Materials Chemistry and Physics 88 (2004) 404–409
Low-temperature synthesis of single-phase Co7Sb2 O12
M.S.L. Britoa , M.T. Escoteb,∗ , C.O.P. Santosc , P.N. Lisboa-Filhod , E.R. Leiteb ,
J.B.L. Oliveiraa , L. Gamae , E. Longob
a
Departamento de Quı́mica, Universidade Federal da Paraı́ba, João Pessoa, PB, Brazil
Laboratório Interdisciplinar de Eletroquı́mica e Cerâmica, Departamento de Quı́mica, Universidade Federal de São Carlos, São Carlos, SP, Brazil
c Laboratório Computacional de Cristalografia e Análises Cristalinas, Instituto de Quı́mica, Universidade Estadual Paulista, Araraquara, SP, Brazil
d Laboratório de Supercondutividade de Magnetismo, Departamento de Fı́sica, Universidade Federal de São Carlos, São Carlos, SP, Brazil
e Laboratorio de Cerâmica, Departamento de Engenharia de Materiais, Universidade Federal de Campina Grande, Campina Grande, PB, Brazil
b
Received 14 April 2004; received in revised form 6 August 2004; accepted 18 August 2004
Abstract
Polycrystalline Co7 Sb2 O12 compounds have been synthesized by a chemical route, which is based on a modified polymeric precursor method.
In order to study the physical properties of the samples, X-ray diffraction (XRD), thermal analyses (TG and DSC), infrared spectroscopy (IR),
specific surface area (BET), and magnetization measurements were performed on these materials. Characterization through XRD revealed
that the samples are single-phase after a heat-treatment at 1100 ◦ C for 2 h, while the X-ray patterns of the samples heat-treated at lower
temperatures revealed the presence of additional Bragg reflections belonging to the Co6 Sb2 O6 phase. These data were analyzed by means of
Rietveld refinement and further analyze showed that Co7 Sb2 O12 displays an inverse spinel crystalline structure. In this structure, the Co2+
ions occupy the eight tetrahedral positions, and the sixteen octahedral positions are randomly occupied by the Sb5+ and Co2+ ions. IR studies
disclosed two strong absorption bands, ν1 and ν2 , in the expected spectral range for a spinel-type binary oxide with space group Fd3m.
Exploratory studies concerning the magnetic properties indicated that this sample presents a spin-glass transition at Tf ∼ 64 K.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Polymeric precursor method; Spinel structure; Rietveld refinement; Spin-glass behavior
1. Introduction
Spinel (Sp) oxides exhibit a wide variety of physical properties, which make them interesting materials both from the
scientific point of view and for technological application [1].
They are known as pigments with high thermal and chemical
ability, and present refractory, magnetic, and catalytic properties [2,3]. In this class of materials, the M7 Sb2 O7 (M = Ni,
Co) compounds display interesting magnetic properties [4].
In fact, depending on the M ion this system can be considered
as a candidate to magnetic storage devices.
The Co7 Sb2 O7 powders crystallize in the general chemical formula (A)[B2 ]O4 , in which A is a divalent metal and
B a trivalent one [5]. The Sp properties are dependent on the
∗
Corresponding author. Tel.: +55 16 3361 5215; fax: +55 16 3361 5215.
E-mail address: [email protected] (M.T. Escote).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2004.08.008
nature of the ions A and B, their charge and their distribution
between tetrahedral and octahedral sites [6]. It is common
to call this normal Sp structure when the cations A are in
the tetrahedral sites and the cations B are in the octahedral
ones. When all the A ions are placed in the octahedral sites
and, correspondingly, half of the B ions are in the tetrahedral
sites, we refer to it as a totally inverted Sp [1,5]. As these
compounds are rather sensible to the synthesis conditions,
they display a great cation disorder [6]. This disorder is characterized by the inversion parameter i, which is the fraction
of tetrahedral sites occupied by trivalent cations. Thus, the
inversion parameter is 0, 2/3, and 1 for normal, random, and
spinel inverse structures, respectively [5].
