Synthesis and characterization of uranium (IV) phosphate-hydrogenphosphate hydrate and cerium (IV) phosphate-hydrogenphosphate hydrate

Article abstract of DOI:10.1016/j.jssc.2005.01.005

A new uranium (IV) phosphate of proposed formula U₂(PO ₄)₂HPO₄·H₂O, i.e. uranium phosphate-hydrogenphosphate hydrate (UPHPH), was synthesized in autoclave and/or in polytetrafluoroethylene closed containers at 150 °C by three ways: from uranium (IV) hydrochloric solution and phosphoric acid, from uranium dioxide and phosphoric acid and by transformation of the uranium hydrogenphosphate hydrate U(HPO₄)₂·nH₂O. The new product appears similar to the previously published thorium phosphate-hydrogenphosphate hydrate Th₂(PO₄)₂HPO₄·H₂O (TPHPH). From preliminary studies, it was found that UPHPH crystallizes in monoclinic system (a=2.1148(7)nm, b=0.6611(2)nm, c=0.6990(3)nm, β=91.67(3)° and V=0.9768(10)nm3). Heated under inert atmosphere, this compound is decomposed above 400 °C into uranium phosphate-triphosphate U₂(PO₄)P₃O₁₀, uranium diphosphate α-UP₂O₇ and diuranium oxide phosphate U ₂O(PO₄)₂. Crystallized cerium (IV) phosphate-hydrogenphosphate hydrate Ce₂(PO₄) ₂HPO₄·H₂O (CePHPH) was also synthesized from (NH₄)₂Ce(NO₃)₆ and phosphoric acid solutions by the same method (monoclinic system: a=2.1045(5)nm, b=0.6561(2)nm, c=0.6949(2)nm, β=91.98(1)° and V=0.9588(9)nm3). When heating above 600 °C, cerium (IV) is reduced into Ce (III) and forms a mixture of CePO₄ (monazite structure) and CeP₃O ₉.

Full text of DOI:10.1016/j.jssc.2005.01.005

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 1054–1063
www.elsevier.com/locate/jssc
Synthesis and characterization of uranium (IV)
phosphate-hydrogenphosphate hydrate and cerium (IV)
phosphate-hydrogenphosphate hydrate
Ã
V. Brandel, N. Clavier, N. Dacheux
´ ´ ˆ
Groupe de Radiochimie, Institut de Physique Nucleaire d’Orsay, Universite Paris-Sud-11, Bat. 100, 91406 Orsay, France
Received 17 October 2004; received in revised form 3 January 2005; accepted 8 January 2005
Abstract
A new uranium (IV) phosphate of proposed formula U2(PO4)2HPO4 ꢀ H2O, i.e. uranium phosphate-hydrogenphosphate hydrate
(UPHPH), was synthesized in autoclave and/or in polytetrafluoroethylene closed containers at 150 1C by three ways: from uranium
(IV) hydrochloric solution and phosphoric acid, from uranium dioxide and phosphoric acid and by transformation of the uranium
hydrogenphosphate hydrate U(HPO4)2 ꢀ nH2O. The new product appears similar to the previously published thorium phosphate-
hydrogenphosphate hydrate Th2(PO4)2HPO4 ꢀ H2O (TPHPH). From preliminary studies, it was found that UPHPH crystallizes in
monoclinic system (a ¼ 2:1148ð7Þ nm; b ¼ 0:6611ð2Þ nm; c ¼ 0:6990ð3Þ nm; b ¼ 91:67ð3Þ1 and V ¼ 0:9768ð10Þ nm3). Heated under
inert atmosphere, this compound is decomposed above 400 1C into uranium phosphate-triphosphate U2(PO4)P3O10, uranium
diphosphate a-UP2O7 and diuranium oxide phosphate U2O(PO4)2.
Crystallized cerium (IV) phosphate-hydrogenphosphate hydrate Ce2(PO4)2HPO4 ꢀ H2O (CePHPH) was also synthesized from
(NH4)2Ce(NO3)6 and phosphoric acid solutions by the same method (monoclinic system: a ¼ 2:1045ð5Þ nm; b ¼ 0:6561ð2Þ nm;
c ¼ 0:6949ð2Þ nm; b ¼ 91:98ð1Þ1 and V ¼ 0:9588ð9Þ nm3). When heating above 600 1C, cerium (IV) is reduced into Ce (III) and forms
a mixture of CePO4 (monazite structure) and CeP3O9.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Uranium (IV); Cerium (IV); Phosphate; Hydrogenphosphate
1. Introduction
reexamined [20]. Among the new solids, thorium
phosphate-diphosphate b-Th4(PO4)4P2O7 (b-TPD) was
found to be a very stable matrix in the field of the
immobilization of radionuclides for an underground
repository (thermal stability, high resistance to aqueous
alteration) [20–22]. It allows the incorporation of large
amounts of other tetravalent actinides leading to the
preparation of solid solutions of general formula b-
Th4ÀxMx(PO4)4P2O7 (MQU, Np, Pu) with xp3:0 for
uranium, xp2:1 for neptunium and xp1:6 for pluto-
nium [21–23]. Taking into account these properties, b-
TPD was proposed for the long-term storage of these
elements [24–26]. This compound or associated solid
solutions was obtained by mixing concentrated aqueous
solutions of actinide chloride, nitrate, etc. (for uranium
chloride only) with phosphoric acid. The mixture was
Tetravalent uranium phosphates were extensively
studied in the past [1–11]. However, their chemistry
required to be revised since the general physico-chemical
properties of these solids (and especially their low
solubility) appear of particular interest for their
potential use as conditioning matrices for radioactive
waste. Syntheses of several new compounds were
reported in the last decade [12–19]. With the same
aim, the chemistry of other tetravalent actinide phos-
phates [20–22], especially that of thorium, was also
Ã
Corresponding author. Fax: +33 1 69 15 71 50.
