Synthesis, magnetic structure, and properties of a layered cobalt-hydroxide ferromagnet, Co5(OH)6(SeO4)2(H 2O)4

Article abstract of DOI:10.1021/ic9025552

The synthesis and nuclear and magnetic structures from the powder diffraction of Co^II₅(OH)₆(SeO₄) ₂(H₂O)₄ and its deuterated analogues as well as their infrared spectral, thermal, and magnetic properties are reported. The nuclear structure consists of brucite-like cobalt hydroxide layers connected by ...OSeO₃-Co(H₂O)₄-O₃SeO... bridges. The two independent cobalt atoms within the layer are arranged in chains along the b axis creating an anisotropy within each layer. The interlayer distance (10.718 A) is the only parameter to increase compared to the sulfate analogue (10.273 A). The infrared spectra and thermal properties are similar to those reported for the sulfate analogue. Due to the ferromagnetic exchange between the nearest-neighbor cobalt atoms within the layer, satisfying the Goodenough-Kanamori rule, and the weak interlayer exchange, an overall ferromagnet is obtained. The ferromagnetic order at 9 K was confirmed by the ac susceptibilities, the saturation magnetization, and most importantly the enhancement of some Bragg diffraction peaks below the Curie temperature. The moments of all the cobalt atoms were found to be aligned along the b axis with a moment of 3.25(8) μ_B each giving the best fit. The increase in layer distance and the electron density by replacing sulfur by selenium lowers the Curie temperature.

Full text of DOI:10.1021/ic9025552

Inorg. Chem. 2010, 49, 3019–3024 3019
DOI: 10.1021/ic9025552
Synthesis, Magnetic Structure, and Properties of a Layered Cobalt-Hydroxide
Ferromagnet, Co (OH) (SeO ) (H O)
5
6
4 2
2
4
†
,†
‡
,§
Wassim Maalej, Serge Vilminot,* Gilles Andr ꢀe , and Mohamedally Kurmoo*
†
D eꢀ partement Chimie des Mat eꢀ riaux Inorganiques, IPCMS, UMR 7504 (CNRS-UDS), 23 rue du Loess,
‡
BP 43, 67034 Strasbourg Cedex 2, France, Laboratoire L eꢀ on Brillouin, CEA-CNRS, CEA Saclay, 91191 Gif
sur Yvette Cedex, France, and Laboratoire de Chimie de Coordination Organique, CNRS-UMR 7140,
§
Universit eꢀ de Strasbourg, Institut Le Bel, 4 rue Blaise Pascal, 67000 Strasbourg Cedex 01, France
Received December 22, 2009
II
5
The synthesis and nuclear and magnetic structures from the powder diffraction of Co (OH) (SeO ) (H O) and its
6
4 2
2
4
deuterated analogues as well as their infrared spectral, thermal, and magnetic properties are reported. The nuclear
structure consists of brucite-like cobalt hydroxide layers connected by ...OSeO -Co(H O) -O SeO... bridges. The
two independent cobalt atoms within the layer are arranged in chains along the b axis creating an anisotropy within
3
2
4
3
˚
each layer. The interlayer distance (10.718 A) is the only parameter to increase compared to the sulfate analogue
˚
10.273 A). The infrared spectra and thermal properties are similar to those reported for the sulfate analogue. Due to
(
the ferromagnetic exchange between the nearest-neighbor cobalt atoms within the layer, satisfying the Good-
enough-Kanamori rule, and the weak interlayer exchange, an overall ferromagnet is obtained. The ferromagnetic
order at 9 K was confirmed by the ac susceptibilities, the saturation magnetization, and most importantly the
enhancement of some Bragg diffraction peaks below the Curie temperature. The moments of all the cobalt atoms were
found to be aligned along the b axis with a moment of 3.25(8) μ each giving the best fit. The increase in layer distance
B
and the electron density by replacing sulfur by selenium lowers the Curie temperature.
3
Introduction
giving rise to ferrimagnetism. Nature seems to have realized
this first by the natural formation of loadstone, ferrite Fe O ,
3
4
Ferromagnetism is a rarely observed phenomenon as it
requires all the moments on every site to be parallel, thus
requiring the very rare occurrence of no overlapping between
which contains three magnetic sublattices with a resultant
uncompensated moment. However, there are very few
cases where ferromagnetic alignment is favored. One of them
1,4
1
magnetic orbitals of nearest neighbor moment carriers. This
5
is when two cobalt(II) atoms share an edge. If this is
is possibleonly ifthemagneticorbitals are orthogonalto each
other. In contrast, antiferromagnetism is most favorable, as
there is enough overlap between orbitals from nearest neigh-
bors in most cases that pairing of the electrons takes place.