The antimonates Zn7 Sb2 O12 and Co7 Sb2 O12 constitute
the first examples of cubic spinel with pentavalent and divalent cations [7]. Several works report on the zinc antimonate
compounds, mainly due to their applications in ceramic varis-
M.S.L. Brito et al. / Materials Chemistry and Physics 88 (2004) 404–409
tors [2,8,9]. However, as far as we know, few studies with
Co7 Sb2 O12 spinel phase have been reported. Such compound
was originally synthesized in 1960, by Dulac and Durif [10]
and in 1996, by Ilic et al. [8]. Recently, Antic et al. [3] have
produced the Co2.50 Sb0.50 O4 phase by means of the ceramic
method. It is important to notice that in all these works the
samples were submitted to heat-treatments in temperatures
ranging from 1200 to 1327 ◦ C for 5–24 h. In addition, none
of these works reported on the preparation of these samples
by means of the chemical route. Moreover, chemical methods like polymeric precursors offer good stoichiometric control, low heat treatment temperatures, and low-cost method
for producing mixed oxides as, for example, antimonates of
Co7 Sb2 O12 .
Within this context, the aim of this work is to study the
Co7 Sb2 O12 compounds synthesized by a chemical route,
which was based on a modified polymeric precursor method
[11–13]. These powders were characterized by studies on the
crystallographic structure, morphology, infrared response,
and magnetic properties.
2. Experimental procedure
2.1. Sample preparation
The Co7 Sb2 O12 compounds were prepared by the polymeric precursor method, as described elsewhere [11]. As
starting materials, we have used: reagent-grade cobalt acetate (Aldrich Chemicals 99%), antimony oxide (Vetec
(99%), citric acid (Synth PA) and ethylene glycol (Synth
PA).
For this purpose, antimony oxide was dissolved in a water solution of citric acid under constant agitation to produce
Sb-citrate. It was used a molar rate of 5:1 mol of citric acid to
Sb oxide. Then, an equimolar amount of cobalt acetate was
dissolved in distilled water and the two solutions were mixed
together under constant stirring and low heating (T ∼ 70 ◦ C).
After the homogenization of the solution containing Sb and
Co cations, ethylene glycol was added to promote the polymerization by polyestirification reaction. With the continuous
heating at temperatures close to temperature of 100 ◦ C, the
solution became viscous and we obtained a homogeneous
resin with metal ions uniformly distributed throughout the
matrix.
In order to study the heat-treatment conditions, the dried
polymeric resin was firstly analyzed by means of thermogravimetric (TGA) and differential scanning calorimetry
(DSC) measurements. They were carried out simultaneously
in Netzsch equipment (STA 409 model), under a nitrogen flow
rate of 50 mL s−1 . The heating rate for TGA and DSC was
10 ◦ C min−1 and the measurements were performed at temperatures ranging from 30 to 1200 ◦ C. Then, the final polymeric resins were calcined at temperature of 350 ◦ C and heattreated at several temperatures ranging from 700 to 1100 ◦ C
for 2 h.
405
2.2. Characterizations
X-ray powder diffraction patterns of Co7 Sb2 O12 compounds were recorded at room temperature using a Rigaku
diffractometer model D/max-IIB and using Cu K␣ radiation:
λ = 1.54059 Å; 40 kV and 30 mA. The data were obtained in
the 2θ range of 10–70◦ , the scan step was 0.02◦ , exposure
time 10 s, and SiO2 was used as standard. Through these patterns, we have studied the thermal evolution of the samples
heat-treated at different temperatures and, also, identified the
crystalline phases presents in these diffractograms.
