New structure type among octahydrated rare-earth sulfates, β-Ce 2(SO4)3·8H2O, and a new Ce 2(SO4)3·4H2O polymorph

Article abstract of DOI:10.1002/zaac.200700003

Syntheses, crystal structures and thermal behavior of two new hydrated cerium(III) sulfates are reported, Ce₂(SO₄) ₃·4H₂O (I) and β-Ce₂(SO ₄)₃·8H₂O (II), both forming three-dimensional networks. Compound I crystallizes in the space group P2 ₁/n. There are two non-equivalent cerium atoms in the structure of I, one nine-and one ten-fold coordinated to oxygen atoms. The cerium polyhedra are edge sharing, forming helically propagating chains, held together by sulfate groups. The structure is compact, all the sulfate groups are edge-sharing with cerium polyhedra and one third of the oxygen atoms, belonging to sulfate groups, are in the S-Oμ₃-Ce₂ bonding mode. Compound II constitutes a new structure type among the octahydrated rare-earth sulfates which belongs to the space group Pn. Each cerium atom is in contact with nine oxygen atoms, these belong to four water molecules, three corner sharing and one edge sharing sulfate groups. The crystal structure is built up by layers of [Ce(H₂O)₄(SO₄)]_nⁿ⁺ held together by doubly edge sharing sulfate groups. The dehydration of II is a three step process, forming Ce₂(SO₄)₃·5H ₂O, Ce₂(SO₄)₃·4H₂O and Ce₂(SO₄)₃, respectively. During the oxidative decomposition of the anhydrous form, Ce₂(SO ₄)₃, into the final product CeO₂, small amount of CeO(SO₄) as an intermediate species was detected.

Full text of DOI:10.1002/zaac.200700003

DOI: 10.1002/zaac.200700003
New Structure Type among Octahydrated Rare-Earth Sulfates,
β-Ce2(SO4)3·8H2O, and a new Ce2(SO4)3·4H2O Polymorph
Barbara M. Casaria,* and Vratislav Langerb
Göteborg / Sweden, a Department of Chemistry, Inorganic Chemistry, Göteborg University and b Environmental Inorganic Chemistry,
Department of Chemical and Biological Engineering, Chalmers University of Technology
Received January 3rd, 2007.
Abstract. Syntheses, crystal structures and thermal behavior of two
new hydrated cerium(III) sulfates are reported, Ce2(SO4)3·4H2O (I)
and β-Ce2(SO4)3·8H2O (II), both forming three-dimensional net-
works. Compound I crystallizes in the space group P21/n. There
are two non-equivalent cerium atoms in the structure of I, one nine-
and one ten-fold coordinated to oxygen atoms. The cerium poly-
hedra are edge sharing, forming helically propagating chains, held
together by sulfate groups. The structure is compact, all the sulfate
groups are edge-sharing with cerium polyhedra and one third of the
oxygen atoms, belonging to sulfate groups, are in the SϪOµ3ϪCe2
bonding mode. Compound II constitutes a new structure type
among the octahydrated rare-earth sulfates which belongs to the
space group Pn. Each cerium atom is in contact with nine oxygen
atoms, these belong to four water molecules, three corner sharing
and one edge sharing sulfate groups. The crystal structure is built
nϩ
up by layers of [Ce(H2O)4(SO4)]n held together by doubly edge
sharing sulfate groups. The dehydration of II is a three step process,
forming Ce2(SO4)3·5H2O, Ce2(SO4)3·4H2O and Ce2(SO4)3, respec-
tively. During the oxidative decomposition of the anhydrous form,
Ce2(SO4)3, into the final product CeO2, small amount of CeO(SO4)
as an intermediate species was detected.
Keywords: Hydrated rare-earth sulfate; Cerium sulfate; Lanthan-
ides; Thermal behaviour; X-ray diffraction
Introduction
tures are those of hydrated binary and ternary sulfates [1].
The most common hydrated binary rare-earth sulfates are
the octahydrates, M2(SO4)3·8H2O, and many of them are
described structurally (M ϭ Y [6], Ce [8], Pr [9], Nd [10],
Sm [11], Eu [7], Gd [12], Tb [13], Dy [8,13], Er [14], Yb
[8], Lu [8]). Binary sulfates with lower water content are
uncommon, the crystal structure have been determined for
M2(SO4)3·5H2O (M ϭ Ce [8], Nd [15]), M2(SO4)3·4H2O
In the chemistry of the rare-earth compounds, complex
anions play an important role. Research interest in the area
of rare-earth sulfates derives from their application in the
separation of rare-earth elements and a large number of
complexes and salts have been described. A comprehensive
review of the lanthanide compounds with complex anions
was presented in 2002 by Wickleder [1] covering all data of
structurally characterized compounds known to that date.
