at
grain boundaries, even after heating up to 800°C,
which could be also a good explanation of the amor-
phous state of P2O47– in sample D.
samples heated for 1 h at 1100°C
Ceria was formed in gels containing high con-
centration of citric acid and ethylene glycol. The oxi-
dation of Ce3+ to Ce4+ in the samples was possible un-
der strongly oxidizing conditions of the self-combus-
tion process, which was observed by heat treatment of
gels. Lowering of the organic precursors content in
the prepared sols has to prevent the Ce(III) oxidation,
as was in sample C. Further calcining the samples up
to 1100°C lowered the cerium dioxide and Na3PO4
content in samples, what indicated that samples cal-
cined at 780°C were not finally reacted.
The phase composition of powders calcined at
780°C was checked also by FTIR spectra (Fig. 2). The
main absorption bands were observed at 412–465,
547–601, 960 and 986–1118 cm–1. The bands at
986–1118 cm–1 were due to the asymmetric stretching
and the band at 960 cm–1 to the symmetric stretching
of PO4 group. The asymmetric and symmetric defor-
mation modes of the PO4 group can be observed at
547–601 and 412–465 cm–1, respectively. The above
mentioned absorption bands are in agreement with the
literature data for IR spectra of Na3Ce(PO4)2 com-
pound [1]. In sample C additional peaks at 860, 1367
and 1470 cm–1 were attributed to Na3PO4 phase [14],
which presence was first recognized by XRD. The
strong absorption at 734 cm–1 in sample B was proba-
bly due to symmetric stretching the POP group ob-
served for pyrophosphates [15]. Absorption at the
same position, but much less intensive, was recog-
nized for sample D, indicating the existence of
pyrophosphate in the obtained powder. The XRD and
FTIR results show that the pyrophosphates are built,
when nitric acid is used (samples B and D). The phe-
nomena of pyrophosphate formation on rare-earth
The average crystallite size of Na3Ce(PO4)2
phase calculated from the reflection broadening tech-
nique is found to be 47–53 nm (Table 1) for
samples B–D. The estimated dimensions are very nar-
row, almost independent on the concentration of citric
acid and ethylene glycol used in initial solutions. The
smallest crystallites of 38 nm dimension in sample A
were found, where EDTA without nitric acid addition
was used during synthesis of this material. This could
be explained by the fact, that stronger formation of
cerium chelates could be achieved in sample A than in
acidic conditions, as in sample B [19]. More efficient
complexing in solution prevented faster crystal-
lization of compounds.
The thermochemistry of phase transition of the
Na3Ce(PO4)2 compound was determined by the
TG/DSC analysis. Szczygiel et al. [6] reported, that
the polymorphic transition of the compound occurs at
constant temperature (i.e. 1060°C) during cooling of
molten samples and on heating of powders synthe-
sized in temperatures up to 1150°C. Two effects
(weak at 920 and strong at 1060°C) were observed
otherwise. Unfortunately the temperature of the first
one coincides with the point of b/d’-transition of
Na3PO4 (914°C). Since sodium orthophosphate could
be present in the binary compound as a secondary
phase or formed on its melting, we investigated firstly
the thermal behavior of a sample containing 60 mol%
of CePO4 and 40 mol% of Na3PO4. The sample exhib-
ited during cooling two exothermic effects in temper-
ature range 920–1060°C. Since in this temperature in-
terval monazite does not show any polymorphic tran-
sitions [17], then both effects are connected to
a/b-phase transition of Na3Ce(PO4)2.
The total enthalpy change of Na3Ce(PO4)2 phase
transition was determined on heating and cooling of
sample containing 50 mol% of monazite and 50 mol%
of sodium phosphate. One strong endothermic peak at
1060°C
with
the
enthalpy
change
of
14.1±0.5 kJ mol–1 was observed (Fig. 3). During cool-
Fig. 2 FTIR spectra of samples A–D calcined at 780°C
J. Therm. Anal. Cal., 93, 2008
691

MATRASZEK, SZCZYGIE˜
The enthalpy change of b-to-a transition of
Na3Ce(PO4)2 phase was determined to be
14.2±0.7 kJ mol–1.
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DOI: 10.1007/s10973-008-9129-5
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