Two Ce(SO4)2·4H2O polymorphs: Crystal structure and thermal behavior

Article abstract of DOI:10.1016/j.jssc.2007.02.017

Syntheses, crystal structures and thermal behavior of two polymorphic forms of Ce(SO₄)₂·4H₂O are reported. The first modification, α-Ce(SO₄)₂·4H₂O (I), crystallizes in the orthorhombic space group Fddd, with a=5.6587(1), b=12.0469(2), c=26.7201(3) A and Z=8. The second modification, β-Ce(SO₄)₂·4H₂O (II), crystallizes in the orthorhombic space group Pnma, with a=14.6019(2), b=11.0546(2), c=5.6340(1) A and Z=4. In both structures, the cerium atoms have eight ligands: four water molecules and four sulfate groups. The mutual position of the ligands differs in (I) and (II), resulting in geometrical isomerism. Both these structures are built up by layers of Ce(H₂O)₄(SO₄)₂ held together by a hydrogen bonding network. The dehydration of Ce(SO₄)₂·4H₂O is a two step (I) and one step (II) process, respectively, forming Ce(SO₄)₂ in both cases. During the decomposition of the anhydrous form, Ce(SO₄)₂, into the final product CeO₂, intermediate xCeO₂·yCe(SO₄)₂ species are formed.

Full text of DOI:10.1016/j.jssc.2007.02.017

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 1616–1622
www.elsevier.com/locate/jssc
Two Ce(SO ) ꢀ 4H O polymorphs: Crystal structure
4
2
2
and thermal behavior
a,
Barbara M. Casari , Vratislav Langer
Ã
b
a
Department of Chemistry, Inorganic Chemistry, G o¨ teborg University, SE–412 96 G o¨ teborg, Sweden
Environmental Inorganic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE–412 96 G o¨ teborg, Sweden
b
Received 27 November 2006; received in revised form 15 February 2007; accepted 18 February 2007
Available online 28 February 2007
Abstract
Syntheses, crystal structures and thermal behavior of two polymorphic forms of Ce(SO
4
)
2
ꢀ 4H
2
O are reported. The first modification,
˚
a-Ce(SO ) ꢀ 4H O (I), crystallizes in the orthorhombic space group Fddd, with a ¼ 5.6587(1), b ¼ 12.0469(2), c ¼ 26.7201(3) A and
4
2
2
Z ¼ 8. The second modification, b-Ce(SO
4
)
2
ꢀ 4H O (II), crystallizes in the orthorhombic space group Pnma, with a ¼ 14.6019(2),
2
˚
b ¼ 11.0546(2), c ¼ 5.6340(1) A and Z ¼ 4. In both structures, the cerium atoms have eight ligands: four water molecules and four
sulfate groups. The mutual position of the ligands differs in (I) and (II), resulting in geometrical isomerism. Both these structures are built
up by layers of Ce(H O) (SO ) held together by a hydrogen bonding network. The dehydration of Ce(SO ) ꢀ 4H O is a two step (I) and
2
4
4 2
4 2
2
one step (II) process, respectively, forming Ce(SO
final product CeO , intermediate xCeO
ꢀ yCe(SO
r 2007 Elsevier Inc. All rights reserved.
4
)
2
in both cases. During the decomposition of the anhydrous form, Ce(SO
4 2
) , into the
2
2
4 2
) species are formed.
Keywords: Ce(SO
X-ray diffraction
4
)
2
ꢀ 4H
2
O; Tetrahydrated ceric sulfate; Cerium sulfate; Sulfate decomposition; Thermogravimetry; Differential scanning calorimetry;
1
. Introduction
Complex anions play an important role in the chemistry
system Ce (SO ) ꢁxH O hydrate numbers from 4 up to 16
2
4 3
2
are reported [1,2]. Udupa [3] made a thermal decomposi-
tion study on Ce (SO ) ꢀ 14H O pointing out two
2
4 3
2
of the lanthanide compounds. A comprehensive review of
the rare-earth compounds with complex anions was
presented in 2002 by Wickleder [1] covering all data of
structurally characterized compounds known so far. Due
to the use of complex anions in the separation of rare earth
elements, the lanthanide sulfates have been intensively
studied and a great number of complexes and salts have
been described. The sulfates decompose at elevated
temperature and cannot be obtained from their melt.
