Structural, spectroscopic, magnetic and thermal properties in the [SrM(C3H2O4)2(H2O)5] · 2 H2O (M = Mn, Fe, Co, Ni) system: Precursors of SrMo(3-x) mixed oxides

Article abstract of DOI:10.1002/(sici)1099-0682(199906)1999:6<935::aid-ejic935>3.0.co;2-m

The [SrM(C₃H₂O₄)₂(H₂O)₇] (M = Mn, Fe, Co, Ni) compounds have been prepared and characterised by chemical analysis and X-ray diffraction. The crystal structure of [SrCo(C₃H₂O₄)₂(H₂O)₅] · 2 H₂O has been solved [monoclinic, P2₁/c, Z = 4, a = 6.880(1), b = 14.360(2), c = 15.800(2) A? β = 101.69(1)°]. The compound exhibits a three-dimensional chain/snake structure consisting of -CoO₆-[SrO₇](∞)-CoO₆-repeating units parallel to the [0 1 1] and [0 -1 1] directions. UV/Vis data are consistent with the cations being in a high-spin octahedral symmetry. Magnetic measurements show antiferromagnetic interactions for all compounds with approximate θ values of -0.5 K, except for the cobalt compound (-24.5 K) where the decrease of the χ(m)T curve can also be attributed to the spin-orbit coupling. Thermal analyses performed in air and under nitrogen show three consecutive steps: dehydration, ligand pyrolysis, and inorganic residue evolution. Thermal treatments in tubular furnaces were performed in order to obtain pure SrMO(3- x) phases. In all cases the oxides are nonstoichiometric and oxygen-deficient with structures related to that of perovskite. Lower temperatures and reaction times than those given in the literature have been used when using the ceramic method. These 'soft' treatments gave rise to homogeneous particles of small grain size.

Full text of DOI:10.1002/(sici)1099-0682(199906)1999:6<935::aid-ejic935>3.0.co;2-m

FULL PAPER
Structural, Spectroscopic, Magnetic and Thermal Properties in the
SrM(C H O ) (H O) ] · 2 H O (M ؍ Mn, Fe, Co, Ni) System:
[
3
2
4 2
2
5
2
Precursors of SrMO Mixed Oxides
3
–x
[
a]
[a]
[a]
[b]
Izaskun Gil de Muro, Maite Insausti, Luis Lezama, Jos e´ L. Pizarro,
[b]
[a]
Maria I. Arriortua, and Te o´ filo Rojo*
Keywords: Malonate / Mixed oxides / Crystal structure / Thermochemistry / Magnetic properties
The [SrM(C
have been prepared and characterised by chemical analysis
and X-ray diffraction. The crystal structure of
SrCo(C (H O) ] · 2 H O has been solved [monoclinic,
P2 /c, Z = 4, a = 6.880(1), b = 14.360(2), c = 15.800(2) A. β =
01.69(1)°]. The compound exhibits a three-dimensional
chain/snake structure consisting of –CoO –[SrO –CoO
repeating units parallel to the [0 1 1] and [0 –1 1] directions.
3
H
2
O
4
)
2
(H
2
O)
7
] (M = Mn, Fe, Co, Ni) compounds
m
compound (–24.5 K) where the decrease of the χ T curve can
also be attributed to the spin-orbit coupling. Thermal
analyses performed in air and under nitrogen show three
consecutive steps: dehydration, ligand pyrolysis, and
inorganic residue evolution. Thermal treatments in tubular
furnaces were performed in order to obtain pure SrMO3–x
phases. In all cases the oxides are nonstoichiometric and
oxygen-deficient with structures related to that of perovskite.
[
3
H
2
O
4
)
2
2
5
2
˚
1
1
6
7
]
ϱ
6
–
UV/Vis data are consistent with the cations being in a high- Lower temperatures and reaction times than those given in
spin octahedral symmetry. Magnetic measurements show
antiferromagnetic interactions for all compounds with
approximate θ values of –0.5 K, except for the cobalt
the literature have been used when using the ceramic
method. These “soft” treatments gave rise to homogeneous
particles of small grain size.
Introduction
In addition to the oxidation state, the physical properties
also depend on either the final particle size or the prepara-
tive methods used to obtain the materials. In the last few
years, “soft” chemical routes have often been employed to
prepare advanced ceramic powders, as an alternative
method to the conventional ceramic method. The solid
solution precursor technique used for the preparation of
fine oxide materials at low temperatures in this way has
Since the discovery of high-temperature superconducting
oxides, much interest has been paid to the 3d transition
metal oxides. In particular, the electronic states of the per-
ovskite-type oxides have been extensively investigated from
[1Ϫ3]
the standpoint of 3d band filling,
those that have tran-
sition metal ions such as Mn , Fe4ϩ, Co , Ni , or Cu
4
ϩ
4ϩ
3ϩ
3ϩ
in unusually high oxidation states, as well as mixed valence
states, being those most studied.
[8]
frequently been reported. This method is quite advan-
tageous, due to the excellent stoichiometry and low im-
The physical properties of these kinds of oxides depend
greatly upon the oxidation state and the location of the cat-
ion at either tetrahedral or octahedral sites. The electronic
and magnetic properties of the SrFeO3Ϫx system vary a
great deal with the oxidation state of the iron ions, ranging
from the antiferromagnetic insulator oxide SrFeO2.5, a
brown millerite-type structure, to the antiferromagnetic
[9]
purity content of the final phases.
We are interested in the study of organometallic com-
pounds with carboxylates, as well as their use as precursor
[10]
materials for obtaining mixed oxides.