One way around this problem for making magnets is to use
different moment carriers in ordered magnetic sublattices
extended in three dimensions, one expects all the moments
to be parallel, thus generating a ferromagnet. This particular
arrangement has not been realized yet. However, it is well-
known for layers where edge-sharing can satisfy all the
structural conditions, as in the layered hydroxide adopting
2
6
the brucite, Mg(OH) , structure. Therefore, Co(OH) has
2
2
been found to consist of ferromagnetic layers, but in this
particular case, the interaction between layers is antiferro-
magnetic, which consequently resulted in a fully compen-
sated antiferromagnet at TN of 10 ( 1 K. This particular
layered structure is very rare and is known in only two other
*
To whom correspondence should be addressed. Fax: 00 33 3 88 10 72 47
(
(
S.V.); 00 33 3 68 85 13 25 (M.K.). E-mail: vilminot@ipcms.u-strasbg.fr
S.V.); kurmoo@chimie.u-strasbg.fr (M.K.).
7
(
1) (a) Morrish, A. H. The Physical Principles of Magnetism; R. E. Krieger:
Huntington, NY, 1980. (b) Chikazumi, S. Physics of Ferromagnetism; Clarendon
Press: Oxford, U. K., 1997. (c) Herpin, A. Th eꢀ orie du Magn eꢀ tisme; Presse
Universitaires de France: Paris, 1968. (d) Blundell, S. J. Magnetism in Condensed
Matter; Oxford University Press: Oxford, U. K., 2001. (e) Carlin, R. L.
Magnetochemistry; Springer: Berlin, 1986. (f) Day, P. J. Chem. Soc., Dalton
Trans. 1997, 701.
(4) Magnetism Fundamentals du Tr ꢀe molet de Lacheisserie, Eꢀ ., Gignoux, D.,
Schelenker, M., Eds; Springer: New York, 2005; Chapter 1.
(5) Kurmoo, M. Chem. Soc. Rev. 2009, 38, 1353.
(6) (a) Zigan, F.; Rothbauer, R. Neues Jahrb. Mineral Monatsh 1967, 137.
(b) Parise, J. B.; Leinenweber, K.; Weidner, D. J.; Tan, K.; Von Dreele, R. B. Am.
Mineral. 1994, 79, 193.
(7) (a) Takada, T.; Bando, Y.; Kiyama, M.; Myamoto, H.; Sato, T. J.
Phys. Soc. Jpn. 1966, 21, 2726. (b) Sorai, M.; Kosaki, A.; Suga, H.; Seki, S. J.
Chem. Thermodyn. 1969, 1, 119.
(
(
2) N ꢀe el, L. Ann. Phys. 1948, 1, 137.
3) (a) Molecular Magnetism, New Magnetic Materials; Itoh, K., Kinoshita,
M., Ed.; Gordon Breach-Kodansha: Tokyo, 2000. (b) Metal-Organic and Organic
Molecular Magnets; Day, P., Underhill, A. E., Eds.; Special Publication, Royal
Society of Chemistry: Cambridge, UK, 2000; vol. 252. (c) Blundell, S. J.; Pratt, F. L.
J. Phys.: Condens. Matter 2004, 16, R771.
r 2010 American Chemical Society
Published on Web 02/12/2010
pubs.acs.org/IC

3
020 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maalej et al.
8,9
3
poured in a Teflon-lined stainless steel bomb of 125 cm capacity
compounds, Co (OH) NO and Co (OH) (SO ) (H O) .
2
3
3
5
6
4 2
2
4
The former contains similar CoO layers, but one in four of
and heated at 150 °C for 48 h. After cooling, the pink micro-
2
5 6 4 2 2 4
crystals of Co (OH) (SeO ) (H O) (1H) having a platelet habit
the hydroxide oxygen atoms is replaced by one of the nitrates.