Thus, due to the use of solvent, usually water, most of the
known structures are those of hydrated binary and ternary
sulfates [1]. The most common hydrated binary lanthanide
sulfates are the octahydrates, M (SO ) ꢀ 8H O. Among the
dehydration steps involving nine and five water mole-
cules, respectively. On cooling, Ce (SO ) rehydrates
2
4 3
to give Ce (SO ) ꢀ 5H O which slowly converted into
2
4 3
2
Ce (SO ) ꢀ 14H O on exposure to open atmosphere for
2
4 3
2
several days [3].
Cerium(IV) sulfate does not show the same diversity as
cerium(III) sulfate regarding the hydration grade and only
two modifications of Ce(SO ) ꢀ 4H O has been structurally
4
2
2
characterized. However, the description of the thermal
behavior of tetrahydrated ceric sulfate is ambiguous even
in the authoritative handbook [4]. Investigations on the
thermal decomposition of tetrahydrated ceric sulfate have
revealed obvious disagreement among different references.
An early study [5] reported that Ce(SO ) ꢀ 4H O decom-
2
4 3
2
lanthanide salts, the cerium(III) sulfate displays the great-
est diversity in the hydrate number. Within the binary
4
2
2
posed into 3CeO ꢀ 4SO already at 195 1C, while studies
2
3
made by Udupa showed [3] that Ce(SO ) ꢀ 4H O decom-
4
2
2
Ã
poses in four stages, losing two molecules of water in the
first stage and in second stage, resulting in the formation of
Corresponding author. Fax: +46 31 772 2853.
E-mail address: casari@chem.gu.se (B.M. Casari).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.02.017

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1617
anhydrous cerium(IV) sulfate at 340 1C, then Ce(SO4)2
converts into CeO ꢀ 2Ce(SO ) which in the final stage,
acid solution according to the method described by
Lindgren [10].
2
4 2
between 700 and 940 1C loses sulfur dioxide and oxygen to
give CeO . Tagawa confirmed [6] the same initial decom-
2.2. Single crystal X-ray analysis
2
position temperature but suggested one single decomposi-
tion step from Ce(SO ) to the final stage: CeO and SO .
Ying and Rudong [7] showed evidence that the dehydration
Data were collected using a Siemens SMART CCD
diffractometer equipped with a Siemens LT–2A low
temperature device, at 22 1C (I) and ꢁ90 1C (II). A full
sphere of the reciprocal space was scanned by 0.31 steps in
o with a crystal-to-detector distance of 3.97 cm and
exposure time per frame, being 1 s for (I) and 20 s for
(II). Preliminary orientation matrices were obtained using
SMART (Siemens, 1995) [17]. The collected frames were
integrated with the orientation matrix updated every 100
frames. Final cell parameters were obtained by refinement
on the position of 6440 (I) and 8192 (II) reflections,
respectively, with I410s(I) after integration of all the data
using SAINT (Siemens, 1995) [17]. The data were corrected
empirically for absorption and other effects using SA-
DABS [18]. The structures were solved by direct methods
4
2
2
3
of Ce(SO ) ꢀ 4H O starts at 98 1C and proceeds in two
4
2
2
steps up to 322 1C. Further, they suggested [7] that the
observed weight lost and endothermic peak in the
temperature range 450–495 1C corresponds to the reduc-
tion of anhydrous cerium(IV) sulfate, giving Ce (SO ) ,
2
4 3
SO and O . This reduction process has also been noticed
2
2
by Zhao [8]. Recent research [9] proposes that the
dehydration of Ce(SO ) ꢀ 4H O occurs between 75 and
4
2
2
5
00 1C and that the initial decomposition temperature is
around 600 1C. The weight loss and temperature range
observed during the dehydration process does not correlate
with the ones reported by other researchers [3–8].