Recently, during investigations of rare-earth frameworks,
advances in the synthesis of new inorganic materials [2]
with interesting applications [3] have been achieved. Al-
though a great number of complexes and salts have been
reported, the structural information on the rare-earth sul-
fates is still limited [1].
(M
ϭ La [16], Ce [17], Nd [16], Er [14]) and
Lu2(SO4)3·3H2O [8]. Sulfates with higher water content,
M2(SO4)3·9H2O, have been characterized for the larger lan-
thanide ions only, La [8] and Ce [18]. Structural information
on anhydrous binary rare-earth sulfates is limited and only
four compounds (M2(SO4)3, where M ϭ Y [5], Sc [5], Nd
[19], Er [4]) have been reported. Among the rare-earth salts,
the cerium(III) sulfate displays the greatest diversity in the
hydrate number. Within the binary system Ce2(SO4)3·xH2O,
solid phases with x ϭ 2 [20] , 4, 5, 8, 9, 12 [20] and 14 [21]
were reported.
The sulfates decompose at elevated temperatures and
cannot be obtained from their melt [1, 4, 5]. Thus, due to
the use of water as solvent, most of the known crystal struc-
The thermal behavior of hydrated binary rare-earth sul-
fates have been studied several times [20Ϫ26], mainly for
the octahydrates. According to these investigations, at first
an endothermic dehydration of M2(SO4)3·xH2O takes place
resulting in the anhydrous form, M2(SO4)3. During the de-
composition of the anhydrous form into the final product
M2O3, an intermediate M2O2(SO4) species is formed [1], ex-
cept for cerium(III) sulfate. The description of the thermal
behavior of Ce2(SO4)3 is ambiguous. Investigations on its
thermal decomposition have revealed several disagreements
[20Ϫ26]. Early studies [22Ϫ24] reported that Ce2(SO4)3 like
* Barbara M. Casari
Inorganic Chemistry, Department of Chemistry
Göteborg University
SE-412 96 Göteborg / Sweden
Phone: ϩ46(0)317722877
Fax: ϩ46(0)317722853
E-mail: casari@chem.gu.se
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
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Octahydrated Rare-Earth Sulfates, β-Ce2(SO4)3·8H2O
other rare-earth sulfates first decomposed into Ce2O2(SO4)
which, during further heating, in the final stage gave Ce2O3.
Udupa [21] made a thermal decomposition study on
Ce2(SO4)3·14H2O pointing out two dehydration steps in-
volving nine and five water molecules, respectively. On cool-
ing, Ce2(SO4)3 rehydrates to give Ce2(SO4)3·5H2O which
slowly converted into Ce2(SO4)3·14H2O on exposure to
open atmosphere for several days [21]. The studies made by
Udupa [21] showed that the anhydrous CeIII sulfate is very
stable and undergoes oxidative decomposition in one single
step giving CeO2. Tagawa [25] confirmed the same final
product, CeO2. Recent research by Poston et al. [26] pro-
poses that the decomposition environment of air or nitro-
gen has an effect on the decomposition process of
Ce2(SO4)3. Poston et al. [26] detected, in ambient atmos-
phere, many intermediate phases, various oxysulfides, ox-
ides and one oxysulfate. On further heating these converted
into CeO2. In nitrogen environment, the decomposition
end-product was identified as Ce2O3 [26].
The thermal behavior of Ce(SO4)2 is ambiguous as well.
Udupa [21] showed that anhydrous cerium(IV) sulfate con-
verts into CeO2·2Ce(SO4)2 which in the final stage loses sul-
fur dioxide and oxygen to give CeO2. Tagawa [25] suggested
one single decomposition step from Ce(SO4)2 to the final
stage: CeO2. Ying and Rudong [27] showed evidence that the
observed weight lost in the temperature range 450Ϫ495 °C
corresponds to the reduction of anhydrous cerium(IV) sul-
fate, giving Ce2(SO4)3. This reduction process has also been
noticed by Zhao [28] and confirmed by Poston et al. [26]
Our studies [29] did not show any evidence for a reductive
decomposition step and proposes that only intermediate
CeIV species, xCeO2·yCe(SO4)2, are formed during the de-
composition of Ce(SO4)2 into the final product CeO2.
In this work two new polymorphic forms of hydrated
rare-earth sulfates, Ce2(SO4)3·4H2O and Ce2(SO4)3·8H2O,
have been structurally characterized and the thermal behav-
iour of Ce2(SO4)3·8H2O has been examined.
Fig. 1 Coordination geometry for Ce1 and Ce2 in I. The displace-
ment ellipsoids are drawn at 50 % probability level.
˚
Table 1 Bond lengths /A for I.