˚
were decanted and washed with water, ethanol, and acetone
before drying at 40 °C. For powder neutron diffraction, the
deuterated compound 1D was prepared using the same proce-
dure as above but replacing H O by D O. Deuterium has a low
Therefore, the interlayer distance is increased from 4.7 A for
˚
Co(OH)2 to 6.95 A for Co (OH) NO . The interaction
2
3
3
between layers is still antiferromagnetic, and the N ꢀe el tem-
perature is 10 ( 1 K, which is not much perturbed by the
2
2
incoherent neutron scattering factor compared to hydrogen and
thus results in a reduction of the background.
increase in interlayer distance and the presence of NO in the
3
8
gallery. Because the interlayer exchange is weak, a moderate
Characterizations. Infrared spectra were recorded by trans-
mission through KBr pellets containing 1% of the crystals using
a Digilab Excalibur Series FTIR spectrometer. TG-DTA ana-
lyses were performed on a TGA92 Setaram apparatus under air
at a heating rate of 5 °C/min in a platinum crucible. Powder
X-ray diffraction patterns were recorded using a D8 Bruker
magnetic field of less than 2 kOe is enough to easily reverse
the moments in all the layers to be parallel. Co (OH) (SO ) -
5
6
4 2
(
H O) , on the other hand, consists of a bridge between the
2 4
layers where the connector is ...OSO -Co(H O) -O SO...
3
2
4
3
˚
resulting in an interlayer spacing of 10.3 A. Interestingly it is
the first ferromagnet in the cobalt-hydroxide series. Exten-
sive magnetic and neutron scattering measurements reveal
that the moments within the layer are ordered first at 14 K,
and thermodynamic measurements indicate that of the bridge
becomes ordered at lower temperatures suggesting that this
particular material behaves as a single-layer magnet within a
˚
diffractometer (Cu KR , 1.5406 A), equipped with a front
1
monochromator. EDX analyses were performed using a Kevex
unit of a Jeol 6700 F SEM apparatus. Magnetization measure-
ments were performed in the temperature range 2-300 K and a
field up to 50 kOe by means of a Quantum Design MPMS-XL
SQUID magnetometer.
The neutron diffraction experiments were performed at the
Laboratoire L ꢀe on Brillouin (CEA Saclay) using the multidetec-
9
limited range of temperatures. The nuclear structure is the
˚
tor (800 cells) G4.1 (λ = 2.4226 A) diffractometer for the deter-
same at all temperatures between 1.6 and 300 K, and the
magnetic structure has all the moments lying within the layer
along the b axis. This characteristic may be associated with the
very weak magnetic interaction between layers. These peculiar
behaviors prompted us to study the selenate analogue, Co5-
mination of the magnetic structure and the thermal evolution
study of the low temperature patterns. Thirteen diffraction
patterns were recorded in the 2θ range 2-82°, at different
temperatures between 1.5 and 12 K. The powder sample was
set in a cylindrical vanadium can and held in a liquid helium
cryostat. Nuclear and magnetic structures were refined using the
(
OH) (SeO ) (H O) , to understand the effect of introducing
6 4 2 2 4
1
3
an electronically more dense element (selenium) in the place of
the less dense one (sulfur) as well as modifying the interlayer
distance. Here, we report the synthesis, its nuclear and mag-
netic structures, and its magnetic properties as part of our
FULLPROF program. The nuclear scattering lengths (b =
Co
-
12
-12
-12
0.2490 ꢀ 10
Se
cm, b =0.7970 ꢀ 10
cm, b =0.5803 ꢀ 10
O
-
12
-12
cm, b
D
=0.6671 ꢀ 10
cm, b
H
=-0.3739 ꢀ 10
cm) and
cobalt(II) magnetic form factor were those included in this
program.
10-12
interest in the magnetic properties of synthetic minerals.
It is a ferromagnet with a slightly lower Curie temperature
than the sulfate, and the neutron scattering reveals ordering of
all the moments along the b axis.
Results and Discussion
Synthesis. Although it has been possible to make the
desired compounds, 1H and 1D, it has been difficult to
obtain them as pure phases. There is always the presence
of a small quantity of the corresponding anhydrous
compound, Co (OH) (SeO ) , and sometimes other re-
Experimental Section
Synthesis. The starting material CoSeO
4
5H
2
O was first pre-
pared by the reaction of cobalt(II) carbonate hydrate (Aldrich,
5 wt % Co) and selenic acid H SeO (Aldrich, 40 wt % water
solution). The resulting red solution was evaporated yielding red
crystals of CoSeO 5H O that were then ground to a pink
powder before further use. CoSeO 5H O (1.2 g, 4.11 mmol)
and NaOH (0.10 g, 2.5 mmol) were separately dissolved in
5 mL of deoxygenated boiling distilled water. The two solu-
tions were mixed to give a blue suspension that was immediately
3
5
6
4 2
lated impurities such as the natrochalcite NaCo (H O )-
2
4
2
4
3
2
1
SeO ) . To obtain 1H or 1D as the major phase requires
4
(
4 2
the following experimental optimized conditions: (a) limit
the reaction temperature to 150 °C, (b) reaction time
should be set to 48 h, (c) use boiled deoxygenated distilled
water to avoid the oxidation of Co(II), and (d) most
importantly, use a NaOH/Co ratio less than 0.6 instead
of the stoichiometric 1.2 according to the chemical for-
mula. These conditions were less severe than those found
suitable in the case of the corresponding sulfate, where the
4
3
2
4
3
2
1
(
8) Kurmoo, M. Metal-Organic and Organic Molecular Magnets; Day, P.,
Underhill, A. E., Eds.; Special Publication, Royal Society of Chemistry: Cam-
bridge, U. K., 2000; vol. 252, p 185.