Furthermore, if the dehydration process is assumed to be
through the 400 1C range, this would correspond to the
starting material Ce(SO ) ꢀ 14H O, while if dehydration
2
and refined by full-matrix least squares on all F data using
SHELXTL (Bruker, 2001) [19]. The non-hydrogen atoms
were refined anisotropically. The hydrogen atoms included
in the water molecules were located from difference Fourier
maps and refined isotropically with no restraints for (I) and
4
2
2
has been assumed to be completed at approximately
3
00 1C, this would closely correlate to a starting material
of Ce(SO ) ꢀ 12H O. On cooling, Ce(SO ) rehydrates to
4
2
2
4 2
˚
give Ce(SO ) ꢀ 4H O [3].
with restrained O–H distances to 0.84 A and a common
4
2
2
Two polymorphic forms of Ce(SO ) ꢀ 4H O have been
temperature factor in case of (II). Details on data
collections and refinements are given in Table 1. Further
details of the crystal structures may be obtained from the
Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany, on quoting the depository
number CSD–417358 for (I) and CSD–417357 for (II).
Molecular graphics: DIAMOND [20].
4
2
2
structurally characterized: one orthorhombic, space group
Pnma [10] (called the b-form), and one monoclinic, space
group C2/c [11]. An additional orthorhombic modifica-
tion, space group Fddd (called the a-form) has been known
to exist for decades [10,12a] and has been discussed [13,14],
but its crystal structure is first reported here.
2.3. Thermogravimetry
2
. Experimental
The thermogravimetry and differential scanning calori-
2
.1. Sample preparations
metry (TG–DSC) measurement was performed by a
NETZSCH STA 409 PC Luxx simultaneous thermal
analyzer. The samples were heated from room temperature
Single crystals of both the orthorhombic modifications
–
1
of the Ce(SO ) ꢀ 4H O product were achieved during
to 1000 1C at a heating rate of 5 1C min , kept at 1000 1C
4
2
2
–1
studies on the Ce(IV)/Ce(III)–Cr(VI)/Cr(III) redox system
15,16].
Ce(SO ) ꢀ 4H O, space group Fddd: A solution of
for 60 min and finally cooled down at 5 1C min . The
sample was measured in a dynamic nitrogen atmosphere
[
–
1
(gas flow rate 20 mL min ).
3. Result and discussion
3.1. Structural description
4
2
2
Ce(OH) (probably containing some CeO ) in concentrated
4
2
sulfuric acid, intended for synthetic use, was prepared.
3
After several months few rather big (2.0 ꢂ 0.18 ꢂ 0.14 mm )
orange-yellow single crystals were formed.
Ce(SO ) ꢀ 4H O, space group Pnma: Dried Ce(OH)
4
2
2
4
(0.83 g, 4.00 mmol), probably containing some CeO , and
CrO (0.80 g, 8.00 mmol) was dissolved in water (4.0 ml).
Here we present the crystal structure of two orthorhom-
bic polymorphs of Ce(SO ) ꢀ 4H O. The first modification
2
3
4 2
2
The solid residue was dissolved in a minimum amount
of sulfuric acid solution. After a month, many small
of Ce(SO ) ꢀ 4H O (I) crystallizes in the space group Fddd,
4
2
2
˚
with a ¼ 5.6587(1), b ¼ 12.0469(2), c ¼ 26.7201(3) A and
Z ¼ 8. The second modification (II), earlier described by
Lindgren [10] but without location of the hydrogen atoms,
crystallizes in the space group Pnma, with a ¼ 14.6019(2),
3
(
0.32 ꢂ 0.06 ꢂ 0.06 mm )
corn-formed
and
light-
yellow colored single crystals were obtained. Crystals of
the Pnma modification can also be achieved by hydro-
thermal treatment of Ce(SO ) ꢀ 4H O dissolved in sulfuric
˚
b ¼ 11.0546(2), c ¼ 5.6340(1) A and Z ¼ 4.