All the single-phase X-ray patterns of the Co7 Sb2 O12
were analyzed by the Rietveld method by using the GSAS
package [14]. The refinements were made assuming Fd3m
space group for a cubic spinel-type structure. We have used a
pseudo-Voight function to fit 18 parameters to the data point:
one scale factor, one zero shifting, three background, one cell
parameters, five shape and width of the peaks, four thermal
factors, two asymmetric factors, and one the oxygen positional parameter u. In such Sp structure, the tetrahedrally
coordinated ions Co2+ are at the 8a site (1/8,1/8,1/8), the
octahedral coordinated ions (Co2+ and Sb5+ ) are at the 16d
site (1/2,1/2,1/2), and the oxygen atoms are in the special position 32e (u,u,u) with u starting from 0.25. These analyses
provide a precise description of the average crystallographic
structure in terms of the intensities, widths and positions of
the Bragg reflections.
An additional structural analysis of the Co7 Sb2 O12 compounds was made by infrared (IR) spectroscopy. These measurements were performed at room temperature using a FT-IR
Bomen, model 102 in the scan range of 1000–270 cm−1 . For
this purpose, small amounts of the samples were mixed with
KBr, and the resultant powder was pressed into pellets. The
scanning number was 50 and the spectra were analyzed by
the software Win-Bomen Easy tm, version 3.02.
The magnetic properties of Co7 Sb2 O12 compounds were
studied by magnetization M(T) as a function of temperature.
The M(T) data were recorded by using a commercial MPMS
superconducting quantum interference device (SQUID) magnetometer from Quantum Design. These measurements were
taken on the zero-field-cooled ZFC and field-cooled FC process, in the temperature range of 5–100 K, and under an external dc magnetic field varying from H = 100 to 1000 Oe.
3. Results and discussion
As discussed above, the calorimetric measurements
DSC/TG performed on polymeric resin revealed thermal behavior of the Co7 Sb2 O12 compounds, as shown in Fig. 1. In
both TG and DSC curves, we verify the presence of three endothermic peaks at temperatures ranging from 150 to 350 ◦ C.
They should be addressed to an initial dehydration reaction
and decomposition of organic materials of the polymeric
resin. The exothermic peak observed at temperatures close to
470 ◦ C is assigned to organic species pyrolysis, which is gen-
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M.S.L. Brito et al. / Materials Chemistry and Physics 88 (2004) 404–409
Fig. 1. TG/DSC curves for the dried gel at 100 ◦ C of the Co7 Sb2 O12 phase.
erally attributed to the rupture of polymeric chain. The almost
linear behavior of the DSC curve above T ∼ 500 ◦ C suggests
the beginning of the crystallization process. In addition, the
TG/DSC data revealed that up to 1200 ◦ C Co7 Sb2 O12 samples maintain their thermal stability and no thermal transformation were observed. Through this result we have chosen
the temperatures of the heat treatment of the Co7 Sb2 O12 compounds.
The structural properties of the samples were characterized by X-ray powder diffraction (XRD) measurements.
Fig. 2 shows the XRD patterns obtained for the Co7 Sb2 O12
powders heat-treated in the temperature range from 700 to
1100 ◦ C for 2 h. These data revealed that all samples heattreated at temperatures close to 1100 ◦ C, are single-phase.
For samples annealed at lower temperatures (T < 1100 ◦ C),
the XRD diagrams display a Bragg reflection close to 2θ ∼
27◦ that is identified as belonging to the additional CoSb2 O6
phase. The diffractograms of the single-phase samples were
indexed as belonging to the Co7 Sb2 O12 crystalline phase, as
could be seen in Fig. 2 [15]. This figure also displays the
Fig. 2. X-ray diffraction patterns of polycrystalline samples of Co7 Sb2 O12
heat-treated at several temperatures: (a) 700 ◦ C; (b) 800 ◦ C; (c) 900 ◦ C; (d)
1000 ◦ C; (e) 1100 ◦ C.
thermal evolution of the crystalline phase formation of the
samples. It was carry out by taking the XRD data in samples
since the early stages of crystallization (see Fig. 2(a)) until
the desired phase was obtained (see Fig. 2(e)). The results of
the sample heat-treated at 700 ◦ C disclosed the beginning of
the Co7 Sb2 O12 phase formation. Most of the peaks were indexed as belonging to the desired phase and to the additional
CoSb2 O6 phase, but they are rather broad. In general, such
broad peaks were related to the small grain sizes obtained
in this low-temperature synthesis. With the increasing of the
heat treatment temperatures, it was observed a clear increase
in the intensity and narrowing of all reflections belonging to
the Co7 Sb2 O12 phase. Increasing the sintering temperature
to 1100 ◦ C, we have obtained single-phase materials, with no
trace of the secondary phase observed in the early stages.