The chemistry of
transition metal complexes involving malonate as a bridg-
ing ligand has been investigated due to a wide variety of
metallic compound SrFeO , with cubic perovskite struc-
[11]
3
physical properties,
the results concerning magnetic
[
4]
ture. The SrMnO3Ϫx phase becomes increasingly anion-
deficient in air with increasing temperature.[ For this rea-
son the physical properties related to the crystal structures
have been studied.[ In the case of the cobalt oxides, the
named cobaltites have attracted special attention due to
their application as low-cost fuel-cell electrodes.[
properties of 1D, 2D, and 3D systems being of special inter-
5]
[12]
est.
6]
Recently, a series of metal malonate complexes has been
[13]
reported.
In this paper, the synthesis and characteris-
7]
ation of the [SrM(C H O ) (H O) ] (M ϭ Mn, Fe, Co, Ni)
3
2
4 2
2
7
compounds is presented. Hereafter, these complexes will be
[
a]
Departamento de Qu ´ı mica Inorg a´ nica, Facultad de Ciencias. denoted as SrMn, SrFe, SrCo, and SrNi, respectively. Spec-
Universidad del Pa ´ı s Vasco,
troscopic and magnetic studies have been performed on
Departamento de Mineralog ´ı a-Petrolog ´ı a, Facultad de Ciencias. these compounds. In order to obtain information concern-
Apdo. 644, E-48080 Bilbao, Spain
[
b]
Universidad del Pa ´ı s Vasco,
ing the formation of pure mixed oxides from the organome-
tallic precursors, a detailed thermal study has been carried
out.
Apdo. 644, E-48080 Bilbao, Spain
Fax: (internat.) ϩ 34-94/464-8500
E-mail: qiproapt@lg.ehu.es
Eur. J. Inorg. Chem. 1999, 935Ϫ943

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ1948/99/0606Ϫ0935 $ 17.50ϩ.50/0
935

I. Gil de Muro, M. Insausti, L. Lezama, J. L. Pizarro, M. I. Arriortua, T. Rojo
FULL PAPER
˚
Results and Discussion
Table 2. Selected bond lengths [A] and angles [°] for the
[
3 2 4 2 2 5 2
SrCo(C H O ) (H O) ] ·2 H O compound; symmetry transforma-
tions used to generate equivalent atoms: (1): Ϫx ϩ 1, y Ϫ 1/2,
Ϫz ϩ 1/2; (2): x, Ϫy ϩ 1/2, z ϩ 1/2; (3): Ϫx, Ϫy, Ϫz ϩ 1; (4):
The synthesis of the complexes was performed as de-
scribed in the Experimental Section. Crystal structure Ϫx ϩ 1, Ϫy, Ϫz ϩ 1; (5): x, Ϫy ϩ 1/2, z Ϫ 1/2; (6): Ϫx ϩ 1,
y ϩ 1/2, Ϫz ϩ 1/2
analyses were performed for the [SrCo(C H O ) (H O) ]
3
2
4 2
2
7
complex. Crystal data and details of the structure determi-
nation are summarized in Table 1. Bond lengths and angles
for non-hydrogen atoms are given in Table 2.
1
SrϪO(8)
SrϪO(2)
2.583(4)
2.595(4)
2.596(4)
2.599(4)
2.665(4)
2.675(4)
2.737(4)
2.752(4)
2.847(4)
2.047(5)
2.050(4)
2.068(4)
2.091(4)
2.096(5)
2.128(4)
1.250(7)
O(2)ϪC(1)
O(3)ϪC(3)
O(4)ϪC(3)
O(5)ϪC(4)
O(6)ϪC(4)
O(7)ϪC(6)
O(8)ϪC(6)
1.249(7)
1.261(7)
1.238(7)
1.259(7)
1.243(7)
1.252(7)
1.246(7)
2.583(4)
2.675(4)
1.504(9)
1.493(9)
0.97
SrϪO(4w)
SrϪO(5w)
SrϪO(3w)
Table 1. Data collection and structure refinement for the
2
3
[SrCo(C H
2 4
O
)
2
(H
2
O)
5
] · 2 H
2
O compound
SrϪO(8)
2
SrϪO(7)
3
6
SrϪO(3w)
SrϪO(1)
CoϪO(3)
CoϪO(5)
CoϪO(1)
CoϪO(7)
O(8)ϪSr
Molecular Formula
C
6
H
18CoO15Sr
5
O(8)ϪSr
C(1)ϪC(2)
C(2)ϪC(3)
˚
a [A]
6.880(1)
14.360(2)
15.800(2)
101.69(1)°
1528.6(4)
4
˚
b [A]
C(2)ϪH(21)
C(2)ϪH(22)
C(4)ϪC(5)
˚
c [A]
0.97
β
CoϪO(1w)
CoϪO(2w)
O(1)ϪC(1)
1.514(8)
1.519(8)
0.97
˚
3
V [A ]
C(5)ϪC(6)
Z
C(5)ϪH(51)
C(5)ϪH(52)
O(2)ϪSrϪO(8)
O(2)ϪSrϪO(1)
Molecular mass [g]
Space group
476.75
0.97
2
P2
1
/c
O(8)1ϪSrϪ
91.42(14)
93.44(14)
119.17(13)
76.86(13)
178.7(2)
91.9(2)
148.71(14)
47.05(12)
67.85(13)
T [°C]
293(2)
2.07(6)
