Concerning the cation distribution in MnFe2O4 synthesized through the thermal decomposition of oxalates

Article abstract of DOI:10.1016/j.jpcs.2003.10.059

A single phase manganese ferrite powder have been synthesized through the thermal decomposition reaction of MnC₂O₄·2H ₂O-FeC₂O₄·2H₂O (1:2 mole ratio) mixture in air. DTA-TG, XRD, Mo?ssbauer spectroscopy, FT-IR and SEM techniques were used to investigate the effect of calcination temperature on the mixture. Firing of the mixture in the range 300-500 °C produce ultra-fine particles of α-Fe₂O₃ having paramagnetic properties. XRD, Mo?ssbauer spectroscopy as well as SEM experiments showed the progressive increase in the particle size of α-Fe₂O ₃ up to 500 °C. DTA study reveals an exothermic phase transition at 550 °C attributed to the formation of a Fe₂O ₃-Mn₂O₃ solid solution which persists to appear up to 1000 °C. At 1100 °C, the single phase MnFe ₂O₄ with a cubic structure predominated. The Mo?ssbauer effect spectrum of the produced ferrite exhibits normal Zeeman split sextets due to Fe³⁺ions at tetrahedral (A) and octahedral (B) sites. The obtained cation distribution from Mo?ssbauer spectroscopy is (Fe_0.92Mn_0.08)[Fe_1.08Mn_0.92]O ₄.

Full text of DOI:10.1016/j.jpcs.2003.10.059

Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
www.elsevier.com/locate/jpcs
Concerning the cation distribution in MnFe2O4 synthesized
through the thermal decomposition of oxalates
a,
M.A. Gabal , S.S. Ata-Allah
b
*
aDepartment of Chemistry, Faculty of Science, Benha University, Benha, Egypt
bReactor and Neutron Physics Department, Nuclear Research Center, Atomic Energy Authority, Cairo, Egypt
Received 12 June 2003; revised 7 October 2003; accepted 15 October 2003
Abstract
A single phase manganese ferrite powder have been synthesized through the thermal decomposition reaction of MnC2O4·2H2O–
¨
FeC2O4·2H2O (1:2 mole ratio) mixture in air. DTA-TG, XRD, Mossbauer spectroscopy, FT-IR and SEM techniques were used to
investigate the effect of calcination temperature on the mixture. Firing of the mixture in the range 300–500 8C produce ultra-fine particles
¨
of a-Fe2O3 having paramagnetic properties. XRD, Mossbauer spectroscopy as well as SEM experiments showed the progressive increase in
the particle size of a-Fe2O3 up to 500 8C. DTA study reveals an exothermic phase transition at 550 8C attributed to the formation of a
Fe2O3–Mn2O3 solid solution which persists to appear up to 1000 8C. At 1100 8C, the single phase MnFe2O4 with a cubic structure
predominated. The Mossbauer effect spectrum of the produced ferrite exhibits normal Zeeman split sextets due to Fe3þions at tetrahedral
¨
¨
(A) and octahedral (B) sites. The obtained cation distribution from Mossbauer spectroscopy is (Fe0.92Mn0.08)[Fe1.08Mn0.92]O4.
q 2003 Elsevier Ltd. All rights reserved.
¨
Keywords: C. Mossbauer spectroscopy
1. Introduction
spinel structure. The determination of the cation distribution
in the tetrahedral and octahedral sites can be achieved using
Transition metal ferrites are a family of oxides having
great technological importance because of their novel
physical properties. They can be used in the fabrication of
magnetic, electronic and microwave devices such as
thermistors and recording media [1]. The basis for the
wide range of applications is related to the variety of
transition metal cations that can be incorporated into the
lattice of the parent magnetite structure. The majority of the
spinel ferrites belong to the space group Fd3m with lattice
¨
neutron diffraction or Mossbauer spectroscopy techniques,
and has been the subject of many studies [3,4].