E-mail addresses: vbrandel@neuf.fr (V. Brandel),
dacheux@ipno.in2p3.fr (N. Dacheux).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.01.005

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V. Brandel et al. / Journal of Solid State Chemistry 178 (2005) 1054–1063
1055
evaporated, the residue was ground, pressed at room
temperature then fired at 1200–1250 1C to obtain the
final ceramics [27]. In order to avoid the evaporation of
volatile products (such as HCl, NO, NO2) during the
heat treatment, another way of preparation of these
ceramics was developed. This new method consists of
the precipitation of thorium phosphate-hydrogenphos-
phate, amorphous Th2(PO4)2HPO4 ꢀ nH2O with n ¼
327 [28] or crystallized monohydrate (using hydrother-
mal conditions) Th2(PO4)2HPO4 ꢀ H2O [29] as precur-
sors. The precipitate was filtered, washed then heated
for dehydration (E200 1C) and final transformation into
b-TPD (950–1000 1C).
The main objective of the studies undertaken is to
obtain solid solutions of thorium-actinide (IV) phosphate-
hydrogenphosphate hydrate Th2Àx/2Anx/2(PO4)2HPO4 ꢀ
H2O (AnQU, Np, Pu), using the same process. These
solid solutions, obtained by precipitation, separation
from the mother liquor then drying, will afterwards be
heated at high temperature to prepare the thorium-acti-
nide (IV) phosphate-diphosphate b-Th4ÀxAnx(PO4)4P2O7
ceramics [30].
were recorded from 5 to 601 (2y with a step of 0.011).
High-temperature XRD data (HT-XRD) were recorded
with a Philips X’PERT PRO—PW 3040/60 apparatus
using an HTK 1200 furnace from Anton Parr Instru-
ment Company.
The infrared (IR) spectra were recorded from 4000 to
400 cmÀ1 with a Hitachi I-2001 spectrophotometer.
Samples of 1–1.5 wt% of the solids were prepared in
KBr pellets. The UV-visible absorption spectrum of the
obtained products was recorded by means of a Varian
DMS 300 spectrophotometer.
The Raman spectra were recorded using a Labram
(Dilor-Jobin Yvon) microspectrometer with an argon
laser working at 514.5 nm and equipped by a Notch
filter. The power varied from 50 to 100 mW and the laser
beam was focused on the sample with an Olympus
microscope.
Thermal gravimetric analysis (TGA) was performed
in argon flow up to 1000 1C by means of a SETARAM
TG-DTA92 apparatus.
Electron probe microanalyses (EPMA) were carried
out by means of a CAMECA SX 50 apparatus working
with a 15 kV voltage and a 10 nA current beam.
Uranium dioxide UO2 (Mb ray of uranium), KTi2(PO4)3
or SmPO4 (Ka ray of phosphorus) and CePO4 (La ray of
cerium) were used as calibration monitors.
The observations of the powders were carried out
with a HITACHI S 2500 scanning electron microscope
(SEM).
In this paper, the synthesis and the characterization of
the pure crystallized uranium (IV) phosphate-hydro-
genphosphate hydrate U2(PO4)2HPO4 ꢀ H2O (UPHPH)
is described. Synthesis of cerium (IV) phosphate-
hydrogenphosphate hydrate Ce2(PO4)2HPO4 ꢀ H2O
(CePHPH) is also reported since Ce (IV) is often used
as a surrogate for tetravalent plutonium.
2.2. Samples preparation
2. Experimental
Crystallized uranium (IV) phosphate-hydrogenphos-
phate hydrate was obtained using three ways:
2.1. Chemicals and apparatus
All reagents, except uranium salt solution, were
supplied by Aldrich-Fluka. Uranium metal chips were
dissolved in 6 M hydrochloric acid [19] to obtain a 1 M
final solution. Cerium-ammonium nitrate (NH4)2Ce
(NO3)6 was also prepared as 1 M solution. All concen-
trations were determined by conventional analytical
methods.
Syntheses were performed under hydrothermal con-
ditions in Parr Instrument Company autoclaves sup-
ported with polytetrafluoroethylene (PTFE) containers
(maximum volume of 23 mL). Samples were heated at
150 1C during one week to two months. Some syntheses
were also carried out in closed PTFE recipients heated
on a sand bath at 150 1C for 2–4 weeks. Heating
treatments were performed in alumina boats in a Pyrox
MDB15 or HM40 tubular furnaces under argon atmo-
sphere (with a rate of 5 1C/min).