2.054
O(2)ϪSrϪO(4w)
O(2)ϪSrϪO(5w)
O(2)ϪSrϪO(3w)
O(3)ϪCoϪO(5)
O(5)ϪCoϪO(1)
Ϫ3
ρ
ρ
obsd. [g cm
]
O(4w)ϪSrϪO(1)
Ϫ3
calcd. [g cm
]
O(4w)ϪSrϪO(3w) 136.36(14)
O(3)ϪCoϪO(1) 88.9(2)
O(3)ϪCoϪO(2w) 88.9(2)
O(1)ϪCoϪO(7) 179.1(2)
Ϫ1
µ [mm
]
4.651
˚
λ [A]
0.71070
0.0427
0.0967
[
a]
R(F
o
)
O(1)ϪCoϪO(1w) 91.4(2)
R
w
(F
o
2)[b]
O(1)ϪCoϪO(2w) 92.9(2)
O(1w)ϪCoϪO(2w) 175.0(2)
3
SrϪO(3 W)ϪSr
C(6)ϪO(7)ϪCo
113.9(2)
126.0(3)
93.2(3)
140.6(2)
149.2(4)
96.3(3)
113.87(14)
121.9(5)
125.2(6)
122.7(5)
C(1)ϪO(1)ϪCo
C(1)ϪO(1)ϪSr
CoϪO(1)ϪSr
127.9(4)
88.7(3)
[
a]
Խ]. Ϫ [b] wR
2) ϭ {Σ[w(F
2
R
1
(F
o
) ϭ [ΣԽԽF
o
Խ Ϫ ԽF
c
ԽԽ/ΣԽF
o
2
(F
o
o
Ϫ
5
2
2
2 2 1/2
C(6)ϪO(7)ϪSr
143.4(2)
100.7(4)
130.4(4)
127.5(4)
123.4(6)
123.4(6)
122.5(6)
118.6(5)
F
c
) ]/Σ[w(F
o
) ]}
5
CoϪO(7)ϪSr
C(1)ϪO(2)ϪSr
C(3)ϪO(3)ϪCo
C(4)ϪO(5)ϪCo
O(4)ϪC(3)ϪO(3)
O(4)ϪC(3)ϪO(3)
O(6)ϪC(4)ϪO(5)
C(4)ϪC(5)ϪC(6)
6
C(6)ϪO(8)ϪSr
The asymmetric unit consists of diaquobis(malonato)co-
baltate(II) anions linked to strontium ions and two water Sr ϪO(8)ϪSr
5
C(6)ϪO(8)ϪSr
6
5
O(2)ϪC(1)ϪO(1)
C(3)ϪC(2)ϪC(1)
O(8)ϪC(6)ϪO(7)
molecules of crystallization, O(6w) and O(7w) (see Figure
). The cobalt(II) centres adopt an octahedral coordination
1
pattern. The polyhedron is formed by the four carboxylate H(21)ϪC(2)ϪH(22) 106.3
groups from the two malonate ions and the O(1w) and
H(51)ϪC(5)ϪH(52) 107.1
O(2w) water molecules, which are located in the axial plane.
The O(1), O(5), O(3), and O(7) atoms are coplanar at aver-
˚
age distances of 2.06 A. The OϪCoϪO angles are close to
9
0°, in good agreement with those observed for other re-
[
13][14]
lated compounds.
The cobalt atom is raised by
˚
0
.016(1) A above the square plane in the direction of O(2w).
The distortion around the cobalt(II) ion has been calculated
by quantification of the Muetterties and Guggenberger de-
[
15]
scription.
The value obtained, ∆ ϭ 0.04, is indicative of
a quite regular octahedron.
The strontium ions are surrounded by nine oxygen atoms,
five of them belonging to three malonate groups, and four
3
water molecules, O(3w), O(3w) , O(4w), O(5w), forming an
˚
irregular polyhedron. The average SrϪO distance is 2.67 A.
The strontium polyhedra are linked between them along the
[
1 0 0] direction via two different common edges,
1
2
3
O(8) ϪO(8) and O(3w)ϪO(3w) , due to the presence of an
inversion centre at the middle point of these edges [at x ϭ
0
tively]. The SrϪO(8)ϪSr and SrϪO(3w)ϪSr angles are
both 113.9(2)°. These [SrO7]ϱ chains, running along the
Figure 1. View of the coordination polyhedra for strontium and
1
2
3
and 1/2, for O(8) ϪO(8) and O(3w)ϪO(3w) , respec- cobalt ions with atom numbering for the [SrCo(C H O ) -
3 2 4 2
2 5 2
(H O) ] ·2 H O compound
[
100] direction, are linked between them by corner-sharing formed by ϪCoO Ϫ[SrO ] ϪCoO Ϫ units, as can be seen
6 7 ϱ 6
2
CoO distorted octahedra, where O(1) and O(7) oxygens in Figure 2. In these chains, the Co···Co direct distance is
are the common corners. These connections form new “per- 7.908(2) A and the exchange distance through the
pendicular” chains parallel to [0 1 1] and [0 Ϫ1 1] directions O(1)ϪSrϪO(7) atoms is 9.74 A. In this way, the structure
6
˚
2
˚
936
Eur. J. Inorg. Chem. 1999, 935Ϫ943

Precursors of SrMO3Ϫx Mixed Oxides
FULL PAPER
could be described as a three dimensional package of stron- count these results, the four phases can be considered to
tium dimers linked through CoO6 polyhedra. (1: 1 Ϫ x, be isostructural.
y Ϫ 1/2, 1/2 Ϫ z; 2: x, 1/2 Ϫ y, 1/2 ϩ z; 3: Ϫx, Ϫy, 1 Ϫ z).