In technologies where ferrites are to be used for magnetic
or electrical applications, high-density materials are gener-
ally required. So, ferrites are often prepared by high-
temperature solid-state reactions between finely ground
powders [5]. Alternatively, several coprecipitated precursor
systems have been used, including oxalates [6], formates [7]
and hydrazinates [8]. The freeze-drying method [9] can be
used also to make various forms such as poly crystalline,
aggregates, thin and thick films and single crystals.
˚
parameters around 8.5 A [2].
The cation distribution of these materials has the form:
(Fed3þM21þ2d)[Md2þFe232þd]O422 where the round and the square
brackets represent the tetrahedral (A) and the octahedral (B)
sites, respectively. Occupation of the tetrahedral site entirely
with a divalent transition metal (i.e. d ¼ 0) yields a normal
spinel structure, while occupation of the octahedral site with
the divalent transition metal (i.e. d ¼ 1) produces an inverse
Manganese ferrite (MnFe2O4) was originally thought to
be an inverse spinel but was later found to be about 80%
normal, 20% inverse. However, since both cations (Mn2þand
Fe3þ) are d5 transition elements, the overall magnetic
moment is insensitive to the degree of inversion. It is a
well-known soft magnetic material due to its low coercivety
and moderate saturation magnetization as well as a remark-
able chemical stability and a mechanical hardness [10].
Extensive studies on the preparation and characterization
of manganese ferrite have been reported giving useful
information about the influence of the preparation method
*
Corresponding author. Present address: Institute of Mechanical
Engineering and Mechanics (IMVM), University of Karlsruhe, Geb.
30.70, Kaiser Str. 12, Karlsruhe 76128, Germany. Tel.: þ20-1010-48600;
fax: þ20-132-22578.
E-mail address: mgabalabdo@yahoo.com (M.A. Gabal).
0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2003.10.059

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M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
on the various physical properties. The solid-solid inter-
action between manganese carbonate and ferric oxide have
been investigated by Deraz and El-Shobaky [11] using DTA
and X-ray powder diffraction (XRD) techniques. It was
found that MnFe2O4 starts to appear at temperatures starting
from 900 8C. The nanocrystalline MnFe2O4 phase [12],
prepared by the ball milling of Mn2O3 and Fe2O3 under an
argon atmosphere, was formed after annealing at
600–700 8C. Using the hydrothermal route at pH 9 and
210 8C, a nanosized manganese ferrite was prepared and
studied using XRD, magnetization measurements and
a muffle furnace for 5 min at temperatures ranging from
300 to 1000 8C with an interval of 100 8C or for 2 h at
1100 8C. Samples were then removed from the furnace,
and cooled down to room temperature in a desiccator.
For the sake of simplicity, these products are denoted in
the text by the notation MnFe followed by the calcination
temperature. Thus, MnFe-300 indicates the decomposition
product of MnFe at 300 8C.
2.2. Techniques
¨
Mossbauer spectroscopy [13]. Burojeanu et al. [14] have
Thermal analysis experiments including differential
thermal analysis (DTA) and thermogravimetry (TG) were
carried out using a Shimadzu DT-40 thermal analyzer
(Japan). The experiment was performed in a dynamic
(30 ml min21) atmosphere of air up to 1100 8C at a
heating rate of 5 8C min21 using a sample mass of
10 mg. Highly sintered a-Al2O3 powder was used as the
reference material for the DTA measurements.
been prepared MnFe2O4 by coprecipitation from a MnO2
and FeSO4·7H2O aqueous solution, and the produced ferrite
was characterized using DTA-TG, DTG, IR, mass spectral,
SEM and XRD measurements.
The present study has focused on the use of an oxalates
system for the preparation of manganese ferrite with high-
purity through the thermal decomposition reaction taking
place between the solid-state oxalates mixture of MnC2-
O4·2H2O–FeC2O4·2H2O (1:2 mole ratio) in air. DTA-TG,
XRD was carried out using a model PW 1710 Philips
diffractometer at ambient temperature. The instrument is
equipped with an iron anode generating Fe Ka1 radiation
¨
XRD, Mossbauer spectroscopy, FT-IR and SEM techniques
˚
were used to characterize the decomposition products
obtained at different temperatures as well as the ferrite
formation. DTA-TG measurements provide information
about the thermal decomposition course of the mixture, and
indicate the temperatures required for the calcinations
(l ¼ 1:9374 A). For identification purposes, the relative
˚
intensities ðI=I0Þ and the d-spacing (A) were compared
with standard diffraction patterns in the ASTM powder
diffraction files [16].