ꢁ By direct mixture of uranium (IV) hydrochloric
solution and phosphoric acid in the mole ratio
P/U ¼ 3/2 with 2% excess of 5 M H3PO4, either in
autoclave or in closed PTFE containers heated on a
sand bath (150 1C). The gel initially formed was
transformed into a precipitate after one day. Crystal-
lized powder was obtained after one week.
ꢁ By mixing uranium dioxide UO2 with phosphoric acid
(5% of excess) in the same initial mole ratio. This
synthesis was performed in autoclave (150 1C) during
several weeks.
ꢁ By heating the amorphous uranium (IV) hydrogen-
phosphate hydrate U(HPO4)2 ꢀ nH2O in autoclave
(150 1C). Instead of the expected crystals of
U(HPO4)2 ꢀ 2H2O, the solid revealed the same XRD
lines and IR spectrum as those prepared by the first
two ways.
X-ray powder diffraction (XRD) data were collected
with a Philips PW 1050/70 or a BRUKER-AXS D8
Advance diffractometer using the monochromatic CuKa
radiation (l ¼ 0:15418 nm) and a nickel filter. Patterns
Cerium (IV) phosphate-hydrogenphosphate hydrate
was obtained in autoclave from a mixture of cerium

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V. Brandel et al. / Journal of Solid State Chemistry 178 (2005) 1054–1063
1056
ammonium nitrate and 5 M H3PO4 in 2 M HNO3
solution.
a ¼ 2:1148ð7Þ nm; b ¼ 0:6611ð2Þ nm; c ¼ 0:6990ð3Þ nm
and b ¼ 91:67ð3Þ1 (calculated density of 5.31)
(Table 3). Taking into account the similarities between
the XRD diagrams of UPHPH and TPHPH, recently
described as an orthorhombic compound with a ¼
Whatever the method considered, the crystallized
products were filtered, washed with de-ionized water
and ethanol, dried at 120 1C then finally stored in a
desiccator. In order to study the behavior of the
obtained products during the heating treatment, the
samples were fired (under argon atmosphere for
uranium samples) from room temperature up to
1000–1100 1C with heating steps of 100 1C and heating
times of 1 hour (HT-XRD experiments) or 14 hours
(room temperature XRD experiments).
2:1368ð2Þ nm;
b ¼ 0:6695ð1Þ nm;
c ¼ 0:7023ð1Þ nm
(calculated density of 5.08) [31,32], it can be accepted
that the structures of both compounds are similar.
However, a slight distortion of the unit cell is observed
between both solids due to the weak difference of the
ionic radii (VIIrTh
4þ ¼ 0:100 nm and VIIrU4þ ¼ 0:095 nm)
leading to a monoclinic system with b ¼ 91:671 [32].
The comparison of the refined unit cell parameters
exhibits a contraction of only 2.8% in volume from
TPHPH to UPHPH, which appears in good agreement
with the small difference between the ionic radii of
3. Results and discussion
Th4+ and U4+
.
3.1. Uranium phosphate-hydrogenphosphate hydrate
Since uranium (IV) can be easily oxidized into the
molecular uranyl ion UO22+, UV-visible spectrum was
recorded in order to check that uranium was only
present in the tetrapositive oxidation state in the
obtained phosphate. All the bands were assigned to
The results of EPMA are gathered in Table 1. The
P/U mole ratios were found to be equal to 3/2 what-
ever the method of preparation applied.
As it can be seen on the SEM micrograph in Fig. 1a,
the crystals do not exceed 2 mm in length and appear as
layered bars. Fig. 1b exhibits bigger aggregates (up to
20 mm) composed of these small crystals. The observa-
tions performed on the same type of aggregates in
thorium-uranium (IV) phosphate-hydrogenphosphate
hydrate (TUPHPH) solid solutions revealed spherulitic
texture with an empty core [30].
The final uranium (IV) phosphate sample exhibited a
similar XRD diagram (Fig. 2b) as thorium phosphate-
hydrogenphosphate hydrate (TPHPH) (Fig. 2a).
The corresponding XRD data are listed in Table 2.
The morphology of the samples leads to preferential
orientations for (00) peaks, which is clearly observed in
the figure. From preliminary studies, it was found that
uranium (IV) phosphate-hydrogenphosphate hydrate
(UPHPH) crystallizes in the monoclinic system with
3
the transition levels of the tetravalent uranium [33]: P1
multiplets (500–560 nm), 1G4, 1D2, 3P0 mixed levels
(650–750 nm) and 3H6 level (800 nm). No absorption
band characteristic of the uranyl ion [34] was evidenced
between 400 and 440 nm.
On the TGA curve (Fig. 3), four steps are observed up
to 540 1C. The substantial weight loss of about 2.5%
occurs at 200 1C, corresponding to about 1.1 molecule of
water per unit formula. Thus, as it was observed for
TPHPH [29,31], it can be accepted that UPHPH is
monohydrated from room temperature to about 150 1C
and fully dehydrated at about 200 1C.