NaOH/Co was limited to 0.17 and the reaction time to
2
for the H O salt clearly promote the formation of an in-
(
9) (a) Ben Salah, M.; Vilminot, S.; Andr ꢀe , G.; Richard-Plouet, M.;
9
4 h. Experiments in D O under the optimized conditions
Bour ꢀe e-Vigneron, F.; Mhiri, T.; Takagi, S; Kurmoo, M. J. Am. Chem.
Soc. 2006, 128, 7972. (b) Ben Salah, M.; Vilminot, S.; Richard-Plouet, M.; Andr ꢀe ,
G.; Mhiri, T.; Kurmoo, M. Chem. Comm. 2004, 2548.
2
2
creased amount of secondary phases. Some phases crystal-
lize as dark red colored microcrystals. For measurements
that need only a few milligrams such as for magneti-
zation using a SQUID, it was possible to eliminate part
of these secondary phases under a microscope. On the
other hand, neutron diffraction requires a few grams,
(
10) (a) Vilminot, S.; Andr ꢀe , G.; Richard-Plouet, M.; Bour ꢀe e-Vigneron,
F.; Kurmoo, M. Inorg. Chem. 2006, 45, 10938–10946. (b) Vilminot, S.; Andr ꢀe ,
G.; Bour ꢀe e-Vigneron, F.; Richard-Plouet, M.; Kurmoo, M. Inorg. Chem. 2007,
4
6, 10079.
(
11) (a) Vilminot, S.; Andr ꢀe , G.; Bour ꢀe e-Vigneron, F.; Richard-Plouet,
M.; Kurmoo, M. Inorg. Chem. 2007, 46, 10079. (b) Vilminot, S.; Richard-
Plouet, M.; Andr ꢀe , G.; Swierczynski, D.; Bour ꢀe e-Vigneron, F.; Marino, E.;
Guillot, M. Cryst. Eng. 2002, 5, 177. (c) Ben Salah, M.; Vilminot, S.; Mhiri,
T.; Kurmoo, M. Eur. J. Inorg. Chem. 2004, 2272. (d) Vilminot, S.; Richard-
Plouet, M.; Andr ꢀe , G.; Swierczynski, D.; Bour ꢀe e-Vigneron, F.; Kurmoo, M.
Dalton Trans. 2006, 1455.
(13) Rodriguez-Carvajal, J. FULLPROF: Rietveld, Profile Matching and
Integrated Intensity Refinement of X-Ray and/or Neutron Data, 3.5d Version;
L ꢀe on-Brillouin Laboratory/CEA Saclay: France, 2005.
(
12) Vilminot, S.; Richard-Plouet, M.; Andr ꢀe , G.; Swierczynski, D.;
(14) Vilminot, S.; Andr ꢀe , G.; Bour ꢀe e-Vigneron, F; Baker, P. J.; Blundell,
Bour ꢀe e-Vigneron, F.; Kurmoo, M. Inorg. Chem. 2003, 42, 6859.
S. J.; Kurmoo, M. J. Am. Chem. Soc. 2008, 130, 13490–13499.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3021
Figure 1. Electron micrographs of crystals of Co (OH)
5
6 4
(SeO )
(H O)
2 2
4
: crystals under short unfocused (left) and long focused (right) periods of
irradiation.
Table 1. Summary of the Rietveld Refinements at Different Temperatures for a
Sample of 1D
a
b
b,c
XRD 300 K
NPD 12 K
NPD 1.6 K
˚
a (A)
5.4958(14)
6.3447(11)
21.436(7)
92.06(3)
747.0(8)
15.7
5.4473(12)
6.3428(9)
21.248(4)
91.82(1)
733.8(5)
11.0
11.9
4.01
3.42
5.4226(13)
6.3475(10)
21.171(6)
90.78(2)
728.6(7)
14.5
16.2
6.01
4.33
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
R
R
R
R
p
(%)
wp (%)
(%)
(%)
16.8
B
F
a
b
Le Bail pattern matching. Rietveld refinement.
c
Rmag = 8.58%.
matching of the X-ray powder diffraction data with con-
sideration of a contribution from the anhydrous salt.