4
2
2

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1618
Table 1
Crystal data and structure refinement for the two orthorhombic
modifications (I) and (II)
antiprisms, shown in Fig. 2. The angles within the
tetragonal and triangular faces of (I) are ranging between
89.02(7)–90.94(7)1 and 55.88(6)–66.42(6)1, respectively. In
modification (II), the angles within the tetragonal and
triangular faces are ranging between 84.58(7)–92.00(11)1
and 54.99(8)–65.14(9)1, respectively. Comparing the co-
ordination environment of the Ce ions within the two
structures, one can see that they show geometrical
isomerism, given by the orientation of the water and
sulfate ligands with respect to the central cerium ion, as
shown in Figs. 1 and 2. In modification (I) the sulfate
ligands lie trans to each other in both the tetragonal faces,
while in modification (II) the sulfate ligands lie trans to
each other in one tetragonal face and cis in the other.
Both structures are build up by layers of Ce(H O) (SO ) ,
where the cerium ions are linked by sulfate bridges.
These layers are normal to the c-axis for (I) and normal
to the a-axis for (II), see Fig. 3. The layers are held together
by a hydrogen-bonding network (Table 3) between the
water molecules in one layer and the sulfate terminal
oxygen atoms in an adjacent layer, shown in Fig. 4. In
structure (I) half of the hydrogen bonds (O10ꢁH11?O2)
are acting within the layers while the other half
Identification code
Empirical formula
Formula weight
Temperature
Wavelength
(I)
(II)
Ce(SO
404.30
4
)
2
ꢀ 4H
2
O
Ce(SO
404.30
4
)
2
ꢀ 4H
2
O
22(2) 1C
0.71073 A
ꢁ90(2) 1C
˚
˚
0.71073 A
Crystal system
Space group
Unit cell
Orthorhombic
Fddd
Orthorhombic
Pnma
˚
˚
a ¼ 5.6587(1) A
a ¼ 14.6019(2) A
˚
˚
dimensions
b ¼ 12.0469(2) A
b ¼ 11.0546(2) A
˚
˚
c ¼ 26.7201(3) A
c ¼ 5.6340(1) A
3
3
˚
1821.50(5) A
˚
909.43(3) A
Volume
Z
8
2.949 Mg/m
4
2.953 Mg/m
3
3
Density
(calculated)
2
4
4 2
–1
–1
Absorption
coefficient
F(0 0 0)
5.516 mm
5.524 mm
1552
776
3
3
Crystal size
y range for data
collection
2.00 ꢂ 0.18 ꢂ 0.14 mm
3.05–32.941
0.32 ꢂ 0.06 ꢂ 0.06 mm
2.79–32.831
Index ranges
ꢁ8php8,
ꢁ22php22,
ꢁ16pkp16,
ꢁ8plp8
15220
ꢁ
18pkp18,
40plp40
ꢁ
Reflections
collected
7079
(
O10ꢁH12?O2) are acting between the layers. In structure
Independent
reflections
837 [R(int) ¼ 0.0314]
99.7%
1731 [R(int) ¼ 0.0380]
99.9%
(II) the O10ꢁH11?O4, O30ꢁH31?O3 and the
O20ꢁH21?O4 hydrogen bonds are all layer-connecting
while only the O20ꢁH22?O3 hydrogen bond is an inter
layer linkage.
Completeness to
y ¼ 30.501
Max. and min.
transmission
Refinement
method
0.5123 and 0.0320
Full-matrix least-
0.7328 and 0.2709
Full-matrix least-
3.2. Structurally related compounds
2
2
squares on F
837/0/45
squares on F
1731/7/88
Data/restraints/
parameters
Goodness-of-fit on
Three polymorphic forms of Ce(SO ) ꢀ 4H O have been
4
2
2
structurally characterized: two orthorhombic modifica-
tions, reported in this work, and one monoclinic. The
two orthorhombic modifications are called the a-form (I)
and the b-form (II) and they crystallize in space group Fddd
and Pnma, respectively. The crystal structure of the
monoclinic modification (space group C2/c) has been
solved and discussed by Filipenko et al. [11]. The structure
of the monoclinic polymorph Ce(SO ) ꢀ 4H O [11] is very
1.001
1.000
2
F
Final R indices
R
1
¼ 0.0186,
wR
¼ 0.0482
¼ 0.0203,
wR
R
1
¼ 0.0335,
wR
¼ 0.0895
¼ 0.0404,
¼ 0.0944
[I42s(I)]
R indices (all data)
2
2
R
1
R
1
2
¼ 0.0493
wR
2
Extinction
coefficient
0.00194(12)
0
Largest diff. peak
and hole
1.206 and
3.618 and
4
2
2
–3
–3
˚
˚
ꢁ1.064 e.A
ꢁ2.490 e.A
similar to the structure of the a-modification, structure (I).