The IR and Raman spectra of the synthesized
uranium compound are presented in Figs. 4a and 5a,
respectively. All the bands of UPHPH (Table 4) were
assigned to the vibrations of the bonds in phosphate,
Table 1
Results obtained from EPMA for uranium (IV) and cerium (IV) phosphate-hydrogenphosphates
Synthesized solid and ways of synthesis
Ce (wt%)
U (wt%)
P (wt%)
O (wt%)b
Ratio P/M
U2(PO4)2HPO4 ꢀ H2O
UO2+H3PO4
UCl4+H3PO4
—
—
—
—
61.0a
11.9a
26.7a
1.50a
61.570.4
61.670.3
61.470.6
12.070.1
11.970.3
12.170.2
26.570.4
26.570.8
25.870.7
1.5070.01
1.4970.04
1.5270.04
U(HPO4)2 ꢀ nH2O (hydrolysis)
a-U4(PO4)4P2O7
63.2a
12.4a
24.4a
1.50a
Heating at 250 1C
62.770.3
12.670.2
24.770.5
1.5470.01
U(UO2)(PO4)2
Heating at 1200 1C (in air)
—
—
68.2a
8.9a
22.9a
1.00a
69.870.4
9.170.2
21.170.6
1.0070.03
Ce2(PO4)2HPO4 ꢀ H2O
47.9a
—
—
15.9a
35.6a
1.50a
(NH4)2Ce(NO3)6+H3PO4
48.770.5
16.270.3
35.171.0
1.5170.04
aCalculated.
bIncluding oxygen from water molecules.

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1057
Fig. 1. SEM micrographs of UPHPH (a, b) and CePHPH (c, d).
hydrogenphosphate and OH groups as well as in
water molecule according to the data reported in the
literature [35–40].
As a summary, all the results presented confirm
the formula proposed for UPHPH, i.e. U2(PO4)2
HPO4 ꢀ H2O. The reactions involved can thus be written
as follows:
(a)
(b)
ꢁ from uranium (IV) in hydrochloric solution
2U4þ þ 3H3PO4 ðaqÞ þ H2O
ð‘Þ
ðaqÞ
(c)
! U2ðPO4Þ HPO4 ꢀ H2O # þ8 Hþ
(1)
ðsÞ
2
ðaqÞ
ꢁ from uranium dioxide
10
20
30
40
50
60
2θ (degrees)
2UO2ðsÞ þ 3H3PO4ðaqÞ ! U2ðPO4Þ HPO4 ꢀ H2O
#
(2)
ðsÞ
2
þ 3H2O
ð‘Þ
Fig. 2. XRD diagrams of TPHPH (a), UPHPH (b) and CePHPH (c).

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V. Brandel et al. / Journal of Solid State Chemistry 178 (2005) 1054–1063
1058
ꢁ from amorphous uranium hydrogenphosphate
Eq. (3) is written after King [41] who observed the
conversion of hydrated plutonium (IV) hydrogenphos-
phate Pu(HPO4)2 ꢀ xH2O, PuHP, into a plutonium
phosphate that formula was presumed as Pu2H(PO4)3 ꢀ
yH2O. It was not specified whether the process is a real
equilibrium. However, PuHP was only stable with large
excess of phosphoric acid.
2UðHPO4Þ þ H2O $U2ðPO4Þ HPO4 ꢀ H2O
#
ð3Þ
ð‘Þ
ðsÞ
2ðsÞ
2
þ H3PO4ðaqÞ
When heating above 200 1C, UPHPH is fully dehy-
drated and forms UPHP according to
Table 2
XRD data of powdered UPHPH
U2ðPO4Þ HPO4 ꢀ H2O ! U2ðPO4Þ HPO4ðsÞ
2ycalcð1Þ
2yexpð1Þ
dexpðnmÞ
I=I0
ðsÞ
2
2
þ H2O
"
(4)
ðgÞ
8.35
14.04
16.78
18.40
19.03
22.62
25.28
25.50
26.64
26.98
28.40
29.33
29.90
30.25
31.10
31.32
31.94
32.60
33.93
34.29
34.67
35.43
36.46
36.75
37.30
38.62
39.29
39.80
41.19
41.27
42.40
42.77
43.49
44.90
45.54
46.14
47.14
47.77
48.37
49.00
49.39
49.76
8.37
14.02
16.79
18.39
19.01
22.61
25.30
25.48
26.64
26.97
28.40
29.33
29.90
30.24
31.09
31.32
31.93
32.60
33.94
34.29
34.72
35.45
36.47
36.76
37.35
38.62
39.27
39.77
41.18
41.42
42.41
42.67
43.50
44.88
45.52
46.12
47.14
47.74
48.33
49.09
49.37
49.72
1.058
0.632
0.528
0.482
0.467
0.393
0.352
0.350
0.335
0.331
0.314
0.304
0.299
0.295
0.288
0.286
0.280
0.275
0.264
0.262
0.258
0.253
0.246
0.244
0.241
0.233
0.229
0.227
0.219
0.218
0.213
0.211
0.208
0.202
0.199
0.197
0.193
0.191
0.188
0.186
0.185
0.183
9
17
100
19
20
16
80
63
18
14
13
8
As it was already observed for TPHPH, the con-
densation of HPO4 groups occurs above 200 1C, leading
to P2O7 entities with loss of one additional water