It is also important to notice that, through the polymeric
precursor method, we have obtained single-phase Co7 Sb2 O12
samples at lower temperature (T = 1100 ◦ C) and at a shorter
period of time (t ∼ 2 h), than those conditions of T ∼
1200–1300 ◦ C and t ∼ 8 h reported for similar compounds
prepared through the conventional ceramic method described
in the literature [3,4,8].
As many physical properties of the spinels oxides are
related to different distributions of cations in the crystallographic sites, XRD patterns of the single-phase Co7 Sb2 O12
were analyzed by Rietveld refinements. In the case, we are
interested in the Sb5+ and Co2+ ion distribution in the two
non-equivalent crystallographic sites of the crystalline structure of Co7 Sb2 O12 sample. A typical example of such analyses is shown in Fig. 3, which presents the experimental and
calculated X-ray patterns obtained through these refinements
of the Co7 Sb2 O12 phase. After the refinement, the atomic occupancies of the unit formula may be written in an expanded
form as (Co)[Co1.33 Sb0.67 ]O4 . The reliability parameters obtained through this refinement are S = 2.854, Rwp = 9.58%,
and RBragg = 7.24%. The low values of the reliability parameters RBragg and S indicate the good quality of these refine-
Fig. 3. Refined powder diffraction patterns at room temperature of
Co7 Sb2 O12 heat-treated at 1100 ◦ C. The open circles represent the experimental data, the dashed line the calculated values, and the line on the bottom
is the difference between the experimental and calculated values.
M.S.L. Brito et al. / Materials Chemistry and Physics 88 (2004) 404–409
Fig. 4. The infrared spectra of samples of Co7 Sb2 O12 heat-treated at 900
and 1100 ◦ C.
ments. The lattice parameter a ∼ 8.5371(3) Å obtained in
this work is in good agreement with that of a ∼ 8.523 and
8.54 Å reported for similar spinels by Dulac and Durif [10]
and JCPDS card [15], respectively. Through this atomic configuration, we have estimated the cation–anion bond length
to compare the first sphere of coordination with that reported
in literature [3]. From the refined parameters (a, atomic positions, thermal factor, and occupation numbers), we found
the average bond length of the tetrahedral Co2+ -oxygen of d
∼ 1.91(5) Å. This value is close to the d ∼ 1.988 Å reported
for a similar Sp structure [3]. This small difference can be attributed to the low resolution of the X-ray powder diffraction
to the presence of the oxygen in these patterns. Meanwhile,
this cation distribution is in good agreement with those reported for Sp compounds in literature [3,8].
An additional structural characterization of this antimoniate was taken by FT infrared spectroscopy. FT-IR measurements were performed on the samples heat-treated at temperatures of 900 and 1100 ◦ C. Fig. 4 displays the FT-IR spectra
obtained for these compounds. In general, all spectra display
a very similar behavior, and we have observed two strong
absorption bands ν1 and ν2 at 626 and 550–390 cm−1 . Such
bands are in agreement with the expected wavelength range
for a spinel-type binary oxide with space group Fd3m-Oh
[17]. However, we have not observed the two other weak
bands expected for Sp compounds in the far infrared region
(300–140 cm−1 ) [16]. We also believe that the frequencies
of such bands could be close to the detection limit of the
spectrometer.