Fourier transform-infrared spectra (FT-IR) were
measured between 4000 and 200 cm21 using a KBr
disc technique with a model FT-IR 310 Jasco spec-
trometer (Japan). In all spectra, the relative transmittance
was represented versus wavenumber (cm21).
¨
processes. XRD, Mossbauer, FT-IR and SEM measure-
ments were used to identify phases formation and the
changing in the particle sizes along the decomposition
¨
course. Mossbauer spectroscopy was also used to study the
cation distribution and hyperfine fields at both (A) and (B)
sites in the formed MnFe2O4 spinel.
The Mo¨ssbauer spectra at room temperature were
recorded on an Austin Science Mo¨ssbauer effect
spectrometer using a constant acceleration derive and a
1024-channel analyzer. The source was 57Co in Rh
matrix with an intial activity of 50 mCi. The absorber
2. Experimental
contained 10 mg of Fe cm22. The Mossbauer parameters
¨
2.1. Materials
were computed using an interactive least square ‘Mos-90’
program [17].
Individual metal oxalates of MnC2O4·2H2O and FeC2-
O4·2H2O were prepared by the coprecipitation technique.
Weighed amounts of high purity MnCl2·4H2O or FeSO4·7-
H2O was dissolved in deionized water, and then oxalic acid
solution (which contains an equivalent amount) was added
slowly with vigorous stirring untill a permanent precipitate
occurred. The precipitate was filtered, washed with distilled
water, and allowed to dry in air.
SEM measurements were performed using a Jeol T 300
(Japan) scanning electron microscope operated at 15 keV.
Mixtures were mounted separately on aluminum substrates
evacuated to 103 Torr, and were precoated (20, 5 min for
each side of the four sides) in a sputter-coater with a thin
uniform gold/palladium film to minimize charging in the
electron beam. The applied voltage was 1.2–1.6 kV.
Mixed metal oxalates (MnC2O4·2H2O–FeC2O4·2H2O
with a 1:2 mole ratio) was prepared by the impregnation
technique described previously [15].
3. Results and discussion
To characterize the products at different decompo-
sition stages and to follow the ferrite formation, samples
of the mixture were calcined at different temperatures
(estimated from DTA-TG results) for different time
durations in a Pt crucibles under static air atmosphere.
Thus samples of the mixture were thermally heated in
3.1. Thermal decomposition course
Typical DTA-TG curves for a MnC2O4·2H2O–FeC2-
O4·2H2O (1:2 mole ratio) mixture at heating rate of
5 8C min21 in air atmosphere is shown in Fig. 1. The TG
curve indicates multistep weight loss when increasing

M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
997
Fig. 1. DTA–TG curves of MnC2O4·2H2O–FeC2O4·2H2O (1:2 mole ratio) mixture in air at a specified heating rate of 5 8C min21
.
the temperature up to 1100 8C. Considerable weight loss
(about 54.5%) occurred in the temperature range 117–
260 8C through two overlapped steps. The first one is
accompanied by a large dehydration endothermic DTA peak
at 193 8C with a slight shoulder on its low-temperature side
(at 170 8C). The weight loss effected in this step (about
20%) accounts for the complete dehydration of the mixture
and the formation of anhydrous oxalate mixture (expected
weight loss ¼ 20%).
If however, the temperature is raised further, the sample
undergoes a further small exothermic weight loss between
437 and 470 8C to yield a product that can be identified by
XRD as Mn2O3. Consistent with this result, a very small
exothermic weight loss (1.5%) was obtained by raising the
temperature up to about 450 8C. The weight loss after this
decomposition step was brought to 56%, which is close to
that expected (55.6%) for the overall conversion of the
mixture to a Fe2O3–Mn2O3 mixture.