Within the monoclinic modification the coordination
geometry about the two non-equivalent cerium ions is
square antiprismatic, the same conformation as the cerium
polyhedron in the a-form has (see Fig. 5) and the CeꢁO
In both orthorhombic modifications the cerium atoms
are surrounded by eight oxygen atoms, a coordination
which is expected for Ce(IV), see Fig. 1. In both structures
every cerium ion is in contact with four water molecules
and four sulfate groups and every sulfate group is in
contact with two cerium ions, the bonding distances are
˚
bonding distances average to 2.31(4) A.
All three polymorphs are built up by layers of
Ce(H O) (SO ) held together by a hydrogen bonding
2
4
4 2
network between the water molecules that are bound to the
central cerium ion and the sulfate group terminal oxygen
atoms. The packing features adopted by the monoclinic
form are very similar to the one adopted by the a-form (I),
while in the b-form (II), both the layers and the packing of
the layers are essentially different, see Fig. 6.
˚
given in Table 2. The CeꢁO distances average to 2.33(3) A,
˚
the CeꢁO(water) are slightly longer, 0.05(1) A, than the
CeꢁO(sulfur). The sulfate groups show a very small
departure from the ideal tetrahedral symmetry, the
˚
bridging oxygen bonds are 0.03(1) A longer than the
terminal oxygen bonds.
The cerium coordination polyhedra within (I) and (II)
are both forming slightly distorted square Archimedian
The Ce(SO ) ꢀ 4H O (I) bulk sample, (space group Fddd)
4
2
2
was analyzed by X-ray powder diffraction. The diffracto-
gram was compared to the only reference powder pattern

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1619
Fig. 1. Coordination geometry for the orthorhombic modifications (I) and (II). The displacement ellipsoids are drawn at 50% probability level.
Table 2
Bond lengths [A] for the two orthorhombic modifications
˚
(I)
(II)
Ce(1)–O(1) ( ꢂ 4)
Ce(1)–O(10) ( ꢂ 4)
2.2969(16)
2.3533(16)
Ce(1)–O(1) ( ꢂ 2)
Ce(1)–O(2) ( ꢂ 2)
Ce(1)–O(10)
2.312(3)
2.299(2)
2.335(4)
2.356(2)
2.375(4)
Ce(1)–O(20) ( ꢂ 2)
Ce(1)–O(30)
S(1)–O(1) ( ꢂ 2)
S(1)–O(2) ( ꢂ 2)
1.4881(16)
1.4620(18)
S(1)–O(1)
S(1)–O(2)
S(1)–O(3)
S(1)–O(4)
1.486(3)
1.491(3)
1.458(2)
1.461(3)
Fig. 3. The cerium polyhedra together with the bridging sulfate ligands
are forming layers parallel to the ab plane for the orthorhombic
modification (I) and parallel to the bc plane for (II).
Table 3
˚
Hydrogen bonds length in [A] and angles [1] for compound (I) and (II)
Compound D–HyA
d(D–H) d(HyA) d(DyA) /(DHA)
i
(
(
(
(
(
(
I)
O(10)–H(11)?O(2) 0.80(4) 2.02(4)
2.776(2) 158(4)
2.692(2) 170(6)
2.716(4) 156(4)
2.688(4) 173(5)
2.742(4) 159(5)
2.692(4) 168(4)
ii
I)
O(10)–H(12)?O(2) 0.77(5) 1.93(5)
iii
II)
II)
II)
II)
O(10)–H(11)?O(4) 0.80(3) 1.97(3)
iv
O(20)–H(21)?O(4) 0.82(3) 1.87(3)
v
O(20)–H(22)?O(3) 0.81(3) 1.97(3)
vi
O(30)–H(31)?O(3) 0.82(3) 1.88(3)
Symmetry transformations used to generate equivalent atoms: (i)
xꢁ1,ꢁy+5/4,ꢁz+5/4 (ii) xꢁ3/4,ꢁy+3/2,zꢁ1/4 (iii) x,y,zꢁ1 (iv) x+1/2,
y,ꢁz+3/2 (v) ꢁx+3/2,ꢁy,z+1/2 (vi) x+1/2,y,–z+1/2.