Table 3. Crystallographic data for the [SrM(C
O) ] · 2 H O (M ϭ Mn, Fe, Ni, Co) compounds
3 2 4 2
H O ) -
(
H
2
5
2
Compound SrMn
˚
SrFe
SrNi
SrCo
6.880(1)
a [A]
6.9517(3)
6.9105(3)
14.4758(7)
15.7808(8)
102.03(3)
1543.9
6.8762(5)
14.296(1)
15.889(1)
101.36(4)
1531.3
˚
b [A]
14.5678(7)
15.7760(6)
102.31(2)
1560.9
14.360(2)
15.800(2)
101.69(1)
1528.6(4)
˚
c [A]
β [°]
˚
3
V [A ]
Space group P2
1
/c
P2
4
1
/c
P2
4
1
/c
P2
4
1
/c
Z
4
The IR spectra of the compounds show a strong broad
Ϫ1
absorption band in the region 3100Ϫ3600 cm , corre-
sponding to the stretching vibration of the hydroxy groups,
ν(OϪH), of the water molecules. The presence of a band at
Ϫ1
around 1580 cm is attributed to the stretching vibration
of the carbonyl group linked to the metal ions. This band
Ϫ
appears together with the antisymmetric υ (COO ) vi-
a
bration. The bands corresponding to the symmetric
Ϫ
Ϫ1
υ (COO ) vibration are localised in the 1380Ϫ1452 cm
s
range. The relative positions of these two bands, ∆ ϭ
Ϫ
Ϫ
υ (COO ) Ϫ υ (COO ), are lower than those correspond-
a
s
ing to the free malonic acid. This fact indicates that the
coordination of the ligands to the metal ions could be
through the formation of a chelate, giving rise to an elec-
Figure 2. perspective view of the [SrCo(C
A
3 2 4 2
H O ) -
(H
2
O) ] · 2 H O compound in the bc plane
5
2
[17]
tronic delocalization in the carboxylate group.
The pres-
Ϫ1
In the malonate ligands, the dihedral angle between the ence of a shoulder at around 1700 cm in the IR spectra
planes O(5)ϪC(4)ϪO(6) and O(7)ϪC(6)ϪO(8) is 143.8(7)° could be attributed to the existence of CϭO groups, which
and between O(4)ϪC(3)ϪO(3) and O(1)ϪC(1)ϪO(2) it is are not linked to any metal ions as was observed in the
9
.4(5)°. The chelated malonate ring formed by the crystal structure of the SrCo compound. Nevertheless, the
O(5)ϪC(4)ϪC(5)ϪC(6)ϪO(7) atoms has an envelope con- appearance of a broad band in this region, probably due to
formation in which only the methylene group is significantly the presence of hydrogen bonds between the water mol-
displaced from the ring plane, while the other ring formed ecules and the carboxylate groups, hampers the localization
by the O(3)ϪC(3)ϪC(2)ϪC(1)ϪO(1) atoms is quite planar. of free carboxylate groups in the IR spectra. Finally, the
Ϫ1
The difference in the C(3)ϪC(2)ϪC(1), 125.2(6)° and band observed for all compounds in the 1100Ϫ1300 cm
C(4)ϪC(5)ϪC(6), 118.6(5)° angles could be explained on range corresponds to the bending vibration of the carbonyl
the basis of these linkages and the conformation of the li- group, δ(CϭO).
gands. In both malonate ions, the average CϪO distances
and OϪCϪO angles are 1.25 A and 122.6°, respectively. [SrM(C H O ) (H O) ] (M ϭ Fe, Co, Ni) compounds. For
Reflectance
spectra
were
recorded
for
the
˚
3
2
4 2
2
7
All the oxygen atoms belonging to the malonate ligand are the nickel phase the bands that appear at frequencies ν˜ ϭ
1
Ϫ1
bonded to one or two metal ions, except the O(4) and O(6) 8930, ν˜ ϭ 13513 and ν˜ ϭ 26315 cm have been assigned
2
3
3
3
3
atoms which are uncoordinated.
In the crystal structure, extensive hydrogen bonds involv-
to the three allowed transitions A Ǟ T , T (F) and
2g 2g 1g
3
3
1
1
T (P). The spin-forbidden transitions A Ǟ E and T
1g 2g g 2g
ing the carboxyl groups and water molecules could be de- were also observed ν˜ 4 ϭ 15570 and ν˜ 5 ϭ 21500 cmϪ1,
˚
scribed by acceptor-donor distances of ca. 3 A. However, respectively. The Dq and Racah parameters, calculated by
the difficulty in locating the water hydrogen atoms prevents fitting the experimental frequencies to an energy level dia-
8
Ϫ1
us from predicting this kind of bonding in the structure.
gram for octahedral d systems are: Dq ϭ 893 cm , B ϭ
Ϫ1
Ϫ1
For the SrMn, SrFe, and SrNi phases, high-quality crys- 957 cm and C ϭ 4264 cm , values usually found for
tals were not obtained. X-ray diffraction patterns of the octahedrally coordinated Ni compounds.