To analyze the two observed bands ν1 and ν2 , we took into
account the interpretation for normal Sp with high-valence
cations [17], the inverse II–III Sp [18,19], and the fact that
antimony occupies only octahedral sites [3]. In this sense, the
band ν1 could be attributed to vibrations of the ions of higher
valence Sb5+ in the octahedral (B) sites. As the Co2+ ions in
this compound occupy both tetrahedral and octahedral sites,
the ν2 band may be due to cobalt vibrations in the octahedral
(B) sites or, perhaps to, a mixed vibration of the cobalt ions
in tetrahedral and octahedral sites.
407
At this point, we observed that we have obtained high quality Co7 Sb2 O12 samples, which are single-phase and present
a high degree of crystallization. In this sense, we have performed an exploratory study of the magnetic properties of
these compounds. For a better understanding of this point, it
is important to consider that the physical properties of systems like M7 Sb2 O12 (M = Zn, Ni, Co) and solid solutions
are considerable changed depending on the M cation and
the occupation of the atomic crystallographic sites [3,4,8].
In fact, some works have observed paramagnetic, ferrimagnetic, antiferromagnetic ordering and spin-glass effects in
such compounds. In most of the cases the magnetic properties are related to the octahedral and/or tetrahedral occupancy
of the M2+ and Sb5+ ions.
In this context, we have studied the magnetic properties of
the Co7 Sb2 O12 samples by using dc magnetization measurements M(T) as a function of temperature. The M(T) curves
for these samples were obtained at several external magnetic
fields and during the ZFC and FC processes. In Fig. 5, a
typical example of the magnetic susceptibility χ(T) curves
obtained for these compounds at applied magnetic field of H
∼ 100 Oe is shown. These data revealed the occurrence of a
sharp cusp at temperature Tc ∼ 60 K, and it was observed a
clear irreversibility of the ZFC and FC curves below Tf . In
fact, above T > Tf these curves are essentially identical and,
below Tf , it was observed a clear difference from the FC and
ZFC M(T) curves.
The χ(T) data of the Co7 Sb2 O12 samples at temperatures
above Tf were fitted to the Curie–Weiss law: χ(T) = χ0 +
C/(T − θ), where C is the Curie constant, θ the Curie–Weiss
temperature, and χ0 the temperature independent term of the
magnetic susceptibility (see the inset of Fig. 5). Such results
revealed a paramagnetic behavior at temperatures T > Tf with
the effective magnetic moment of µeff ∼ 4.23µB , in good
agreement with the µeff of ∼ 4.25µB listed for analogous
compounds in [8]. Through this analysis, we also obtained
a positive Curie–Weiss temperature of θ ∼ 63 K that sug-
Fig. 5. Magnetic susceptibility χ(T), as a function of temperature, for
Co7 Sb2 O12 heat-treated at 1100 ◦ C for 2 h and using magnetic field H ∼
100 Oe. The inset shows the 1/χ(T) vs. temperature curve (open circles) and
a Curie–Weiss law fitting (dashed line).
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M.S.L. Brito et al. / Materials Chemistry and Physics 88 (2004) 404–409
Fig. 6. The ZFC magnetic susceptibility, as a function of the temperature,
for the Co7 Sb2 O12 for several external applied magnetic fields.
gested the development of ferromagnetic interactions close
to Tf .
Below Tf , we have to noticed that the increase of the external applied magnetic field from H ∼ 1 Oe to 1 kOe resulted in
significant differences of the behavior of the χ(T) curve obtained during the ZFC processes (see Fig. 6). In fact, χZFC (T)
data obtained at H ∼ 1 Oe revealed only the transition from a
paramagnetic to ferromagnetic ordering at Tf ∼ 66 K. When
H is increased to 10 Oe, it was noticed the beginning of a
cusp formation at Tc ∼ 62.4 K, which seems to be related
to an additional antiferromagnetic AF ordering just below Tf
∼ 64 K. With the increase of H to ∼100 Oe, such cusp became rather broader and more intense at Tc ∼ 63 K, and to
H ∼ 1 kOe, which suggests a bigger fraction of the AF contribution. Finally, at H ∼ 1 kOe, the AF peak is even broad
when compared to that observed at H ∼ 100 Oe. From these
χZFC (T) curves, we believe that there is an coexistence of
ferromagnetic and AF interactions below Tf . At the same
time, we have observed that the irreversibility temperature
at Tf seems to shift to lower temperature as the field was
increased, varying from Tf ∼ 66.7 to 58 K for the χZFC (T)
data obtained at H ∼ 1 Oe to 1 kOe, respectively (not shown).