Ce1ϪO11
Ce1ϪO31ii
Ce1ϪO32
Ce1ϪO33
Ce1ϪO34iii
Ce1ϪO41
Ce1ϪO44
Ce1ϪO51i
Ce1ϪO52
Ce1ϪO53
Ce2ϪO21
Ce2ϪO22
Ce2ϪO23
Ce2ϪO32v
Ce2ϪO33iv
Ce2ϪO41v
Ce2ϪO42
Ce2ϪO43
Ce2ϪO53iv
2.582(2)
2.456(2)
2.542(2)
2.787(2)
2.561(2)
2.879(2)
2.551(2)
2.447(2)
2.618(2)
2.563(2)
2.506(2)
2.490(3)
2.478(2)
2.585(2)
2.599(2)
2.504(2)
2.683(2)
2.551(2)
2.465(2)
S3ϪO31
S3ϪO32
S3ϪO33
S3ϪO34
S4ϪO41
S4ϪO42
S4ϪO43
S4ϪO44
S5ϪO51
S5ϪO52
S5ϪO53
S5ϪO54
1.461(2)
1.504(2)
1.490(2)
1.458(2)
1.491(2)
1.473(2)
1.485(2)
1.476(2)
1.466(2)
1.480(2)
1.513(2)
1.457(2)
Symmetry transformations for equivalent atoms:
(i): Ϫxϩ1,Ϫyϩ2,Ϫz (ii): Ϫxϩ1/2,yϪ1/2,Ϫzϩ1/2
(iii): Ϫxϩ1/2,yϩ1/2,Ϫzϩ1/2 (iv): xϪ1/2,Ϫyϩ3/2,zϪ1/2
(v): Ϫx,Ϫyϩ1,Ϫz
The three sulfate groups show a small departure from the
ideal tetrahedral symmetry and the SϪO distances (Table
1) average to 1.48(2) A. There is only one terminal oxygen
˚
Result and Discussion
atom (O54) within the sulfate groups, all other sulfate oxy-
gen atoms are bridging to one or two cerium atoms. The
S3Ϫgroup is edge sharing with the Ce1Ϫpolyhedron, with
the edge sharing oxygen atoms (O32, O33) in the µ3 bond-
ing mode and the other two oxygen atoms in the µ2 bonding
mode. The S4Ϫgroup is edge sharing with both the Ce1Ϫ
and the Ce2Ϫpolyhedra, with one oxygen atom (O41) in
the µ3 bonding mode and three oxygen atoms in the µ2
bonding mode. The S5Ϫgroup, with one terminal, one µ3
bonding (O53) and two µ2 bonding oxygen atoms, is edge
sharing with the Ce2 polyhedron. As expected, the SϪO
Structural description of Ce2(SO4)3·4H2O (I)
The tetrahydrated cerium(III) sulfate (I) crystallizes in the
space group P21/n with a ϭ 12.3119(2), b ϭ 7.1368(1), c ϭ
˚
14.2321(3) A, β ϭ 90.629(1) and Z ϭ 4. There are two ce-
rium atoms, three sulfate groups and four water molecules
in each asymmetric unit. The Ce1 atom is in contact with
ten oxygen atoms: they belong to one water molecule, three
corner sharing and three edge sharing sulfate groups (Fig.
1). For Ce2 the coordination sphere consists of nine oxygen
atoms from three water molecules, four corner sharing and
one edge sharing sulfate groups (Fig. 1). Coordination
numbers such as ten (Ce1) and nine (Ce2) are expected for
the large Ce3ϩ ion. The Ce1ϪO and Ce2ϪO distances aver-
˚
distances (average: 1.47(1) A) within the SϪOµ2ϪCe bonds
˚
are slightly shorter than SϪO distances (average: 1.50(1) A)
within the SϪOµ3ϪCe2 bonds. This compound forms an
intricate three dimensional network (Fig. 2a), each Ce1
atom is connected through sulfate bridges to five Ce1 and
six Ce2 atoms and each Ce2 atom is bridging through sul-
˚
age to 2.60(14) and 2.54(7) A, respectively (Table 1). The
bond-valence sums are 2.90 and 2.94 v.u. for Ce1 and Ce2,
respectively (using parameter from Trzesowska et al. [30]).
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B. M. Casari, V. Langer
Fig. 3 Helical strings of edge sharing cerium polyhedra intra con-
nected by the S3 sulfate groups, within structure I.
˚
Table 2 Hydrogen bonds for I/A and °.