molecule for two formula units [31]. For UPHPH, the
band located at 1235 cmÀ1 associated with the d (P–O–
H) vibration disappears progressively in the IR spec-
trum (Fig. 6) with the temperature rise. Simultaneously,
a band located at 752 cmÀ1 (IR, y ꢂ 400 1C; Fig. 6) and
at 760 cmÀ1 (Raman, y ꢂ 250 1C; Fig. 7) associated with
the ns (P–O–P) vibration is observed [35,39]. This fact
suggests the conversion of UPHP into a-UPD, analo-
gous to a-TPD (low-temperature form of TPD) [31] as
follows:
56
15
28
17
9
22
18
22
37
19
4
2U2ðPO4Þ HPO4ðsÞ ! a-U4ðPO4Þ P2O7ðsÞ þ H2O " (5)
ðgÞ
2
4
Nevertheless, the band associated with the H2O
bending mode at 1622 cmÀ1 remains present even after
6
8
1
5
11
9
U2(PO4)2HPO4 . H2O
0
-1
-2
-3
-4
-5
19
18
13
61
19
7
U2(PO4)2HPO4
α
-U4(PO4)4P2O7
9
α
U (PO )P O
2 4 3 10
-UP2O7
+
+ U2O(PO4)2
12
8
32
12
13
15
11
100 200 300 400 500 600 700 800 900 1000 1100 1200
Temperature (°C)
Fig. 3. Results of TGA obtained for UPHPH.
Table 3
Unit cell parameters of TPHPH [31], UPHPH and CePHPH [32]
Compound
a (nm)
b (nm)
c (nm)
b (1)
V (nm3)
Th2(PO4)2HPO4 ꢀ H2O
U2(PO4)2HPO4 ꢀ H2O
Ce2(PO4)2HPO4 ꢀ H2O
2.1368(2)
2.1148(7)
2.1045(5)
0.6695(1)
0.6611(2)
0.6561(2)
0.7023(1)
0.6990(3)
0.6949(2)
—
91.67(3)
91.98(1)
1.0048
0.9768
0.9588

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1059
respectively (Fig. 7). According to Grending et al. [42],
these frequencies can be assigned to n1 (UQO) bond in
the UO22+ ion. The oxidation process of U4+, probably
due to the presence of small amounts of oxygen (or air)
in argon, seems to take place at the surface of the grains
during the heat treatment. Another experiment, per-
formed in air atmosphere at 1200 1C revealed a band of
very high intensity at 877 cmÀ1 (Fig. 7) as previously
observed in the Raman spectrum of U(UO2)(PO4)2 [11].
Thus, UPHPH was transformed into U(UO2)(PO4)2 in
air at 1200 1C (which was also confirmed from the
results of EPMA, Table 1):
(a)
(b)
4U2ðPO4Þ2HPO4 ꢀ H2OðsÞ þ 2O2ðgÞ
! 4UðUO2ÞðPO4Þ þ P4O10ðgÞ " þ6H2O
"
(6)
ðgÞ
2ðsÞ
4000
3500
3000
2500
2000
1500
1000
500
In the XRD diagram (Fig. 8), the lines of uranium
(IV) phosphate-triphosphate (UPTP) U2(PO4)P3O10 [43]
(formerly known as b-UP2O7) appear above 400 1C. Its
transformation into the more stable a-UP2O7 is ob-
served at higher temperature (800–1000 1C). Simulta-
neously, the diffraction lines of U2O(PO4)2 also appear.
The decomposition of UPHPH (in argon atmosphere)
may be approximately written as
σ (cm-1)
Fig. 4. IR spectra of UPHPH (a) and CePHPH (b).
(a)
4U2ðPO4Þ HPO4ꢀH2O !2a-UP2O7ðsÞþU2ðPO4ÞP3O10ðsÞ
ðsÞ
2
þ 2U2OðPO4Þ2ðsÞ
þ 6H2O
"
(7)
ðgÞ
In these conditions, a b-UPD compound analogous to
b-TPD [20] was not obtained at high temperature as
already described for the other ways of synthesis [21,22].
Finally, a-UP2O7 was transformed into U2O(PO4)2
when heating at higher temperature (about 1250 1C)
and/or when extending the heating time [15].
(b)
200
400
600
800
1000 1200 1400 1600 1800
σ (cm-1)
3.2. Cerium phosphate-hydrogenphosphate hydrate
Fig. 5. Raman spectra of UPHPH (a) and CePHPH (b).
Several cerium (IV) phosphates were mentioned in the
literature. King [41] reported the preparation of
Ce2H(PO4)3 ꢀ yH2O from a Ce (IV) salt and sodium
phosphate in acidic solution. However, no XRD data
or any other result were published for this compound
that formula may be also written as Ce2(PO4)2HPO4 ꢀ
yH2O.