II
[18]
In the case of
microcrystalline products were performed in the 2θ range the [SrCo(C H O ) (H O) ] compound, the positions of the
3
2
4 2
2
7
II
of 5Ϫ70°. Indexation of the diffraction profile and refine- bands are in the range usually found for Co ions in octa-
ment of the cell parameters were made by F (pat- hedral geometries, ν˜ ϭ 8550, ν˜ ϭ 16670 and ν˜ ϭ 20200
1
2
3
tern-matching analysis)[ on the basis of the space group, cm which have been assigned to the T Ǟ T (F), A
16]
Ϫ1
4
4
4
1g
2g
Ϫ1
2g
4
P2 /c, and the cell parameters found for the SrCo phase. (F) and T (P) transitions. The Dq ϭ 966 cm and B ϭ
The calculated values are given in Table 3. Taking into ac- 851 cm values confirm the octahedral geometry. For the
1
1g
Ϫ1
Eur. J. Inorg. Chem. 1999, 935Ϫ943
937

I. Gil de Muro, M. Insausti, L. Lezama, J. L. Pizarro, M. I. Arriortua, T. Rojo
5
FULL PAPER
5
iron compound the band corresponding to the T2g Ǟ Eg
For the SrCo compound the high-temperature data (T >
transition is split into two, due to the JahnϪTeller distor- 60 K) were also fitted to a CurieϪWeiss law with θ ϭ
5
3
Ϫ1
tion of the E energy level. From the position of these Ϫ24.5 K and Cm ϭ 3.77 cm Kmol of metal ion (Figure
g
Ϫ1
bands, 11235 and 6875 cm , a value of (8/3)dσ ϭ 4360 4b). The continuous decrease in the χ T curve must be as-
cm was calculated. This fact is indicative of the presence cribed to the antiferromagnetic exchange interactions, to-
m
Ϫ1
of a slight distortion around the metal ion in this com- gether with the effect of the spin-orbit coupling (λ ϭ Ϫ170
Ϫ1
2ϩ
[19]
pound.
cm for the free Co ion),
which leads to an effective
The X-band EPR spectra of polycrystalline samples of spin S ϭ 1/2 for the system at low temperatures (T < 30 K).
the [SrM(C H O ) (H O) ] (M ϭ Mn, Co) compounds This effect is qualitatively similar to those of the antiferro-
3
2
4 2
2
7
have been performed in the temperature range 4.2Ϫ300 K. magnetic interactions, making difficult to distinguish be-
The spectra at 4 K are shown in Figure 3. In the case of the tween the contributions of these two effects on the whole
manganese phase broad and isotropic signals appear at magnetic behaviour of this compound.
room temp., and these signals are narrower at low tempera-
tures. The corresponding giso parameter at 4 K has a value
of 2.00, in good accordance with that corresponding to the
II
Mn ions in octahedral geometry. The spectrum of the co-
balt phase shows the strong anisotropy of the g factor. The
signal presents a very broad linewidth due to the spin-orbit
coupling which hampers the determination of good g values
for this compound.
Figure 3. Powdered X-band ESR spectra for the
[
SrM(C
3 2 4 2 2 5 2
H O ) (H O) ] · 2 H O compounds at 4 K (a) M ϭ Mn,
(
b) M ϭ Co
Susceptibility
magnetic
measurements
of
the
[
SrM(C H O ) (H O) ] (M ϭ Mn, Co, Fe, Ni) compounds
3
2
4 2
2
7
were carried out in the temperature range of 1.8Ϫ300 K. A
similar magnetic behaviour was observed for the SrFe,
Ϫ1
Figure 4. Thermal variations of the magnetic susceptibility (χ
and (χ T) for [SrM(C (H O) ] · 2 H O compounds a) M ϭ
Mn, b) M ϭ Co
Ϫ1
)
SrNi, and SrMn phases. The χ
and χ T vs T curves for
m
m
m
m
3
H
2
4
O )
2
2
5
2
the Mn compound is represented in Figure 4a. The χmϪ1
vs T curves for these compounds show a CurieϪWeiss be-
haviour in the range 5Ϫ300 K with negative intercepts of
II
Thermal treatments in air and under nitrogen were per-
Ϫ0.68 K (SrFe), Ϫ0.36 K (SrNi), and Ϫ1.8 K (SrMn). formed for all compounds. The TG curves are shown in
These negative θ values and the overall appearance of the Figure 5. The decomposition steps can be explained as
χmT vs T curves are indicative of the presence of antiferro- three consecutive processes: namely dehydration, ligand
magnetic interactions. The values for the Curie constants pyrolysis, and evolution of the inorganic residue. The dehy-
3
are 3.65, 1.18, and 4.36 cm K/mol, respectively. The χ T dration process occurs in the 70Ϫ190°C range, and it is in
m
products are practically temperature-independent for T > good accordance with the loss of the seven water molecules
5
0 K, reaching values of 2.45 (SrFe), 1.12 (SrNi), 3.7 present in the complexes. This process seems to take place
3
(SrMn) cm K/mol when cooling from this temperature to in only one step, except for the SrNi compound. However,
4
.2 K. Taking into account the crystallographic description, this fact does not allow the presence of different water mol-
the magnetic behaviour could be described on the basis of ecules to be deduced, as was previously observed from the
either a Heisenberg square-planar system or a 3D network. structural data. After a period of stability, ligand pyrolysis
Nevertheless, in all cases the best least-squares fit leads to occurs at around 350°C. As can be expected, and consider-
J/k values less than 1 K.