Therefore, 1H and 1D are assumed to be isostructural to
their sulfate analogues. However, the full Rietveld refine-
ments for the structures were hampered by the preferential
orientation of the Bragg-Brentano geometry of the appa-
ratus, though this is less of a problem for the neutron
diffraction data (Table 1) where better averaging is possi-
ble. It is to be noted that the basic structural motif is correct
as found by the Rietveld refinement of the neutron data
at 12 K. As expected, the replacement of the sulfate ion
Figure 2. Pattern matching of the X-ray powder diffraction at 300 K of
H. The two sets of ticks correspond to those of 1H (upper) and to the
anhydrous salt (lower).
1
and therefore this large-scale purification was not possi-
ble. In order to try identifying the possible phases, SEM
observations and EDX analyses have been performed on
samples from few batches.
SEM Observations and EDX Analyses. Figure 1 shows
a view at low magnification of a sample that appears homo-
geneous in color and crystal habit and with the X-ray
diffraction pattern exhibiting primarily the Bragg reflec-
tions of 1H. The crystals appear as platelets that can reach a
few tens of micrometers. EDX analyses on different crystals
give results in agreement with the 1H formula: observed
˚
(mean S-O=1.48 A) by the bigger selenate (mean Se-O=
1.65 A) results in a shift of the 00l Bragg reflections toward
˚
lower 2θ values. While the a and b unit cell parameters
remain constant on going from sulfate to selenate, the c axis
9
increases by more than 4% from 20.547 to 21.436 A. The
˚
increase of the volume cell is therefore only due to the
expansion along the c axis. This suggests that the layer is
very rigid to whatever modifications are made in the
gallery. The structure consists of edge-sharing octahedra
within brucite layers having an anisotropic arrangement of
chains of two (Co1 and Co2) of the three independent
cobalt atoms along the b axis (Figure 3). The ab plane is the
(
calculated, atom%) Se, 9.4 (8); Co, 19.8 (20); O, 70.8 (72).
Moreover, when the electron beam is focused on the crystals
for a long period, they are damaged at the surface. The
curving and flaking of the plates may be due to local heating
resulting in the loss of water.
EDX analyses on a few crystals having a different habit
led us to conclude a possible NaCo (H O )(SeO ) for-
mulation. For the samples prepared with D O, SEM
brucite layer that involves one oxygen atom of the SeO
four hydroxides of Co(OH) . These layers are then bridged
bythe octahedraof thethirdcobalt atom (Co3) throughthe
two-trans coordinated SeO , thus pillared by ...OXO -Co-
(H O) -O XO... where X = S or Se. The increase of the c
in
2
3
2
4 2
4
2
2
observations reveal the presence of other types of crystals.
X-Ray Powder Diffraction. The XRD patterns of 1H
and 1D appear similar to the ones for the corresponding
sulfate analogues. They are dominated by a progression of
strong Bragg reflections based on 00l due to preferential
orientations of the plate crystals in the Bragg-Brentano
geometry used to record the data (Figure 2). Using the unit
4
3
2
4
3
˚
axis by about 0.22 A per X-O bond is close to the expected
˚
0.18 A for replacing sulfate by selenate. The crystal system
is not changed as a function of the temperature from 1.6 to
300 K, but there is a contraction of the lattice on cooling
3
3
˚
˚
cell parameters for the sulfate, monoclinic system P2 /c,
1
their cell parameters have been refined by Le Bail pattern
(V = 747 A at 300 K and 729 A at 1.6 K). While the b axis
parameter is not altered on cooling, those of a, c, and

3
022 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maalej et al.
Figure 4. Temperature dependence of χ (red) and χT (blue) of Co
5
-
(
OH) (SeO (H O)
6
4
)
2
2
4
.
Thermal Properties. The TGA trace (Figure S2, Sup-
Figure 3. Views of the layered structure of Co
showing the two different octahedral sites (left) within a layer and the
way the layers are pillared by the OSeO -Co(H O) -O SeO (right).