DϪH···A
d(DϪH)
d(H···A)
d(D···A)
<(DHA)
O11ϪH11A···O22i
O11ϪH11B···O42ii
O21ϪH21A···O54iii
O21ϪH21B···O44iv
O22ϪH22A···O11v
O22ϪH22B···O43v
O23ϪH23A···O52iv
O23ϪH23B···O54vi
0.76(2)
0.76(2)
0.76(2)
0.78(2)
0.77(2)
0.76(2)
0.78(2)
0.77(2)
2.31(3)
2.05(3)
2.03(3)
2.31(3)
1.99(3)
2.24(5)
1.97(3)
1.94(3)
3.055(4)
2.737(3)
2.759(3)
3.076(3)
2.705(4)
2.758(3)
2.741(3)
2.699(3)
166(5)
151(5)
162(5)
171(5)
154(5)
126(5)
172(5)
169(6)
Symmetry transformations used to generate equivalent atoms:
(i): Ϫx,Ϫyϩ1,Ϫz (ii): x,yϩ1,z (iii): Ϫxϩ1,Ϫyϩ1,Ϫz
(iv): Ϫxϩ1/2,yϪ1/2,ϪzϪ1/2 (v): x,yϪ1,z (vi): xϪ1/2,Ϫyϩ1/2,zϪ1/2
The crystal structure for one polymorph and three
chemical analogues to I have been described earlier:
M2(SO4)3·4H2O (M ϭ Ce [17], La [16], Nd [16], Er [14]).
These compounds, except for the erbium [14] salt, all crys-
tallizes in the same space group (P21/n) but they belong
to two different structure types. The structure of I and the
lanthanum salt are isomorphic (structure type A). The ce-
rium polymorph described by Deregine et al. [17] is isomor-
phic with the neodymium [16] salt (structure type B). All
these M2(SO4)3·4H2O compounds form three dimensional
networks.
Fig. 2 Packing diagrams, views along 21 axes. a) Compound I
(type A) showing the three dimensional network. b) Structure of
Ce2(SO4)3·4H2O, type B [17].
fate groups to six Ce1 and three Ce2 atoms. The cerium
polyhedra are edge sharing as well, each Ce1 polyhedron
shares two edges with two Ce2 polyhedra and vice versa,
shown in Figure 2a as groups of four cerium polyhedra in
the ac plane. The edge sharing cerium polyhedra form
chains propagating helically in the b direction, shown in
Figure 3. The S3 sulfate groups are binding within those
chains only, while the S4 and S5 sulfate groups are con-
necting each chain to neighboring chains. In structure of I
most of the hydrogen bonding contacts are connecting
chains. The O11ϪH11AϪO22 and O22ϪH22BϪO11 hy-
drogen bonds are acting within the chains while the all the
remaining contacts are inter-chain links (Table 2). No hy-
drogen contacts are found to the µ3 bonding oxygen atoms.
¯
The structure of Er2(SO4)3·4H2O [14] (space group P1)
differs essentially from the other structures, it is less com-
pact and does not show any edge sharing between the metal
polyhedra. There are two nonequivalent erbium ions, one
with seven-fold coordination binding to six sulfate groups
and one water molecule and one with eight-fold coordi-
nation binding to five sulfate groups and three water mol-
ecules. The three sulfate groups are bridging by corner shar-
ing only.
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Octahydrated Rare-Earth Sulfates, β-Ce2(SO4)3·8H2O
˚
Table 3 Bond lengths /A for II.
Ce1ϪO11
Ce1ϪO12
Ce1ϪO13
Ce1ϪO14
Ce1ϪO31
Ce1ϪO32
Ce1ϪO51
Ce1ϪO52i
Ce1ϪO54
Ce2ϪO21
Ce2ϪO22
Ce2ϪO23
Ce2ϪO24
Ce2ϪO33iii
Ce2ϪO34iii
Ce2ϪO41
Ce2ϪO43ii
Ce2ϪO44
2.474(3)
2.492(3)
2.610(3)
2.653(3)
2.540(3)
2.767(3)
2.437(3)
2.423(3)
2.506(3)
2.463(3)
2.524(3)
2.695(3)
2.595(3)
2.669(3)
2.555(3)
2.498(3)
2.427(3)
2.432(3)
S3ϪO31
S3ϪO32
S3ϪO33
S3ϪO34
S4ϪO41
S4ϪO42
S4ϪO43
S4ϪO44iv
S5ϪO51
S5ϪO52
S5ϪO53
S5ϪO54v
1.478(3)
1.485(3)
1.475(3)
1.475(3)
1.477(3)
1.470(3)
1.468(3)
1.473(3)
1.471(3)
1.470(3)
1.475(3)
1.479(3)
Fig. 4 Coordination geometry for Ce1 and Ce2 in II. The displa-
cement ellipsoids are drawn at 50 % probability level.