ing the nature of the cations coordinated to the malonate
938
Eur. J. Inorg. Chem. 1999, 935Ϫ943

Precursors of SrMO3Ϫx Mixed Oxides
FULL PAPER
Figure 5. TGA curves for the [SrM(C
and under nitrogen (- - -)
H
3 2
O
4
)
2
(H
2
O)
5
] · 2 H
2
O compounds, a) M ϭ Mn, b) M ϭ Fe, c) M ϭ Ni, d) M ϭ Co, in air (Ϫ)
ligand,[20] the weight loss is in good accordance with the
formation of the products obtained from its decar- tallic precursor at 650°C for 10 h gave rise to the non-stoi-
The thermal decomposition of the manganese organome-
boxylation, such as the corresponding transition metal ox- chiometric SrMnO3Ϫx oxide.[
23]
This product exhibits a
ide and SrCO . This fact can be attributed to the very high dark brown color. The results of the titration analysis are
3
stability of the alkaline-earth metal carbonates. This step consistent with the formula SrMnO2.95. The X-ray diffrac-
remains stable until temperatures above 650°C, when car- tion pattern of this phase has been indexed on the basis of
bonates undergo decomposition to yield mixed oxides. In an orthorhombic cell with parameters a ϭ 9.437(3), b ϭ
˚
fact, it is in this process where some differences have been 5.453(2) and c ϭ 9.083(4) A (Figure 6). It is important to
observed between the treatments in air and under nitrogen. note the difference observed between this oxide and that
The formation of mixed oxides is obviously favoured in air. reported in the literature which was obtained from the ce-
In this way, the SrMnO3Ϫx, SrCoO2.52, and SrFeO3Ϫx ox- ramic method. This latter compound is stoichiometric and
ides have been identified by X-ray diffraction as pure crystallises in the hexagonal system. The presence of mixed
[
21]
3ϩ
4ϩ
phases. However, in the case of the nickel compound, the valence Mn /Mn , together with a distortion to an or-
final residue is formed by the mixture of NiO and SrCO3. thorhombic symmetry, appears at temperatures above
The decomposition processes under nitrogen gave rise to a 1035°C. This fact can be due to the creation of oxygen vac-
mixture of phases (carbonates, metal oxides, mixed oxides) ancies in the compound.[ The cell parameters of this non-
24]
in all compounds.
stoichiometric oxide are similar to those given for
In order to improve the above results and to obtain more SrMnO2.95 which was obtained by using the organometallic
information about the mixed oxides, thermal treatments in method. Taking into account these results, and with the aim
tubular furnaces were performed in air and under nitrogen. of confirming the presence of a mixed valence in the last
In this case, the malonate precursors were fired at 400°C phase, we performed further IR and ESR experiments. The
Ϫ1
for 12 h. to remove the organic part, followed by treatments IR spectrum shows two broad bands at around 600 cm
Ϫ1
at higher temperatures. The resulting products were charac- and 400cm (Figure 7a), which could be assigned to the
3ϩ
terised by IR, X-ray powder diffraction (Figure 6) and ν stretching vibration corresponding to the Mn ϪO and
3
ϩ
differential thermal analysis (DTA).
Mn4 ϪO bonds, respectively, as usually appears in perov-
The metallic ions exhibit mixed valence state in all com- skites with mixed valences.[25] The split of the bands is in-
pounds obtained. In order to propose a chemical formula dicative of a lowering in the symmetry of the manganese
for all the oxides the oxidation states for the different cat- environment, probably due to the presence of oxygen vac-
ions have been determined by a titration method. The ancies in the surrounding of these ions.
samples were dissolved under nitrogen in a solution of HCl,
The ESR spectra of this oxide at different temperatures
which reacts with metals in the formal ϩ3 and ϩ4 oxidation are given in Figure 8a. As can be observed, an isotropic
state. The evolving gas is collected in a solution of KI and signal appears at room temp. with an ∆Hpp value of 960
after that, titrated with thiosulfate.[
22]
G. The intensity of the signal decreases with decreasing
Eur. J. Inorg. Chem. 1999, 935Ϫ943
939

I. Gil de Muro, M. Insausti, L. Lezama, J. L. Pizarro, M. I. Arriortua, T. Rojo
FULL PAPER
3 2 4 2 2 5 2
Figure 6. X-ray diffraction patterns of the phases obtained at different temperatures from the [SrM(C H O ) (H O) ] · 2 H O precursors,
a) M ϭ Mn, b) M ϭ Fe, c) M ϭ Ni, d) M ϭ Co
temp., being undetectable at temperatures less than 250 K. isotropic signal with a g value of 2.006 (Figure 8b). This
These results are characteristic of compounds with antifer- result, together with the value of 620 G for the linewidth,
romagnetic interactions. Taking into account that the ESR confirms the presence of mixed valence Fe3 /Fe in this
ϩ
4ϩ
spectra of Mn4 ions show a very broad signal, the results phase.
ϩ
obtained for this phase can be explained as due to the pres-
ence of a small amount of Mn ions in the compound. (H O) ] · 2 H O
The
decomposition
precursor
These results are in good agreement with those obtained (500Ϫ700°C) yielded a mixture of SrCO and NiO. The
of
the
[SrNi(C H O ) -
3 2 4 2
3
ϩ
at
low
temperatures
2
5
2
3
from chemical analyses.