5 6 4 2 2 4
(OH) (SeO ) (H O)
porting Information) recorded under air reveals a four-step
evolution of the sample weight. They correspond to water
loss (170 to 260 °C ΔP/P=7.5%, calc.=9.5%), OH decom-
3
2
4
3
β are contracted by 1.3%. It is interesting to note that the
magnetostriction is along the same contracted axes, while
the b axis remains unaltered and is the easy axis of this
ferromagnet (see below).
position as H O(260to350°C, ΔP/P=8.9%, calc.=7.2%),
2
selenate ion decomposition as SeO with concomitant oxi-
3
dation of Co (350 to 730 °C, ΔP/P=30.1%, calc.=29.8%),
and finally reduction of Co O in CoO (910 to 925 °C, ΔP/
P = 3.5%, calc. = 3.3%). Apparently the water loss and
3
4
For most samples, extra Bragg reflections are present,
and they are also made up of a progression of 00l Bragg
lines characteristic of a layered structure (Figure 2). We
believe they belong to an impurity phase consisting of the
anhydrous form. These lines are related to those found for
the equivalent anhydrous cobalt hydroxy-sulfate, for
which the single crystal diffraction reveals a hexagonal
the OH decomposition are concomitant, since their sum
(16.4%) is closer to the expected value (16.7%) than the
individual values. The total weight loss, 48.9%, is close to
the expected one, 50.5%, the difference being attributed to
the impurities detected by powder X-ray diffraction and
EDX analyses. The weight losses are accompanied by
endothermal effects on the TDA trace at 222, 337, 474,
and 920 °C.
Magnetic Properties. The temperature dependence of
the magnetic susceptibility and its product with the
temperature are shown in Figure 4. In the paramagnetic
region, the susceptibility follows a Curie-Weiss law, χ =
C/(T - θ), with C = 14.98(4) emu K/mol and θ = þ3.2(5)
K as deduced from a fit of 1/χ versus the temperature data
in the temperature region 150-300 K. The Curie constant
translates to 2.996 emu K/mol of cobalt. From this value,
˚
system with unit cell parameters a = 6.530(1) A and c =
3
˚
2.05(3) A.
Infrared Spectroscopy. The infrared spectra (Figure S1,
Supporting Information) of 1H and 1D reveal the presence
of two families of bands; one is related to O-H and H O
2
vibrations that are shifted from 1H to 1D by a factor of
1/2
around (18/(17 ꢀ 2)) , and the other is related to the SeO4
tetrahedron vibrations that remain constant in position. In
-
1
-1
the 3600-3200 cm (3000-2300 cm ) region, the five
observed bands are attributed to OH(D) stretching vibra-
-
1
tions. While the first three at 3602, 3557, and 3488 cm
(
-
1
2690, 2629, and 2592 cm ) are sharp, the other two at 3333
we get an effective moment of 4.90 μ . This value is larger
B
-
1
-1
than expected assuming only the spin quantum numbers
but is within the range observed for most Co(II) systems.
and 3183 cm (2469 and 2351 cm ) are broad. A similar
result was evidenced in the case of the corresponding sulfate
compounds, Co (OH) (SO ) (H O) and Co (OD) (SO ) -
For the corresponding sulfate, a value of 4.82 μ was
B
5
6
4 2
2
4
5
6
4 2
observed. This enhancement is due to the presence of
spin-orbit coupling. At low temperatures, the effective
spin reduces to 1/2, and a virtual Weiss constant of -20 K
has been proposed by Mabbs and Machin for noninter-
(D O) . According to the structural results that evidenced
2 4
much shorter hydrogen bonds engaged by water molecules,
the broad bands were attributed to H(D) O stretching
2
vibrations, whereas the sharp ones correspond to OH(D)
1
6
1
5
acting cobalt(II). In our case, θ = þ3.2(5) K charac-
terizes ferromagnetic interactions between nearest Co
neighbors due to the edge-sharing arrangement leading
to Co-O-Co angles less than 98°, satisfying the Good-
vibrations. It appears reasonable to draw the same con-
clusion concerning the selenate even if the structure determi-
-
1
-1
nation is less precise. At 1615 cm for 1H (1192 cm for
D), the band is related to the deformation of the water
1
1
7,18
-
1
-1
enough and Kanamori rule.
molecules. A broad band around 680 cm in 1H (498 cm
for 1D) has been attributed to other OH modes. For the bands
showing no deuteration effects, those at 889 and 865 cm are
At around 9 K, the susceptibility sharply increases,
and it is accompanied by the presence of one peak for
both the real and imaginary parts of the ac susceptibilities
-
1
attributed to the ν of selenate, the presence of a doublet being
3
(
Figure 5). According to the temperature of the maximum
in relation with the distorted local symmetry that also permits
-
1 15
the ν vibration to be IR active at 813 cm . The ν mode is
1
4
-
1
observed at 425 cm . Very similar results were evidenced for
(16) Mabbs, F. E.; Machin, D. J. Magnetism and Transition Metal
Complexes; Chapman and Hall: London, 1973.