The endothermic dehydration peak was swamped by two
large exothermic peaks (at 210 and 260 8C) closely
corresponding to the second step (observed weight loss
34.5%) which follows immediately after the completion of
the first one. This indicates that the decomposition of the
anhydrous mixture occurs virtually in two unresolved
stages. On the basis of the DTA peak’s values, the first
one is attributed to the oxidative decomposition of FeC2O4
to Fe2O3 and the second one is attributed to the oxidative
decomposition of MnC2O4 [18]. Using X-ray diffraction,
Macklen [18] proposed that the decomposition product of
manganese oxalate at this temperature range possesses a
poorly defined structure and identification is not possible.
The exothermic DTA peak which appears at 550 8C can be
attributed to the formation of iron-manganese oxide solid
solution [(Fe,Mn)2O3]. This solid solution is thermally stable
up to the last decomposition step in which the Mn2O3
dissociated to Mn3O4. This step is accompanied by a weight
loss of 1.5% (theoretical weight loss is 1.5%) at 1050 8C. A
small endothermic DTA peak at 1020 8C characterized this
step. Macklen [18] showed that the conversion of Mn2O3 to
Mn3O4 occurs between 910 and 950 8C. The higher
conversion temperature obtained in this work is attributed
to the presence of Mn2O3 in a solid solution with Fe2O3. The
heating process was continued up to 1100 8C, and then the
mixture was cooled in air up to 800 8C. The cooling DTA-TG

998
M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
curves were represented by the dotted lines (see Fig. 1). A
reverse behavior for that obtained on heating was obtained
where the TG curve shows a weight gain accompanied by a
small exothermic DTA peak at 920 8C. This behavior is
attributed to the conversion of Mn3O4 to Mn2O3, and
indicates the inability of obtaining MnFe2O4 spinel without
isothermal calcination at temperature higher than 1050 8C.
the calcination temperature on the composition and crystal-
linity was significant. XRD of the parent mixture
shows broad peaks characteristic for the individual oxalates
of MnC2O4·2H2O (JCPDS file no. 25-544) and FeC2O4·2-
H2O (JCPDS file no. 23-293). These characteristic peaks
disappeared by raising the calcination temperature to
300 8C. A nearly amorphous XRD pattern was obtained
for MnFx-300 mixture (see Fig. 2b) which suggested that
the mixture at this decomposition stage possesses a very
weak crystallinity. A few diffused and broad peaks were
observed by raising the calcination temperature to 400 8C
(see Fig. 2c). However, the considerable broadening of all of
the diffraction peaks suggests that the particle size of
3.2. X-ray powder diffraction
XRD patterns taken at room temperature of MnFx
mixture and its calcined products at different temperatures
are illustrated in Fig. 2. In this figure, the effect of
Fig. 2. Characteristic parts of XRD patterns of MnC2O4·2H2O–FeC2O4·2H2O (1:2 mole ratio) mixture calcined at different temperatures. (a) Parent mixture,
(b) mixture calcined at 300 8C, (c) mixture calcined at 400 8C, (d) mixture calcined at 500 8C, (e) mixture calcined at 600 8C, (f) mixture calcined at 1000 8C
and (g) mixture calcined at 1100 8C. Phases: (K) MnC2O4·2H2O, (W) FeC2O4·2H2O, ( p ) a-Fe2O3, ( £ ) g-Mn2O3, (þ) (Fe,Mn)2O3.

M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
999
the mixture is expected to remain very small. It is evident
that the increase in the annealing temperature will yield a
decrease in the broadening of the major peaks with an
increase in their intensities which reflects an evolution in the
crystallinity. Consistent with these the MnFe-500 mixture
gave an XRD pattern (see Fig. 2d) with improved crystal-
linity, showing characteristic peaks of a-Fe2O3 (JCPDS file
no. 13-534) and g-Mn2O3 (JCPDS file no. 18-803). The
sharpness and intensity of the characteristic peaks of a-Fe-
O3 and g-Mn2O3 increase with increasing calcination
temperature beyond 500 8C due to improved crystallinity
and particle sizes.