From the decomposition of the [SrFe(C H O ) - treatments up to 800°C;
Sr Ni O mixed oxide was obtained only after thermal
5 4 11
[28]
however, the presence of NiO
3
2
4 2
(
H O) ] · 2 H O phase, the SrFeO
oxide[26] was obtained appears as an impurity phase in the final product (Figure
2
5
2
2.86
II
at 650°C for 10 h. The oxygen content was deduced from 6). Further attempts to oxidise the total amount of Ni
titration. The X-ray diffraction pattern (Figure 6) has been were unsuccessful.
indexed on the basis of a tetragonal unit cell, P4mm, with
parameters a ϭ 3.853(4) and c ϭ 3.861(2) A. In order to (H O) ] · 2 H O compound at 700°C for 10 h., gave rise to
Thermal
treatments
of
the
[SrCo(C H O ) -
3 2 4 2
˚
2
5
2
observe the presence of non-stoichiometry in this phase, the formation of the SrCoO2.56 phase. Several days at
ATD measurements under argon were carried out up to 1100°C were necessary to obtain the mixed oxide from the
100°C. An endothermic peak was observed at around ceramic method.[ The X-ray diffraction pattern of the
29]
1
6
20°C, which would correspond to the reduction of the SrCoO2.56 phase shows the presence of peaks related to the
phase giving rise to the SrFeO2.5 stoichiometric oxide. 2H-Perovskite-type structure. However, not all the diffrac-
When cooling this sample to room temp. an ordered brown tion peaks were indexed with this kind of structure. The
millerite-like structure is obtained. This ordered phase un- diffraction patterns do not undergo any significant changes
dergoes a reversible transition to the disordered cubic struc- when the compounds are obtained from different thermal
ture at 840°C. These results are in good agreement with treatments under both oxygen and nitrogen. The chemical
those given in the literature for this phase.[ In order to analysis indicates the presence of a small amount of Co
27]
IV
study the presence of a mixed valence state for the tran- in the sample. In order to corroborate this result, further
sition metal, IR spectra and ESR measurements in the spectroscopic and magnetic measurements were carried out.
SrFeO2.86 oxide were carried out. As can be seen in the IR The IR spectrum shows the characteristic bands at 600 and
Ϫ1
spectrum (Figure 7b), no absorption band was observed in 400 cm (see Figure 7c), corresponding to the ν stretching
3
Ϫ1
3ϩ
4ϩ
the range 600Ϫ400 cm , as would be expected for a per- vibration of Co ϪO and Co ϪO bonds, respectively.
ovskite with mixed valence Fe /Fe . This result can be Magnetic measurements on this SrCoO2.56 oxide were car-
3
ϩ
4ϩ
explained on the basis of a short range hopping between ried out in the temperature range of 1.8Ϫ300 K. The ther-
the Fe3 /Fe positions.
ϩ
4ϩ
[27]
The ESR spectrum shows an mal evolution of the χ T and χm is shown in Figure 9.
Ϫ1
m
940
Eur. J. Inorg. Chem. 1999, 935Ϫ943

Precursors of SrMO3Ϫx Mixed Oxides
FULL PAPER
Figure 8. X-band ESR spectra for the a) SrMnO2.95 and b)
SrFeO2.86 phases
Figure 7. IR spectra of the a) SrMnO2.95 b) SrFeO2.86, and c)
SrCoO2.55 oxides
The continual decrease observed in the χ T vs T curve is
m
indicative of the presence of an antiferromagnetic ordering,
with a Neel temperature value at around 25 K. It is to note
Ϫ1
an increase of the χm values at low temperatures, which
can be explained as due to the presence of a small amount
IV
III
of Co , hampering the total compensation of the Co
spin moments.
A scanning electron microscopic photograph of one of
the oxide pellets is shown in Figure 10. In all cases, powders
consisting of small homogeneous particles were observed,
due to the use of soft synthesis methods. The SrFeO2.86 ox-
ide was obtained at the lowest temp., 650°C, and particles
with a very small size (diameter less than 0.3 µm) have
been favoured.
The more the temperature increases, the higher the size
Ϫ1
m
of the grains. Furthermore, particles with a prismatic form Figure 9. Thermal variation of the χ and χ T product for the
SrCoO2.55 compound
have been observed for the SrCoO2.55 compound.
been synthesised and characterised. The structure of
SrCo(C H O ) (H O) ] · 2 H O consists of a 3D network
of metal ions coordinated by malonate ligand bridges. The
[
Conclusions
3
2
4 2
2
5
2
With the aim of obtaining SrMO3Ϫx oxides using soft cobalt(II) centres adopt an octahedral coordination while
synthesis routes, four isostructural complexes with the for- the strontium ions are surrounded by nine oxygen atoms.
mula [SrM(C H O ) (H O) ] (M ϭ Mn, Fe, Co, Ni) have Magnetic measurements indicate the presence of weak anti-
3
2
4 2
2
7
Eur. J. Inorg. Chem. 1999, 935Ϫ943
941

I. Gil de Muro, M. Insausti, L. Lezama, J. L. Pizarro, M. I. Arriortua, T. Rojo
FULL PAPER
lettϪPackard 5352B microwave frequency counter. Nickel(II) chlo-
ride, iron(II) chloride, sodium carbonate, and malonic acid were
purchased from Aldrich Co. Strontium chloride was purchased
from Merck and cobalt(II) and manganese(II) chlorides from
Fluka. All of them were used without further purification.
Preparation of the Compounds: The corresponding chloride of the
transition metal (3.0 mmol) was added to an aq. solution of
6
.0 mmol (0.624 g) of malonic acid, neutralized with sodium car-
bonate. The reaction mixture was stirred without heating for about
h, and then an equimolar amount of strontium chloride added.