Fig. 2. Square antiprismatic coordination geometry about the Ce ions for
the orthorhombic modifications (I) and (II), respectively.
in the database [12a] on the Ce(SO ) ꢀ 4H O with space
an evidence of a pure material. The best fit matching all
experimental peaks was achieved with Ref. [12b], which is a
calculated pattern of the monoclinic modification of
Ce(SO ) ꢀ 4H O [11]; this is further stressing the similarity
4
2
2
group Fddd, but there was not a good agreement as several
peaks in the reference were missing. The diffractogram was
then compared with the calculated powder pattern from
the structure solution of (I) and the agreement was perfect,
4
2
2
among the two structures and also that there is a possibility

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1620
that the monoclinic modification may be a result of an
overlooked higher symmetry.
[12c,21] show the same space group for Hf(SO ) ꢀ 4H O. The
4
2
2
structure of U(SO ) ꢀ 4H O was the first of these structures
4
2
2
Another five heavy quadrivalent ions forming chemical
analogous orthorhombic compounds exist: Zr(SO ) ꢀ 4H O,
solved by X-ray single crystal diffraction [22] and it crystal-
lizes in space group Pnma as well as the structure of
Np(SO ) ꢀ 4H O, solved recently [23]. Pu(SO ) ꢀ 4H O is the
4
2
2
Hf(SO ) ꢀ 4H O, U(SO ) ꢀ 4H O, Np(SO ) ꢀ 4H O and Pu
4
2
2
4 2
2
4 2
2
4 2
2
4 2
2
(
SO ) ꢀ 4H O. Zirconium and hafnium disulfate tetrahydrate
only compound apart from Ce(SO ) ꢀ 4H O that has
4
2
2
4 2
2
both crystallize in space group Fddd. The structure of
Zr(SO ) ꢀ 4H O has been published [13] and it is isomorphic
proven to crystallize in both the orthorhombic space groups,
Fddd and Pnma. The structure of b-Pu(SO ) ꢀ 4H O, space
4
2
2
4 2
2
with the structure of a-Ce(SO ) ꢀ 4H O. Indexed powder data
group Pnma, has been determined [24] and it is isomorphic
with the structure of b-Ce(SO ) ꢀ 4H O and even the
4
2
2
4
2
2
hydrogen bonding patterns are the same within these two
structures. Indexed powder data [12d,24] confirm the
existence of the orthorhombic modification with space group
Fddd, which is isomorphic with the structure of
Zr(SO ) ꢀ 4H O, as shown by Jayadevan et al. [24], and to
4
2
2
the structure of a-Ce(SO ) ꢀ 4H O. As expected, the
4
2
2
infrared spectra of the a- and b-Pu(SO ) ꢀ 4H O did not
4
2
2
exhibit substantial differences [24]. The differences between
the a- and b-structure have been discussed by Singer and
Cromer [13] and by Jayadevan et al. [24]. Singer and Cromer
[13] have suggested the operation necessary to relate the Fddd
structure of Zr(SO ) ꢀ 4H O to the Pnma structure of
4
2
2
U(SO ) ꢀ 4H O, resulting in a puckering of the flat layers
4
2
2
of the Fddd-structure, shown in Figs. 4 and 6, and as a
consequence, the hydrogen bonding contacts being also
altered, see Fig. 4.