Such set of features suggested that the magnetic behavior in
these curves could be related to a spin-glass dynamic transition, as observed in a previous work [4]. Although, it should
be stressed that this Tf value found for the sample measured
at H ∼ 100 Oe differs from the Tf ∼ 32 K for the same H reported for Co7 Sb2 O12 in [4], but is quite similar than that of
Tf ∼ 60 K listed for Co2.5 Sb0.5 O4 obtained at H ∼ 6 kOe [3].
We believe that this difference can be attributed to different
degree of crystallization and atomic site occupations in the
spinel structure.
Concerning the spin-glass SG transition, in general, it requires the existence of magnetic frustration, which could be
related to randomness of both spin fluctuations and signs of
magnetic coupling, and due to a competition between different magnetic interactions [20,21]. For the Co-antimoniates
the SG transition was attributed to a competition between
ferromagnetic FM and AF interactions, as discussed for this
compound by Lisboa-Filho et al. [4]. In addition, magnetic
measurements for the Zn7 Sb2 O12 samples revealed the presence of only paramagnetic interactions [4,9] and it was believed that magnetic behavior of the Co7 Sb2 O12 compound
could be attributed only to the Co2+ sub lattice. In this context, we believed that there are at least two factors related
to the spin-glass transition: (a) a competition between the
diamagnetic Sb5+ and the ferromagnetic Co2+ ions both occupying the same crystallographic site; (b) the different crystallographic sites occupied by the Co2+ ions, as we observe
in the structural analysis. Within this scenario, it is believed
that there are several possible magnetic interactions that could
contribute to the SG transition. Firstly, the SG transition could
be related to diamagnetic and ferromagnetic interactions between the Sb5+ and Co2+ ions. Also, the SG behavior could
be attributed to AF interaction between the Co2+ ions in the
(i) octahedral–octahedral or (ii) tetrahedral–tetrahedral sites
or (iii) the ferromagnetic interaction between Co2+ ions in
the octahedral–tetrahedral sites. In this sense, the magnetic
frustration in the spinel structure could be a consequence of
the competition among these magnetic interactions and result
in the spin-glass transition observed in these measurements.
Meanwhile, it is important to stress that it is only a preliminary study of the magnetic properties of these Co7 Sb2 O12 ,
such results need a more accurate investigation and such magnetic analysis could be improved by, for example, ac magnetic
susceptibility measurements.
4. Conclusions
We have produced high quality Co7 Sb2 O12 samples by the
polymeric precursor method. Single-phase samples were obtained at heat treatment temperatures of 1100 ◦ C. A comparison between the sinterization temperatures showed a decrease
in the phase formation temperature between the sample produced in this work and those prepared by conventional synthesis procedures described in the literature. X-ray powder
diffraction and Rietveld analysis revealed that the samples
were single phase and exhibit a high degree of crystallization. It was also possible to estimate the cation occupation
as being (Co)[Co1.33 Sb0.67 ]O4 and the lattice parameter obtained is in accordance to the literature. The infrared analyses
presented characteristics of spinel oxides-bands. Magnetization measurements of the Co7 Sb2 O12 compounds revealed
the presence of a spin-glass transition, which should be a result of the competition between several magnetic interactions
depending on the Co2+ site occupation in this system.
Acknowledgments
The authors would like to thank the financial support of the
Brazilian agencies FAPESP, CNPq/PRONEX, and CAPES.
M.S.L. Brito et al. / Materials Chemistry and Physics 88 (2004) 404–409
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