XRD of the 600 8C heated mixture (see Fig. 2e) shows
˚
new sharp and diffused peaks at 3.87, 2.01, 1.66 and 1.42 A
which are attributed to a phase transformation occurs in the
mixture (in agreement with the exothermic DTA peak at
550 8C). This phase transition is attributed to the formation
of iron-manganese oxide solid solution; (Fe,Mn)2O3 [19].
The only change which can be observed by raising the
annealing temperature up to 1000 8C is the increase in the
intensities of the characteristic peaks of the (Fe,Mn)2O3
solid solution. This is obvious from the XRD pattern of the
MnFe-1000 mixture (see Fig. 2f).
The XRD pattern of theMnFe-1100 mixture (see Fig. 2g)
indicates the formation of a single phase spinel with the
cubic structure of MnFe2O4 (jacobsite) (JCPDS file no. 10-
˚
319). The calculated lattice parameter (a ¼ 8:45 A) agree
well with that reported [20].
¨
3.3. Mossbauer spectral studies
57
¨
Fe Mossbauer absorption spectra measured at room
temperature for the non-calcined MnFe mixture and
mixtures calcined at different temperatures are shown in
Fig. 3. The black dots represent experimental points and the
continuous lines through the data points are results of the
¨
least square fit to the data. The Mossbauer parameters
obtained such as the isomer shift ðdÞ; the quadrupole
splitting ðDEQÞ; the spectral line width ðGÞ and the hyperfine
fields ðHÞ are reported in Table 1.
¨
The Mossbauer spectrum of the parent mixture; MnFe
(see Fig. 3a) do not reveal any magnetic interaction, and
shows a doublet with a large positive isomer shift and
quadrupole splitting as indication of divalent iron. Brady
and Duncan [21] reported an isomer shift and quadrupole
splitting of 1.02 and 1.7 mm s21, respectively, for FeC2-
O4·2H2O at room temperature which are in excellent
agreement with the values obtained in this paper (see
Table 1).
The spectra of all of the samples annealed at tempera-
tures ranging from 300 to 1000 8C (see Fig. 3b–f) consist of
a six-line subspectra and an absorption of a central
quadrupole doublet having a very small magnetically split
pattern. The thermal analysis data inferred that the early
stages of the decomposition involved the oxidation of
iron(II) to iron(III). This is clearly proven by the spectrum
¨
Fig. 3. The Mossbauer spectra of MnC2O4·2H2O–FeC2O4·2H2O (1:2 mole
ratio) mixture calcined at different temperatures: (a) Parent mixture, (b)
mixture calcined at 300 8C, (c) mixture calcined at 500 8C, (d) mixture
calcined at 600 8C, (e) mixture calcined at 800 8C, (f) mixture calcined at
1000 8C and (g) mixture calcined at 1100 8C.

1000
M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
Table 1
¨
Mossbauer parameters for MnC2O4·2H2O–FeC2O4·2H2O (1:2 mole ratio) mixture calcined at different temperatures
Mixture
Subspectra
Area
Isomer shift, d
(^0.02 mm s21
Quadrupole splitting, DEQ
(^0.02 mm s21
Line width, G
(mm s21
)
Hyperfine field, H
(^2 kOe)
)
)
MnFe
Doublet
Doublet
Sextet
100
57
43
36
64
31
69
22
78
27
73
30
70
33
67
41
59
46
54
1.01
0.42
0.47
0.42
0.44
0.42
0.44
0.46
0.44
0.47
0.44
0.46
0.45
0.46
0.45
0.46
0.52
0.41
0.48
1.70
0.87
0.13
0.88
0.20
0.87
0.23
0.95
0.23
0.95
0.23
0.97
0.23
0.98
0.23
0.99
0.17
0.01
0.01
0.62
0.63
0.75
0.66
0.58
0.68
0.50
0.58
0.48
0.53
0.45
0.54
0.42
0.56
0.42
0.40
0.37
0.76
1.08
–
MnFe-300
–
486
–
MnFe-400
MnFe-500
MnFe-600
MnFe-700
MnFe-800
MnFe-900
MnFe-1000
MnFe-1100
Doublet
Sextet
502
–
Doublet
Sextet
508
–
Doublet
Sextet
510
–
Doublet
Sextet
510
–
Doublet
Sextet
510
–
Doublet
Sextet
510
–
Doublet
Sextet
516
424
469
(A) site
(B) site
shown in Fig. 3b. The figure shows that the iron(III) oxide
formed at 300 8C is of a paramagnetic and ferromagnetic
nature, simultaneously. Since it is well known that the
magnetic properties are related to the variation of the