The b-Pu(SO ) ꢀ 4H O can be obtained by very slow
4
2
2
Fig. 4. Hydrogen bonding contacts within (I) and (II) mediated by the
cerium coordinating water molecules and the terminal sulfate oxygen
atoms.
heating of the a-Pu(SO ) ꢀ 4H O to 120 1C [24]. This a-b
4 2
2
transformation can also be achieved by hydrothermal
equilibration [24] similarly to the a-b transformation of
Ce(SO ) ꢀ 4H O.
4
2
2
3.3. Thermal behavior
The thermal behavior of the two orthorhombic mod-
ifications of Ce(SO ) ꢀ 4H O, the a-form (I) and the b-form
4
2
2
(II), have been studied by thermogravimetry (TG),
differential scanning calorimetry (DSC) and X-ray powder
thermodiffractometry, in situ and ex situ.
Both the modifications of Ce(SO ) ꢀ 4H O were heated up
4
2
2
Fig. 5. Comparison of the coordination polyhedra within the orthor-
hombic a-modification (I) and the monoclinic modification, space group
C2/c.
to 140 1C, in sealed autoclaves, in an attempt to obtain a
transformation between the two modifications, a2b, similar
Fig. 6. View of adjacent layers within: the a-form (I), the b-form (II) and the monoclinic modification, space group C2/c [11]. The b-form displays
puckered layers while the a-form and the monoclinic modification both have more compact and flatter layers.

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1621
to the a-b transformation observed for Pu(SO ) ꢀ 4H O
On the basis of TG results the thermal decomposition of
tetrahydrated cerium sulfate proceeds by the following
reactions:
4
2
2
at 120 1C [24]. This transformation was not observed, but a
a-b transformation was achieved by hydrothermal equilibra-
tion at 100 1C. Both the a- and the b-form can be gently heated
up to 90 1C to lose water giving crystalline Ce(SO ) (space
CeðSO Þ ꢀ 4H O ! CeðSO Þ þ 4H O
(1)
(2)
(3)
(4)
4
2
2
4 2
2
4
2
group Pbca) [12e] on cooling. After exposure to open
atmosphere for several days Ce(SO4)2 turns into b-Ce
2
CeðSO4Þ2 ! Ce2OðSO4Þ3 þ SO3
Ce2OðSO4Þ3 ! 2Ce3O2ðSO4Þ4 þ SO3
(
SO ) ꢀ 4H O. In the structure of Ce(SO ) each cerium atoms
4
2
2
4 2
3
is attached to eight sulfate groups and each sulfate group is
bridging to four cerium atoms [25]. Then it is not surprising
that Ce(SO ) is easily hydrated, considering the reluctance of
Ce O ðSO Þ ! 3CeO þ 4SO
3
2
4 4
2
3
4
2
the sulfate group towards a four fold bridging position [1,25].
The thermodiffractometry in situ showed that both the
a- and the b-forms turned into amorphous phases on slow
heating up to 300 1C. On cooling and exposure to open
atmosphere also these phases slowly turned into
b-Ce(SO ) ꢀ 4H O. The preference to form the b-phase
Due to the disagreement among many references on the
thermal decomposition of Ce(SO ) , its thermal behavior
was studied by heating in furnace (N gas flow) to different
temperatures in the range of 200–500 1C. The decomposi-
tion products were analyzed by X-ray powder diffracto-
metry. These experiments do not reveal any other
crystalline decomposition products than CeO(SO4)
[12g,26].
4
2
2
4
2
2
upon hydration of the Ce(SO ) (Pbca) [12e,25] may
4
2
arise from the breaking of the four weakest Ce–O–S bonds
forming four new Ce–OH2 bonds. Then an adjust-
ment of the coordination polyhedron will result in the
antiprismatic geometry found for cerium in b-Ce(SO ) ꢀ
The results from the X-ray powder diffraction analysis
suggest that also the following thermal decomposition
reactions may take place:
4
2
4
H O.