The structure type A shows many similarities with the
structure type B. Both these structure types are rather com-
pact and contain edge sharing metal polyhedra. They both
also contain edge sharing sulfate groups, but the structure
of type B does not contain any doubly edge sharing sulfate
group as the structure of type A. These structures also dif-
fer in the coordination geometry of the metal ions: they are
ten and nine coordinated in the structure type A, while both
non-equivalent metal ions are nine coordinated in the struc-
ture type B. One metal ion is binding to five sulfur groups
(one is bidentate) and two water molecules, the other metal
ion is binding to six sulfate groups (one is bidentate) and
one water molecule, both ions are also sharing one water
molecule. These differences result in different packing fea-
tures for these two structure types, compare Figure 2a (type
A) and Figure 2b (type B).
Symmetry transformations for equivalent atoms:
(i): xϩ1/2,Ϫyϩ1,zϩ1/2 (ii): xϪ1,y,z (iii): xϩ1,y,zϩ1
(iv): xϩ1/2,Ϫyϩ2,zϪ1/2 (v): xϪ1/2,Ϫyϩ1,zϩ1/2
˚
Table 4 Hydrogen bonds for II/A and °.
DϪH···A
d(DϪH)
d(H···A)
d(D···A)
<(DHA)
O11ϪH11A···O53vi
O11ϪH11A···O31iv
O11ϪH11B···O14iv
O12ϪH12A···O22
O12ϪH12B···O42ii
O13ϪH13A···O54i
O13ϪH13B···O53
O14ϪH14A···O33vii
O14ϪH14B···O13iii
O21ϪH21A···O42
O21ϪH21B···O23viii
O22ϪH22A···O32vii
O22ϪH22B···O53i
O23ϪH23A···O24v
O23ϪH23B···O32vii
O24ϪH24A···O41ix
O24ϪH24B···O42v
0.80(3)
0.80(3)
0.80(3)
0.81(2)
0.81(2)
0.83(3)
0.82(3)
0.82(3)
0.84(3)
0.81(3)
0.80(3)
0.82(3)
0.82(2)
0.81(3)
0.82(3)
0.82(2)
0.80(2)
2.14(5)
2.35(6)
2.12(3)
2.08(3)
1.93(3)
2.06(4)
1.90(3)
1.97(3)
2.15(3)
2.07(4)
2.03(3)
1.91(3)
1.89(3)
2.13(4)
2.02(3)
2.04(3)
1.96(3)
2.789(5)
2.886(5)
2.881(5)
2.871(5)
2.724(4)
2.830(4)
2.672(4)
2.785(4)
2.971(5)
2.782(5)
2.806(5)
2.727(5)
2.681(4)
2.911(5)
2.816(4)
2.827(4)
2.729(4)
138(7)
125(6)
159(6)
163(7)
169(6)
156(7)
156(6)
171(7)
165(7)
146(6)
165(8)
172(7)
162(6)
161(6)
163(7)
163(7)
163(6)
Structural description of Ce2(SO4)3·8H2O (II)
The octahydrated cerium(III) sulfate, (II) crystallizes in the
space group Pn with a ϭ 6.8502(2), b ϭ 17.2693(5), c ϭ
˚
6.8747(2) A, β ϭ 92.587(1) and Z ϭ 2. There are two ce-
Symmetry transformations used for equivalent atoms:
(i): xϩ1/2,Ϫyϩ1,zϩ1/2 (ii): xϪ1,y,z (iii): xϪ1/2,Ϫyϩ1,zϩ1/2
(iv): xϩ1,y,z (v): xϪ1/2,Ϫyϩ2,zϩ1/2 (vi): xϩ1/2,Ϫyϩ1,zϪ1/2
(vii): x,y,zϩ1 (viii): x,y,zϪ1 (ix): xϪ1/2,Ϫyϩ2,zϪ1/2
rium atoms, three sulfate groups and eight water molecules
in each asymmetric unit. Each cerium atom is in contact
with nine oxygen atoms, these belong to four water mol-
ecules, three corner sharing and one edge sharing sulfate
groups (Fig. 4). Nine-fold coordination is the most com-
mon coordination for Ce3ϩ ions. The CeϪO distances aver-
in the b direction, by the doubly edge sharing S3Ϫgroups,
forming together the three dimensional network (Fig. 6).
There are sixteen different hydrogen bonding contacts
within the network (Table 4, Fig. 7), ten of them acting
within the layers while the rest of them are acting between
the layers.