particle size, then part of Fe2O3 possesses an ultrafine
particles which give rise to the superparamagnetic relax-
ation that causes the collapse of the sextet pattern with the
formation of a doublet. The other part has an enough large
size to maintain the ferromagnetic properties that yields the
sextet pattern. A similar results were obtained for CoC2O4–
FeC2O4 [15] and NiC2O4–FeC2O4 [22] mixtures calcined at
280 and 350 8C, respectively. Since the ratio of the different
magnetic phases is based on the calculated absorption ratio
of the two subspectra, thus by calculating the area under the
doublet subspectra, it was found that (see Table 1) the ratio
of the doublet decreases with increasing the annealing
temperature up to 500 8C. This is mainly attributed to the
effect of the annealing temperature on the crystallinity of a-
Fe2O3 in agreement with the previous results obtained using
XRD analysis. The observed hyperfine magnetic splitting of
all of the mixtures up to 500 8C is significantly less than the
normal value of iron(III) oxide of 515 kOe [22].
reacts with g-Mn2O3 to form (Fe,Mn)2O3 solid solution
upon raising the calcination temperature. Alternatively, the
ratio of the sextet subspectra (related to the unreacted a-
Fe2O3 with g-Mn2O3) decreases upon increasing the
calcination temperature. The hyperfine magnetic splitting
obtained at 1000 8C (see Table 1) is close to that reported in
the literature for a-Fe2O3 [22].
¨
The Mossbauer spectrum of the mixture annealed at
1100 8C (see Fig. 3g) shows only a well defined hyperfine
Zeemann spectra which can be resolved into two sextets
corresponding to tetrahedral and octahedral sites in the
MnFe2O4 spinel. The isomer shift and the line width as well
as the hyperfine field are a useful parameters in identifying
the subspectra due to tetrahedral and octahedral sites of
Fe3þions. The Zeeman pattern with the smaller isomer shift
exhibits the smaller hyperfine fields and for most magne-
tically ordered spinel ferrites, the magnetic hyperfine field
due to Fe3þ at A-site is usually smaller than that at B-site.
Consequently, the A-site Fe3þ isomer shift is found to be
more negative than that for the B-site Fe3þ ions due to the
larger covalency of Fe3þ in the A-site. The line width of
the tetrahedral component must be less than that of the
octahedral component due to the presence of multiple
hyperfine fields at the Fe3þnuclei on the octahedral
component [23]. On the basis of these points and with
reference to Table 1, the sextets with hyperfine fields of 424
and 469 kOe are assigned to Fe3þ ions at tetrahedral and
octahedral sites, respectively, in agreement with the results
obtained by Geller et. al. [24].
The mixture heated at 600 8C (see Fig. 3d) shows a sextet
¨
superimposed on a doublet which possess new Mossbauer
parameters other than those obtained for samples calcined at
#500 8C indicating formation of a new phase. The sextet
¨
possessing Mossbauer parameters is consistent with those
obtained at #500 8C and attributed again to a-Fe2O3. The
doublet obtained for this heat-treated mixture is attributed to
the reaction between a-Fe2O3 and g-Mn2O3 to form a
(Fe,Mn)2O3 solid solution [19]. This doublet persists even
upon raising the calcination temperature up to 1000 8C with
increasing its ratio, which suggested that more a-Fe2O3
The fraction of Fe3þions at (A) and (B) sites were
¨
determined using the areas of Mossbauer subspectra.
Thus, considering Table 1, the probable cation distribution
of the manganese ferrite is suggested as (Fe0.92Mn0.08
)

M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
1001
[Fe1.08Mn0.92]O4. The difference in the cation distribution
of MnFe2O4 spinel in this work than that reported in Ref.