Konig and Meyn [44] precipitated a crystallized
cerium (IV) phosphate with mole ratio P/CeE1.5 from
cerium sulfate and phosphoric acid solutions. These
authors proposed the (Ce–O–Ce)(HPO4)3 ꢀ H2O formula
based on the known (Ce–O–Ce)6+ ion in aqueous
solution. They also postulated that this compound was
transformed into (Ce–O–Ce)(P2O7)3 by condensation of
HPO4 entities when heating above 900 1C. However, the
initial (Ce–O–Ce)(HPO4)3 ꢀ H2O can be also written as
Ce2(PO4)2HPO4 ꢀ 2H2O (without excluding the presence
of the P2O7 entity after heating). Furthermore, the
temperature of condensation, mentioned by the authors,
heating the sample at 400–600 1C, which is probably due
to the hygroscopic behavior of the product(s) formed
above 200 1C.
As it was already described for TPHPH and a-TPD
[31], the XRD diagrams of UPHP and a-UPD remain
almost unchanged compared to that of UPHPH
showing that these compounds are isostructural. When
heated under argon atmosphere at 400 1C, a-UPD is
decomposed into other uranium compounds as it can be
observed on the XRD diagram (see below in the
following section). On this basis, the band located at
1308 cmÀ1 in the IR spectrum (Fig. 6c) can be attributed
to a polyphosphate grouping [35] and that at 752 cmÀ1
to the ns (P–O–P) of a-UP2O7 and/or of a-UPD.
Moreover, a weak band located at 872 cmÀ1 is observed
in the IR spectrum obtained for 800–1000 1C (Fig. 6d).
Two other weak bands appear in the Raman spectrum
at 865 and at 877 cmÀ1 for y ¼ 250 and 1200 1C,

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Table 4
Assignment of frequencies in the IR and Raman spectra of MPHPH compounds
Frequencies (cmÀ1
)
Assignment
Refs.
UPHPH
CePHPH
IR
3700–2600
1622
1235
3640–2930
1615
1232
n (O–H), n ((P)–O–H)
d (H–O–H)
[35]
[35]
dip (P–O–H)a
nas (P–O)
[37]
1100, 1072, 1014
950
854
1088,1034
944
[35,36]
[35,36]
[35–38]
[35–39]
[35–39]
ns (P–O)
812 sh
612, 560, 512
–
ns (P–O) and/or dop (P–O–(H))b
das (P–O)
620, 560, 520
444
ds (P–O)
Raman
236
280
241
284
External modes and/or n (U–O), n (Ce–O)
External modes
[36,39]
[36]
314–421
568, 591, 617
820–865
925
373, 391, 424
587, 612
–
ds (P–O)
das (P–O)
[35–39]
[35–39]
[35–39]
[35–39]
[35–39]
ns (P–O) and/or dop (P–O–(H))b
ns (P–O)
927, 935
1008, 1034
1019, 1049
nas (P–O)
aIn the plane deformation, which could also include the hydrogen bond deformation P–O–H?O–H (40) or P–O–H?O–P– [36]
bOut of the plane deformation, which could also include the hydrogen bond deformation P–O–HyO–H (40) or P–O–HyO–P– [36]
(d)
(d)
(c)
(c)
(b)
(b)
(a)
(a)
200
400
600
800
1000 1200 1400 1600 1800
σ (cm-1)
4000
3500
3000
2500
2000
1500
1000
500
σ (cm-1)
Fig. 7. Variation of the Raman spectrum of UPHPH in terms of
heating temperature: (a) initial solid, (b) heated at 180 1C, (c) heated at
1200 1C under Ar-atmosphere (d) heated at 1200 1C in air.
Fig. 6. Variation of the IR spectrum of UPHPH in terms of heating
temperature: (a) initial solid, (b) heated at 200 1C, (c) heated at
400–600 1C, (d) heated at 800–1000 1C.
seems to be doubtful taking into account the behavior of
tetravalent cerium at high temperature. Indeed, when
heating a-CeP2O7 at 900 1C, Botto and Baran [45]
observed the complete reduction of cerium (IV) into
Cerium (III), leading to the formation of CePO4
(monazite structure).
Alberti et al. [46] synthesized a cerium (IV) phosphate
with a mole ratio PO4/Ce equal to 1.55, but its chemical
formula was not suggested. Nevertheless, the XRD data
of this product (with estimated intensities) are similar to
that of Konig and Meyn’s [44].
In this study, the synthesis of cerium (IV) phosphate-
hydrogenphosphate hydrate was undertaken in the same
conditions as UPHPH using (NH4)2Ce(NO3)6 and
H3PO4 solutions. For this sample, the EPMA results
are given in Table 1 while the SEM micrographs are
presented in Fig. 1c and d. The crystals present variable
length up to 10 mm. Although the samples appear
homogeneous and of the expected stoichiometry, some
small impurities of CePO4 (monazite structure), prob-
ably formed consequently to the reduction of small
amounts of tetravalent cerium into trivalent cerium,

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1061
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
Ce2(PO4)2HPO4 . H2O
+
+
(d)
*
+
*
+
+
*
*
*
Ce2(PO4)2HPO4
˚
(c)
(b)
(a)
*
+
˚
˚
*
α
-Ce4(PO4)4P2O7
CePO4 + CeP3O9
˚
+
100 200 300 400 500 600 700 800 900 1000 1100 1200
Temperature (°C)
10
20
30
40
50
60
Fig. 9. Results of TGA obtained for CePHPH.