The octahydrates prevail in the system of M2(SO4)3·xH2O
and many octahydrated rare-earth sulfates have been struc-
turally characterized [6Ϫ14]. The previously reported com-
pounds are all isostructural members of the well known
rare-earth octahydrates sulfate structure type (type α) and
they all crystallizes in space group C2/c. Temporary interest
in these compounds was inspired by the supposed existence
of pyroelectricity in crystals of rare-earth octahydrates, indi-
cating that some crystals may belong to a non-centrosym-
metric space group [6, 31] The structure of II is non-centro-
˚
age to 2.54(10) A, see Table 3. The bond-valence sums are
2.97 and 2.98 v.u. for Ce1 and Ce2, respectively (using par-
ameter from Trzesowska et al. [30]). The SϪO distances
within the three sulfate groups (Table 3) average to
˚
1.475(5) A. The S3Ϫgroup is edge sharing both with the
Ce1 and with the Ce2 polyhedra. The S4- and S5-groups
are corner sharing to three Ce1 and three Ce2 polyhedra,
respectively. Two of the sulfates (S4, S5) show almost ideal
tetrahedral symmetry while the S3-group shows smaller
angles 105.04(17)° and 106.00(18)° towards the edge shar-
ing Ce1Ϫ and Ce2 polyhedra, respectively. The crystal
nϩ
structure is built up by layers of [Ce1(H2O)4(S5O4)]n
alternating with layers of [Ce2(H2O)4(S4O4)]nnϩ, parallel to
the ac plane, see Figure 5. These layers are held together,
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B. M. Casari, V. Langer
Fig. 7 Packing diagram, showing part of the hydrogen bonding
network. There are sixteen different hydrogen bonding contacts
within compound II most of them are contacts toward terminal
sulfate oxygen atoms or to water molecules.
nϩ
Fig. 5 Layers of [Ce2(H2O)4(S4O4)]n
Fig. 6 Packing diagram of compound II showing the three dimen-
nϩ
Fig. 8 Packing diagram, structure of α-Ce2(SO4)3·8H2O [8].
sional network, from the top layers of: [Ce2(H2O)4(S4O4)]n
,
[Ce1(H2O)4(S5O4)]nnϩ and [Ce2(H2O)4(S4O4)]nnϩ, held together by
(S3O4)2Ϫ groups.
cerium atoms in II are in contact with nine oxygen atoms.
The sulfate groups in structure type α are all linking by
corner sharing, while one of the sulfate groups is doubly
edge sharing with two metal polyhedra in structure type β.
As a consequence, two very different structural packing
are featured. The structure type α forms layers of
symmetric (space group Pn), and represents a new structure
type among the rare-earth octahydrates, type β. The struc-
ture types α and β differ significantly. In the structure type
α, the metal atoms have eight-fold coordination while the
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Z. Anorg. Allg. Chem. 2007, 1074Ϫ1081

Octahydrated Rare-Earth Sulfates, β-Ce2(SO4)3·8H2O
process the heat capacity increases, possibly to give crystal-
line Ce2(SO4)2, up to 450 °C, then the heat capacity de-
creases. At 730 °C the decomposition process starts, pro-
ceeding to 920 °C. During this distinct oxidative decompo-
sition step, 31.4 % of the weight is lost, see Figure 9 and
Table 5. The end product, CeO2, was confirmed by X-ray
powder diffraction [32b]. On the basis of TG results the
thermal decomposition of II proceeds by the following reac-
tions:
Ce2(SO4)3·8H2O Ǟ Ce2(SO4)3 ϩ 3H2O ϩ H2O ϩ 4H2O
Ce2(SO4)3 Ǟ 2CeO2 ϩ 2SO3 ϩ SO2
(1)
(2a)
(2b)
Fig.
9 TG and DSC curves for 29 mg sample of
1
β-Ce2(SO4)3·8H2O, in flowing high purity N2 at a heating rate of
SO3 Ǟ SO2 ϩ /2O2
10 °C minϪ1
The ex situ X-ray powder thermodiffractometry (by heat-
ing in furnace with N2 flush, Table 5) showed traces of
CeO(SO4) [32c, 34] (also observed by Poston et al. [26]) al-
ready at 350 °C and traces of CeO2 [32b] at 480 °C. An
oxidation encountered at lower temperature in the furnace
experiment may be due to the lower heating rate or to the
more oxidizing environment of the furnace. On the other
hand, the presence of SO3 itself might be enough to contrib-
ute to the CeIII oxidation in both cases. Based on results
from the furnace experiments and X-ray powder diffraction,
the thermal decomposition of II may proceed by the follow-
ing reactions:
Table 5 Thermal behaviour
TGϪDSC
reaction
1a
1b
1c
Weight Temperature
Peak max Peak area
loss
range /°C
50Ϫ100
°C
kJ/g
0.17
Ϫ
7.8 %
2.6 %
9.7 %
105
135
250
890
105Ϫ150
150Ϫ270
0.19
1.4
2a and 2b
31.4 % 730Ϫ920
Ex situ reaction
1a and 2b
Temperature range /°C
22
1c
3
150Ϫ250
350Ϫ480
480Ϫ
4 and/or 5
Ce2(SO4)3 Ǟ 2CeO(SO4) ϩ SO2
Ce2(SO4)3 Ǟ 2CeO2 ϩ 2SO3 ϩ SO2
CeO(SO4) Ǟ CeO2 ϩ SO3
(3)
(4)
(5)
M2(SO4)3·8H2O connected by hydrogen bonds (Fig. 9),
while the structure of II forms a three-dimensional network,
Figures 6 and 7. Compound II has a slightly higher density
than α-Ce2(SO4)3·8H2O.