(17) Goodenough, J. B. Magnetism and the Chemical Bond; John Wiley and
Sons: New York, 1963.
the corresponding H and D sulfates.
(
18) (a) Kanamori, J. J. Phys. Chem. Solids 1959, 10, 87. (b) Kanamori, J. In
(
15) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
Magnetism; Rado, G. T., Suhl, H., Eds.; Academic Press: New York, 1963; vol. I,
ordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986.
ch. 4, p 127.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3023
Figure 5. Temperature dependence of the ac (1 Oe, 17 Hz) and dc (FC,
on cooling.
Figure 7. Observed neutron diffraction patterns of Co
(D O) above (12 K, red) and below (1.5 K, blue) the magnetic ordering
temperature (9 K) and the difference (black).
5 6 4 2
(OD) (SeO ) -
H = 1.6 Oe) susceptibilities of Co (OH)
5
6 4
(SeO )
(H O)
2 2
4
2
4
5 6 4 2 2 4
Figure 8. Magnetic structure of Co (OD) (SeO ) (D O) .
5 6 4 2 2 4
Figure 6. Isothermal magnetization of Co (OH) (SeO ) (H O) at 2 K.
method applied to the P2 /c space group with propaga-
1
tion vector k=(0, 0, 0) and Wyckoff positions (4e) for Co1
and Co2 and (2a) for Co3.
of the peak of the real part and the nonzero imaginary
component, the Curie temperature of the ferromagnet
can be deduced as T =9 K.
1
9-21
Four one-dimensional
irreductible representations, Γ to Γ , are evidenced for
1
4
C
the (4e) position and only two Γ and Γ for (2a). Only Γ
1
The isothermal magnetization at 2 K reveals a hyster-
esis loop with a remnant magnetization around 4 μ and a
coercive field of 100 Oe (Figure 6). The magnetization at
1
3
and Γ can describe a ferromagnetic structure, with the
3
B
moments oriented along b and a, respectively. For Γ , a
3
possible component of the moment along the c axis is
theoretically possible, but refinement with the moment
5
moments are almost aligned with the field. This value
0 kOe has a value of 9.65 μ , which indicates that all the
B
lying in the ac plane results in a value of zero for Mz.
is very close to the value for the corresponding sulfate,
9
9
Consideration of the nuclear structure reveals that Co1
.67 μ . The presence of very small crystals did not
B
and Co2 belong to different chains running along the
b axis within the brucite layers. The octahedra around
Co1 and Co2 share corners with each other, and therefore
it can be assumed that the pathways between Co1-Co1,
Co2-Co2, and Co1-Co2 operate with exchange inter-
actions of similar values. Furthermore, we can also
assume that the magnetic moments are equal for Co1
and Co2. Interestingly, refinements with independent
values for Co1 and Co2 severely diverge. Concerning
the moment of Co3, three possibilities exist: (a) M(Co3)
is equal to M(Co1) and M(Co2), (b) M(Co3) is indepen-
dent of M(Co1) and M(Co2), and (c) M(Co3) = 0. The
last model was adopted for the corresponding sulfate.
Table 2 summarizes the results.
From Table 2, it appears that, for the same model,
alignment of the magnetic moment along the b axis gives a
slightly better result than that along the a axis. Similar
allow measurements on aligned samples as was possible
for the sulfate compound.
Magnetic Structure Determination. A comparison of
the neutron powder patterns recorded at 1.6 and 12 K
(Figure 7), i.e., below and above T reveals a strong increase
C
of the Bragg reflection at 13.10° indexed as (002). On the
difference pattern, three other contributions clearly appear at
2
2.09°, 26.38°, and 39.98°. The last two can be attributed to
(
004) and (006) planes of 1D, while the first one is attributed
to an impurity peak (Figures S3 and S4, Supporting In-
formation). The fact that no new lines appear compared to
the pattern taken above the magnetic ordering temperature
and that only an increase in intensity of some nuclear lines is
observed is characteristic of a ferromagnetic structure.
The associated magnetic structure (Figure 8) has been
determined employing Bertaut’s representation analysis
9
results are obtained with a nonzero value for M (Co3)
y
(
(
19) Bertaut, E. F. Acta Crystallogr., Sect. A 1968, 24, 217.
either equal to M (Co1)=M (Co2) or independent from
y
20) KAREP-A Program for Calculating Irreductible Space Group
y
Representations: Hovestreydt, E.; Aroyo, I.; Sattler, S.; Wondratschek, H.