1
The addition of ethanol (3 mL) afforded polycrystalline powders
of all phases. In the case of the iron compound a closed system
with nitrogen was employed. For the cobalt compound, pale pink
crystals in the form of needles were collected after leaving the re-
maining solution for some days. The compounds were washed with
2 5
water and acetone, and dried over P O . The crystals were of good
quality for X-ray single-crystal studies. Attempts to obtain single
crystals for the other compounds have been unsuccessful. Elemen-
Figure 10. SEM photograph of the SrMnO2.95 oxide
ferromagnetic interactions in all compounds. For the cobalt tal analysis, together with Inductively Coupled Plasma measure-
compound, the effect of a spin-orbit coupling must also be ments, were carried out in order to determine the C, H, and metal
contents. Ϫ C
1.62, Sr 18.54; found C 14.94, H 3.21, Mn 11.51, Sr 17.91. Ϫ
18FeO15Sr (473.65): calcd. C 15.22, H 3.83, Fe 11.79, Sr 18.50;
found C 14.93, H 3.52, Fe 11.52, Sr 19.00. Ϫ C 18CoO15Sr
476.75): calcd. C 15.12, H 3.81, Co 12.36, Sr 18.38; found C 15.34,
H 3.49, Co 12.11, Sr 18.47. Ϫ C 18NiO15Sr (476.51): calcd. C
5.12, H 3.81, Ni 12.32, Sr 18.39; found C 15.40, H 3.59, Ni 12.86,
Sr 17.62.
6
H18MnO15Sr (472.75): calcd. C 15.24, H 3.84, Mn
considered when discussing the magnetic behaviour. The
decomposition steps of the metallo-organic phases, as well
as the temperatures in which the evolution of the inorganic
residue occurs, were deduced from the TGA studies. This
information was utilised in order to perform the thermal
treatments, in tubular furnaces, with the aim of obtaining
mixed oxides. Depending on the reaction conditions (tem-
perature, reaction time, atmosphere employed, etc.) control
1
C H
6
6
H
(
6
H
1
of the stoichiometry of these mixed oxides has been X-ray Crystal.Structural Determination of [SrCo(C
3
H
2
O
4
)
2
(H
2 7
O) ]:
Pale pink crystals of C H18SrCoO15 were sealed in a glass capillary
6
achieved. In this way, the oxides with stoichiometry
and used for data collection. Preliminary cell dimensions were cal-
culated by Weissenberg photographs. Diffraction experiments were
performed with a crystal of dimensions 0.6 ϫ 0.1 ϫ 0.05 mm using
SrFeO2.86, SrCoO2.56, and SrMnO2.95 were synthesised at
IV
6
00, 700, and 650°C, respectively. The appearance of Co
III
in SrCoO2.56 and the presence of Mn in the SrMnO2.95
phase is of note. This last oxide was obtained as a stoichio-
metric phase when synthesised using ceramic methods. The
use of soft synthesis methods have also afforded homo-
an Enraf-Nonius CAD4 automatic diffractometer with graphite-
˚
α
monochromatized Mo-K radiation (λ ϭ 0.7107 A) and the ω-scan
technique. Orientation matrix and lattice parameters were deter-
mined by least-squares refinement of the diffraction data from 25
geneous and small size particles of oxides, which have been machine-centred high-angle reflections in the range 10° < 2θ < 26°.
visualised by scanning electron microscopy. In conclusion, Intensities and angular positions of two standard reflections were
it has been shown that from a metallo-organic precursor measured every 2 h, and these showed neither decrease nor mis-
alignment during data collection. The observed systematic extinc-
tions were consistent with the space group P2 /c. Lorentz and pola-
1
non-stoichiometric mixed oxides have been obtained at
much lower temperatures and reaction times than those em-
ployed using the ceramic method
rization corrections and an empirical absorption correction using
XABS2 program were applied to the data.[ The structure was
30]
[
31]
solved by Patterson interpretation using the program S86.
2
It was then refined by full-matrix methods based on F , using
Experimental Section
[32]
S93.
Non-hydrogen atomic scattering factors were taken
[33]
General: IR: M FTIR 1000 spectrophotometer between
from International Tables for X-ray Crystallography
and aniso-
Ϫ1
4
2
00Ϫ4000 cm , KBr disks. Ϫ Diffuse reflectance spectra: C
tropic thermal parameters were assigned. Difference Fourier syn-
thesis did not allow the location of H atoms. Therefore, the hydro-
gen atoms of the malonate groups were calculated and isotropic
thermal parameters were assigned. The final difference Fourier
Ϫ1
415 spectrometer in the range 5000Ϫ45000 cm , room temp. Ϫ
Thermogravimetric measurements: PerkinϪElmer System-7 DSC-
TGA, crucibles containing 20 mg of sample were heated at 5°/min
under dry nitrogen and in air. Ϫ DTA: S  92 equipment
under argon. Ϫ Powder X-ray diffraction: Philips X-PERT dif-
˚
Ϫ3
map showed no peaks higher than 0.70 or lower than Ϫ0.83 e A
The geometric calculations were performed with P
.
[
34]
and
fractometer with Cu-Kα1 radiation, data were collected by scanning B[
35]
and molecular illustration were drawn with P-
36]
[37]
in the range 5° < 2θ < 120° with increments of 0.02°(2θ). Ϫ Mag-
94[ and A.
Crystallographic data (excluding structure
netic susceptibilities and field dependences of magnetization (up to
factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplemen-
tary publication no. CCDC-106908. Copies of the data can be ob-
tained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: int. code ϩ 44-1223/336-033; E-
mail: deposit@ccdc.cam.ac.uk].
7
Tesla): Q D MPMS-7 S magnetometer, tem-
perature range 1.8Ϫ300 K. Ϫ ESR: Bruker ESP300 spectrometer,
equipped with a standard Oxford low-temperature device operating
at X and Q bands, magnetic field was measured with a Bruker 
200 gaussmeter, and the frequency was determined with a Hew-
942
Eur. J. Inorg. Chem. 1999, 935Ϫ943

Precursors of SrMO3Ϫx Mixed Oxides
FULL PAPER
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Acknowledgments
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