The diffractogram on the bulk material of II was com-
pared with the calculated powder pattern from the structure
solution of II and the agreement was perfect, an evidence
of a pure material. The best fit matching the experimental
peaks was achieved with a powder pattern reference [32a]
of an assumed orthorhombic modification, indexed by Pan-
netier and Dereigne [33] (space group Cmca); we believe that
this compound is the same as II.
According to these experiments, Ce2(SO4)3 [32d] un-
dergoes an oxidative decomposition and no other crystal-
line compounds than traces of CeO(SO4) as an intermedi-
ate, and CeO2 as the only final product, were detected.
Since there were indication on that the decomposition of
Ce(SO4)2 could take place through a reductive step, forming
Ce2(SO4)3 as an intermediate [27, 28], we also studied and
compared the thermal decompositions of Ce2(SO4)3 and
Ce(SO4)2 by TG and DSC. The DSC curves as well as the
enthalpy changes differ and the end product, CeO2, is
formed at a lower temperature for the tetravalent cerium
sulfate.
Thermal behaviour
The thermal behaviour of β-Ce2(SO4)3·8H2O (II) has been
studied by thermogravimetry (TG), differential scanning
calorimetry (DSC) and X-ray powder thermodiffractome-
try, in situ and ex situ.
The TG and DSC analysis showed that the endothermic
dehydration of II takes place in three steps, on heating up
to 300 °C, giving approximately 7.8 %, 2.6 % and 9.7 %
weight loss, corresponding to three, one and four water of
hydration, respectively (Fig. 9, Table 5). However, we
know from our earlier work with the α- and the
β-Ce(SO4)2·4H2O, that the dehydrations steps may be dif-
ferent for the α- and β-forms [29]. After the dehydration
The thermodiffractometry in situ (evacuated sample
chamber) on compound II showed some traces of its poly-
morphic form αϪCe2(SO4)3·8H2O [32e, 8] at 70 °C. Small
amounts of Ce2(SO4)3·5H2O [32f, 8] started to form at
80 °C and traces of Ce2(SO4)3·4H2O [32g, 17] (type B) was
detectable at 85 °C, together with the former hydrates. The
hydrates turned into amorphous phases on slow heating up
to 100 °C. Due to the detection of the polymorphic form,
several experiments were carried out to obtain a β Ǟ α
phase transition, but all of them resulted in
a
Ce2(SO4)3·4H2O [32g, 17] (type B) ϩ 4H2O. At ambient
Z. Anorg. Allg. Chem. 2007, 1074Ϫ1081
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1079

B. M. Casari, V. Langer
the position of 8192 reflections with I > 10σ(I) after integration of
all the data using SAINT (Siemens, 1995) [37]. The data were cor-
rected empirically for absorption and other effects using SADABS
(Sheldrick, 2002) [38]. The structures were solved by direct methods
and refined by full-matrix least squares on all F2 data using
SHELXTL (Bruker, 2001) [39]. The non hydrogen atoms were re-
fined anisotropically. The hydrogen atoms were located from differ-
ence Fourier maps and refined isotropically with restrained com-
mon OϪH distances and a common temperature factor. The
crystal of compound (II) was twinned, twin law: [(0 0 1) (0 Ϫ1 0)
(1 0 0)], and the twin volume ratio was refined to 0.9046(6) /
0.0954(6).
temperature and humidity, β-Ce2(SO4)3·8H2O easily dehy-
drates to form Ce2(SO4)3·4H2O [32g, 17] (type B).
Experimental Section
Sample preparations
Ce2(SO4)3·4H2O, space group P21/n: Single crystals of
Ce2(SO4)3·4H2O were achieved during studies on the CeIV/CeIII
Ϫ
CrVI/CrIII redox system [35, 36]. Ce(SO4)2·4H2O (3.0 g, 7.5 mmol)
was dissolved in water (20 mL) and Ce(OH)4 was precipitated with
15 M ammonia. Dried Ce(OH)4 (0.83 g, 4.00 mmol), probably con-
taining some CeO2, and CrO3 (0.80 g, 8.00 mmol) was dissolved in
water (4.0 mL). Sulfuric acid was added until the solid residue was
dissolved. After a month many small corn-formed and light-yellow
colored single crystals of Ce(SO4)2·4H2O (Pnma) were obtained
[29]. This sample was then left covered and unguarded and after a
long period many very many small (0.08 ϫ 0.06 ϫ 0.02 mm) rhom-
bic-formed and uncolored single crystals were obtained.