M (Co1)=M (Co2), with nearly the same R
y y
fac-
magnetic
J. Appl. Crystallogr., 1992, 25, 544.
tors. However, refinements for data recorded at higher
temperatures are all significantly worse for independent
M (Co3). Therefore, the model with the same moment for
(
21) BASIREPS - A Program for Calculating Non-Normalized Basis
Functions of the Irreductible Representations of the Little Group Gk for Atom
Properties in a Crystal; Rodriguez-Carvajal, J., Ed.; Laboratoire L ꢀe on Brillouin
y
(
CEA(CNRS), CEA Saclay: Gif sur Yvette, France, 2004.
the Co atoms has been selected. The last model with

3
024 Inorganic Chemistry, Vol. 49, No. 6, 2010
Maalej et al.
A further observation in the neutron diffraction experi-
ments is the occurrence of an increased background at
low angles that may be associated with short-range
magnetic ordering. The background is almost constant
and weak between 1.6 and 7 K. It increases to a maximum
at the Curie temperature (Figure S5, Supporting In-
formation). The intensity above the Curie temperature
suggests that there is already short-range order of a
reasonable length, while the decrease at low temperatures
may be interpreted as an increase of the diffraction
volume of the long-range order at the expense of that of
the short-range ordering. Further work will be needed to
fully characterize this effect. However, the short-range
ordering appears to be antiferromagnetic and does not
adopt the same crystal system as found in the case of
Figure 9. Temperature dependence of the moment per cobalt ion for
Co (OD) (SeO (D O) derived from the neutron diffraction data.
5
6
4
)
2
2
4
2
2,23
Table 2. Results from the Rietveld Refinements of the Magnetic Structures of 1D
M (OH) (SO ) (H O) , M = Co and Mn.
3 2 4 2 2 4
M (Co1, Co2, Co3)
x
M
y
(Co1 and Co2)
(μ
M
y
(Co3)
(μ
R
magnetic
Conclusion
(
B
μ )
B
)
B
)
(%)
The reaction conditions for the formation of pure Co5-
(OH) (SeO ) (H O) appear to be slightly different from those
3
.21(9)
10.29
8.59
8.62
9.69
3
3
3
.25(8)
.23(9)
.00(8)
3.25(8)
2.8(7)
0
6
4 2
2
4
of Co (OD) (SeO ) (D O) and even more different from
5
6
4 2
2
4
those of the sulfate analogues. All of these compounds are
stable and isostructural for all temperatures between 2 and
300 K. Their magnetic properties are those of ferromagnets
M (Co3) = 0 has a slightly higher R value. In the
case of the sulfate, the same model with M (Co3) being
independent from M (Co1) = M (Co2) yielded M =
y
y
magnetic
˚
y
with Curie temperatures of 9 K (selenate, d =10.72 A) and
00l
˚
y
y
14 K (sulfate, d =10.27 A). The Curie temperature correlates
00l
0
.184(368) μ for Co3, and therefore it was considered
well with the established linear relation with the content
(number of tetrahedral to octahedral sites) within each layer
in the case of X-ray determined structures. The moments lie
along the b axis, that is, one of the anisotropic axes of the layer
or along the chains of Co1 and Co2, in both cases.
B
9
that the moment of Co3 was random. It is interesting
to note that the moments are aligned along the b axis
in both compounds, which corresponds to the chain
direction of the anisotropic layer (Figure 8). The moment
for the three Co atoms, 3.25(8) μ (3.33(3) μ for the
sulfate), is slightly higher than the 3 μB expected for
Co(II), and it is explained by a contribution of orbital
moments. Refinement of the data collected between 3
and 8.5 K shows that the same magnetic structure is
5
B
B
Acknowledgment. This work was funded by the CNRS
France) and the CEA (France).
(
Supporting Information Available: Infrared spectra, TG-DTA,
and neutron diffraction at different temperatures. This material
is available free of charge via the Internet at http://pubs.acs.org.
observed for all temperatures below T . The correspond-
C
ing temperature dependence of the M component of
y
(
22) Ben Salah, M.; Vilminot, S.; Andr ꢀe , G.; Richard-Plouet, M.; Mhiri,
T.; Kurmoo, M. Chem. Mater. 2005, 17, 2612.
23) Ben Salah, M.; Vilminot, S.; Andr ꢀe , G.; Richard-Plouet, M.; Bour ꢀe e-
Vigneron, F.; Mhiri, T.; Kurmoo, M. Chem. Eur. J. 2004, 10, 2048.
Co1=Co2=Co3 is reported in Figure 9. The moment
gradually decreases from 1.6 K down to 9 K where it
becomes zero.
(