2
CeðSO4Þ2 ! CeOðSO4Þ þ SO3
Ce2OðSO4Þ3 ! 4CeOðSO4Þ þ 2SO3
Ce O ðSO Þ ! 3CeOðSO Þ þ SO
(5)
(6)
(7)
The thermal behaviors of both modifications were also
studied by TG and DCS. The endothermic dehydration of
2
a-Ce(SO ) ꢀ 4H O takes place in two steps giving approxi-
4
2
2
mately 13.5% and 4% weight loss, corresponding to three
and one water of hydration, respectively (Fig. 7). This
differs from the dehydration of b-Ce(SO ) ꢀ 4H O, which
3
2
4 4
4
3
4
2
2
Ying and Rudong [7] suggested that the observed weight
lost and the endothermic peak in the temperature range
450–550 1C may correspond to the reduction of anhydrous
takes place in one single step with about 19% lost of its
weight. After the dehydration process the heat capacity for
both the a- and b-forms increase, giving crystalline
Ce(SO ) . On further heating (from 380 1C up to 510 1C),
1
2
cerium(IV) sulfate, giving Ce (SO ) , SO and O . We also
2
2
4 3
3
4
2
studied and compared the thermal decompositions of
Ce (SO ) and Ce(SO ) by TG and DSC. The DSC curves
a two step endothermic process takes place where 8% plus
.5% of the weight is lost in the transformation of
Ce(SO ) into Ce O (SO ) and SO , with Ce O(SO ) as
2
4 3
4 2
5
as well as the enthalpy changes differ and the end product,
CeO2 [12f], is formed at a higher temperature for the
trivalent cerium sulfate. On the basis of all our experi-
ments, there are no indications that any reduction of the
4
2
3
2
4 4
3
2
4 3
a possible intermediate species. Finally, at 840 1C the end
products CeO [12f] and SO are formed, see Fig. 7.
2
3
–
1
Fig. 7. TG and DSC curves for 15 mg sample of a-Ce(SO
4
)
2
ꢀ 4H
2 2
O, in flowing high purity N at a heating rate of 5 1C min .

ARTICLE IN PRESS
B.M. Casari, V. Langer / Journal of Solid State Chemistry 180 (2007) 1616–1622
1622
Ce(IV) species takes place during the thermal decomposi-
tion process.
[10] O. Lindgren, Acta Chem. Scand. A 31 (1977) 453–456.
[11] O.S. Filipenko, L.S. Leonova, L.O. Atovmyan, G.V. Shilov, Dokl.
Akad. Nauk 360 (1) (1998) 73–76.
From a structural point of view the formation of
Ce O(SO ) during the first decomposition step is reason-
able. Within Ce(SO ) , the cerium atoms are arranged in
[
12] ICDD, International Centre for Diffraction Data, PDF-4, 2005
(
2
4 3
a) PDF 00-024-1250, (b) PDF 01-089-0823, (c) PDF 00-022-0621,
(d) PDF 00-015-0414, (e) PDF 01-070-2097, (f) PDF 00-043-1002,
(g) PDF 00-039-0515.
4
2
˚
pairs with a shorter Ce–Ce distance (o5 A) suitable for the
[
[
13] J. Singer, D.T. Cromer, Acta Crystallogr. 12 (1959) 719–723.
14] O.S. Filipenko, L.S. Leonova, L.O. Atovmyan, G.V. Shilov, Russ.
J. Coord. Chem. 25 (11) (1999) 804–810.
formation of an oxygen bridge between Ce atoms upon
losing SO molecules.
3
[
[
[
[
[
[
15] B.M. Casari, A.K. Eriksson, V. Langer, Z. Anorg. Allg. Chem. 632
Acknowledgment
(
2006) 101–106.
16] B.M. Casari, E. Wingstrand, V. Langer, J. Solid State Chem. 179
2006) 296–301.
(
The authors wish to thank Dr. Christopher S. Knee at
¨
the Department of Chemistry, Goteborg University, for
valuable supports and assistance with the TG–DSC
experiments.
17] Siemens SMART and SAINT, Siemens Analytical X-ray Instruments
Inc., Madison, Wisconsin, USA, 1995.
¨
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Germany, 2002.
19] Bruker, SHELXTL, Version 6.10, 2003, Bruker AXS Inc., Madison,
Wisconsin, USA.
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