2θ (degrees)
Fig. 8. Variation of XRD diagram versus heating temperature
recorded for UPHPH: (a) initial product, (b) heated at 400 1C, (c)
heated at 800 1C, (d) heated at 1000 1C. Diffraction lines of
U2(PO4)P3O10 (o), U2O(PO4)2 (*) and a-UP2O7 (+).
between UPHPH and CePHPH are also noted in the
Raman spectrum (Fig. 5). Considering the similarities
between the XRD patterns and IR spectra of UPHPH,
TPHPH and the obtained cerium phosphate-hydrogen-
phosphate hydrate (CePHPH), the following formula:
Ce2(PO4)2HPO4 ꢀ H2O can be proposed. The refined unit
cell parameters are presented in Table 3. As already
described for uranium, a slight distorsion of the unit cell
resulting from the weak difference of the ionic radii
Table 5
XRD data of powdered CePHPH
2ycalcð1Þ
2yexpð1Þ
dexpðnmÞ
I=I0
8.41
14.14
16.86
18.98
19.18
22.30
22.80
25.41
27.95
28.63
30.13
31.36
34.10
34.91
35.69
37.07
37.55
39.51
40.11
43.00
43.34
43.84
47.55
48.68
49.66
49.91
8.41
14.12
16.86
18.98
19.18
22.29
22.80
25.41
27.96
28.62
30.14
31.35
34.11
34.93
35.71
37.06
37.56
39.51
40.09
43.01
43.34
43.85
47.54
48.67
49.67
49.89
1.052
0.626
0.526
0.468
0.463
0.399
0.390
0.350
0.319
0.312
0.296
0.285
0.263
0.257
0.252
0.243
0.239
0.228
0.225
0.210
0.209
0.206
0.191
0.187
0.183
0.183
10
6
VII
VII
Th4þ
Ce4þ
(
r
¼ 0:100 nm and
r
¼ 0:092 nm) leads to a
81
3
monoclinic system with b ¼ 91:981 instead of an
orthorhombic one [32].
4
4
On the TGA curve (Fig. 9) several steps of weight loss
are observed in the range of 160–1200 1C. Since no
plateau was observed, it seems that the process was not
complete even at 1200 1C. The first significant weight
loss of 3.1% (y ꢂ 300 1C), corresponding to one
molecule of water per unit formula, can be assigned to
the complete dehydration of CePHPH leading to
CePHP. An additional weight loss of 1.5% is observed
at about 400 1C corresponding to 0.5 mole of H2O per
unit formula. Thus, the condensation reaction of HPO4
group seems to take place leading to the formation of a-
Ce4(PO4)4P2O7 (a-CePD), analogous to a-TPD [31] and
a-UPD. The XRD diagrams of CePHPH, CePHP and a-
CePD are very close showing only small structure
modifications during dehydration, then condensation
of hydrogenphosphate groups which was already shown
for TPHPH, TPHP and a-TPD [31].
5
100
4
11
10
10
12
20
11
3
4
4
4
46
8
8
4
5
13
9
Above 700 1C, the decomposition process begins (Fig.
10), which agrees well with the impossibility to prepare
b-CePD, analogous to b-TPD, even in oxidizing
conditions, as previously reported [21]. Plutonium (IV)
phosphate in mole ratio PO4/Pu ¼ 1.5 behaves in the
same way [21]. The diffraction lines of CeP3O9 and
CePO4 (monazite type) appear at 700 1C while CePO4
seems to be the major phase at 900–1200 1C. Correla-
tively, the modification of the color of the sample (from
dark yellow to light yellow then white) confirms the
were also found in the raw samples. The XRD data are
listed in Table 5 and the diagram is drawn in Fig. 2c.
The diffraction lines of the solid agree well with those
observed for TPHPH and UPHPH allowing to propose
the formula Ce2(PO4)2HPO4 ꢀ nH2O. The IR spectrum
(Fig. 4b) of this solid, similar to that of UPHPH, can be
interpreted in the same way (Table 4). The similarities

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V. Brandel et al. / Journal of Solid State Chemistry 178 (2005) 1054–1063
1062
nium (IV) hydrochloric solution with phosphoric
+
acid in autoclave or in PTFE closed containers at
150 1C. This product was also obtained in autoclave at
150 1C from uranium dioxide and phosphoric acid.
In the same conditions, uranium hydrogenphosphate
U(HPO4)2 ꢀ nH2O was converted into UPHPH.