Details on data collections and refinements are given in Table 6.
Further details of the crystal structures may be obtained from the
Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopold-
shafen, Germany, on quoting the depository number CSDϪ417417
for I and CSDϪ417418 for II. Molecular graphics: DIAMOND
(Brandenburg, 2001) [40].
Ce2(SO4)3·8H2O, space group Pn: Recrystallization at ambient at-
mosphere and temperature of Ce2(SO4)3·xH2O (P.A. quality, manu-
factured by Aldrich).
Thermogravimetry
The thermogravimetry and differential scanning calorimetry
(TGϪDSC) measurement was performed by a NETZSCH STA 409
PC Luxx simultaneous thermal analyzer. The samples were heated
from room temperature to 1000 °C at a heating rate of 10 °C
minϪ1, kept at 1000 °C for 60 min and finally cooled down at 10 °C
minϪ1. The sample was measured in a dynamic nitrogen atmos-
phere (gas flow rate 20 ml minϪ1).
Single crystal X؊ray analysis
Data were collected using a Siemens SMART CCD diffractometer
equipped with a Siemens LTϪ2A low temperature device, at
Ϫ100 °C. Full spheres of the reciprocal space were scanned by 0.3°
steps in ω with a crystal-to-detector distance of 3.97 cm and expo-
sure time per frame, being 15 s. Preliminary orientation matrices
were obtained using SMART (Siemens,1995) [37]. The collected
frames were integrated with the orientation matrices updated every
100 frames. Final cell parameters were obtained by refinement on
X-ray powder diffraction
X-ray powder diffraction was made using a Siemens D 5000 dif-
fractometer in parallel beam geometry, with X-ray source and
Table 6 Crystal data and structure refinement for I and II.
Empirical formula
Formula weight
Temperature
Ce2(SO4)3·4H2O
640.48
Ce2(SO4)3·8H2O
712.55
Ϫ100 °C
Ϫ100 °C
˚
˚
Wavelength
0.71073 A
0.71073 A
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/n
a ϭ 12.3119(2) A
b ϭ 7.1368(1) A
monoclinic
Pn
a ϭ6.8502(2) A
b ϭ 17.2693(5) A
˚
˚
˚
˚
˚
˚
c ϭ 14.2321(3) A
c ϭ 6.8747(2) A
β ϭ 90.629(1)°
β ϭ 92.587(1)°
3
3
˚
˚
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
θ range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to θ ϭ 32.00°
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (alldata)
Largest diff. peak and hole
Absolute structure parameter
1250.46(4) A
4
812.44(4) A
2
3.402 Mg/m3
7.780 mmϪ1
1200
2.913 Mg/m3
6.020 mmϪ1
680
0.08 ϫ 0.06 ϫ 0.02 mm3
2.18 to 32.00°.
0.06 ϫ 0.04 ϫ 0.02 mm3
1.18 to 33.10°.
Ϫ18Յ h Յ18, Ϫ10Յ k Յ10, Ϫ21Յ l Յ21
20400
Ϫ10Յ h Յ10, Ϫ26Յ k Յ25, Ϫ10Յ l Յ10
14756
4329 [R(int) ϭ 0.0370]
99.7 %
5764 [R(int) ϭ 0.0294]
100.0 %
0.8599 and 0.5749
0.8891 and 0.7140
Full-matrix least-squares on F2
4329 / 12 / 217
Full-matrix least-squares on F2
5764 / 26 / 278
0.999
1.017
R1 ϭ 0.0236, wR2 ϭ 0.0491
R1 ϭ 0.0361, wR2 ϭ 0.0538
0.763 and Ϫ1.009 e.A
R1 ϭ 0.0236, wR2 ϭ 0.0580
R1 ϭ 0.0249, wR2 ϭ 0.0588
0.682 and Ϫ1.898 e.A
Ϫ3
Ϫ3
˚
˚
Ϫ0.052(13), 2823 Friedel pairs
1080
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Z. Anorg. Allg. Chem. 2007, 1074Ϫ1081

Octahydrated Rare-Earth Sulfates, β-Ce2(SO4)3·8H2O
sample in fixed position and a moving detector (2θϪscan) in order
to identify material during thermodiffractometry both, in situ and
ex situ.
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Z. Anorg. Allg. Chem. 2007, 1074Ϫ1081
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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