Cerium (IV) phosphate-hydrogenphosphate hydrate,
Ce2(PO4)2HPO4 ꢀ H2O (CePHPH) was also prepared
from cerium (IV)—ammonium nitrate and phosphoric
acid using the same procedure. The structures of both
compounds seem to appear similar to that of the already
described TPHPH, even though a slight distortion leads
to a monoclinic unit cell (with b near to 901) instead of
the orthorhombic one [32]. When heating, both solids
lead to dehydrated UPHP and CePHP, then to a-UPD
and a-CePD (analogous to TPHP and a-TPD, respec-
tively), which are rapidly decomposed when increasing
the heating temperature. For the uranium compound, a
mixture of U2(PO4)P3O10, a-UP2O7 and U2O(PO4)2 is
obtained in Ar-atmosphere. In air, uranium (IV) is
oxidized leading to U(UO2)(PO4)2. On the contrary,
cerium (IV) is partly reduced to cerium (III) even in
oxidizing conditions leading to CePO4 (as one of the
phases) and CeP3O9. For both elements, neither the
orthorhombic b-UPD nor b-CePD can be prepared,
which agrees well with the results already reported in
literature [21].
(c)
+
*
+
*
.
.
+
o
+
+
+
+
o
+
*
o
(b)
.
+
*
o
.
+
(a)
10
20
30
40
50
60
2θ (degrees)
Fig. 10. Variation of XRD diagram versus heating temperature
recorded for CePHPH: (a) heated at 600 1C, (b) heated at 700 1C, (c)
heated at 900 1C and 1100 1C. Main diffraction lines of: a-Ce4(PO4)4-
P2O7 (o), CeP3O9 (*), CePO4 (+) and non-identified (ꢁ).
partial reduction of cerium (IV) into cerium (III). This
reduction becomes quantitative by extending the heating
time. No difference was observed between the XRD
diagram recorded at 1100 1C and that obtained after
cooling the sample to room temperature.
Taking into account these results, the crystallized
thorium, plutonium (IV) and cerium (IV) phosphates
(in nitric acid solutions) previously identified as
M2H(PO4)3 ꢀ yH2O [41] should be certainly considered
as Th2(PO4)2HPO4 ꢀ yH2O , Pu2(PO4)2HPO4 ꢀ yH2O and
Ce2(PO4)2HPO4 ꢀ yH2O, respectively.
From the results obtained, the following chemical
transformations can be suggested:
ꢁ progressive volatilization of water molecules start-
ing from 160 1C leading first to the dehydration of
the initial product:
The formation of M2(PO4)2HPO4 ꢀ H2O phosphates
seems to be characteristic of M4+ ions with large ionic
radii. Consequently, it should be also obtained for
neptunium and plutonium which appears consistent
with some recent results describing the preparation of
Np2(PO4)2HPO4 ꢀ H2O and Pu2(PO4)2HPO4 ꢀ H2O in
polyphase systems [47]. On the contrary, this phenom-
Ce2ðPO4Þ HPO4 ꢀ H2O ! Ce2ðPO4Þ HPO4ðsÞ
ðsÞ
2
2
þ H2O
"
(8)
ðgÞ
ꢁ then condensation of HPO4 groups:
2 Ce2ðPO4Þ2HPO4ðsÞ ! a-Ce4ðPO4Þ4P2O7ðsÞ
þ H2O "
enon was not observed for smaller cations such as Hf4+
,
(9)
Sn4+, etc. For zirconium, some products of formulae
H3OZr2(PO4)3 ꢀ 0.1H2O (or rather Zr2(PO4)2HPO4 ꢀ
H2O) and HZr2(PO4)3, both crystallized in the cubic
system, were published by Ono and Yajima [48,49].
However, the results presented by these authors remain
ambiguous and the existence of such compounds is
rather doubtful.
ꢁ and finally, decomposition process, which can be
written as follows:
a-Ce4ðPO4Þ4P2O7ðsÞ
! 3CePO4ðsÞ þ CeP3O9ðsÞ þ O2ðgÞ
"
(10)
Acknowledgments
4. Conclusion
The authors are very grateful to Dr. Renaud Podor,
Alain Kohler and Lionel Aranda from the Laboratoire
de Chimie du Solide Mineral, Universite Henri Poincare-
Nancy I, for the EPMA results, TGA/DTA experiments
Crystallized uranium phosphate-hydrogenphosphate
hydrate (UPHPH) with the proposed chemical formula
U2(PO4)2HPO4 ꢀ H2O, was synthesized by mixing ura-

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V. Brandel et al. / Journal of Solid State Chemistry 178 (2005) 1054–1063
1063
and SEM observations. They would like to thank
Therese Lhomme from the CREGU (Universite Henri
Poincare-Nancy I) for recording the Raman spectra.
They also wish to express their gratitude to Prof. M.
Quarton and Dr. G. Wallezfrom Universite de Paris VI,
Pierre et Marie Curie for their contribution in the
determination of the unit cell parameters of TPHPH,
UPHPH and CePHPH.
[24] N. Dacheux, A.C. Thomas, B. Chassigneux, V. Brandel, M.
Genet, in: J.C. Marra, G.T. Chandler (Eds.), Environmental
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p. 373.
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