[20] ((Fe0.20Mn0.80)[Fe1.80Mn0.20]O4) can be attributed to
the method of preparation used in our work which
involves a quenching process at 1100 8C.
The disappearance of all of the characteristic bands of the
oxalate moiety by raising the calcination temperature up to
300 8C (see Fig. 4b) indicates the complete decomposition
of the oxalates. The degree of crystallinity strongly affects
the infrared spectra of the hematite. The infrared spectrum
of poorly crystalline hematite is characterized by three
strong absorption bands at 530, 445 and 308 cm21 which are
assigned to O22 displacement [26]. Thus, the broad bands
obtained at 540 and 450 cm21 (see Fig. 4b) can be assigned
to the presence of poorly crystalline (or possibly amor-
phous) a-Fe2O3. The characteristic O–H bands still
appeared at 3421 and 1637 cm21 and are attributed to the
adsorption of water molecules by ultra-fine granules of a-
Fe2O3. These bands persist even upon raising the calcination
temperature up to 600 8C.
With progressive increase in the calcination temperature,
the resolution and sharpness of the bands increase. This can
be ascribed to better crystallinity. The appearance of the
bands at 532, 460, 350 and 305 cm21 (see Fig. 4c), on
increasing the calcination temperature to 500 8C, indicated
that iron(III) oxide crystallinity was improved. Shwertmann
and Taylor [27] reported the characteristic infrared bands
for well crystallized a-Fe2O3 at 540, 470, 345 and
308 cm21. The small shift in the locations of these
characteristic bands obtained can be attributed to the
presence of foreign ions (manganese ions) in the a-Fe2O3
structure.
3.4. FT-IR spectroscopy
The vibrational spectroscopy is considered to be a very
useful technique for detection of chemical and structural
changes. Fig. 4 displays the FT-IR spectra of the heat-
treated MnFe mixture at different temperatures.
Fig. 4a shows the characteristic bands for symmetric and
asymmetric O–C–O stretcheing modes at (1317 and
1362 cm21) and (1632 cm21), respectively. Also the
characteristic bending modes for CyC–O and O–C–O
were observed at 812 and 493 cm21, respectively. The
appearance of these above mentioned bands agrees with the
presence of the oxalate moiety [25]. The presence of water
within the crystal structure of oxalates was revealed by the
characteristic bands at 3421 and 1632 cm21, identified as
O–H stretching and bending modes of vibration,
respectively.
The FT-IR spectrum of MnFe-600 mixture (see Fig. 4d)
shows that the band at 305 cm21 was replaced by three
small bands in the temperature range 300–350 cm21. This
behavior can be attributed to the formation of a new phase
besides the a-Fe2O3 phase.
The bands appeared at 556 and 380 cm21 for MnFe-1100
mixture are assigned to the stretching vibrations of Fe3þ
–
O22 in the tetrahedral and octahedral complexes of
MnFe2O4, respectively [20].
3.5. Scanning electron microscopy
In this section, SEM micrographs illustrating the change
in morphology and texture taking place during the thermal
decomposition of MnFe mixture are shown in Fig. 5. It is
evident from the micrographs that the particle sizes and
shape change throughout the decomposition course. Tex-
tural feature resolution of the parent mixture is unsatisfac-
tory because of the very small crystallites size (,0.5 mm).
For this reason, the SEM micrograph of the parent mixture
is not reproduced here.
The decomposition of the mixture at 300 8C (see Fig. 5a)
produces a large number of a small and fine granules with
superficial roughening of the crystal edges. This result
agrees with the amorphous X-ray diffraction pattern
obtained for the mixture at this calcination temperature.
Consistent with the FT-IR spectral data, some fine granules
of iron oxide absorb moisture from the atmosphere and
produce a gelatinous appearance.
Fig. 4. FT-IR spectra of MnC2O4·2H2O–FeC2O4·2H2O (1:2 mole ratio)
mixture calcined at different temperatures: (a) Parent mixture, (b) mixture
calcined at 300 8C, (c) mixture calcined at 500 8C, (d) mixture calcined at
600 8C and (e) mixture calcined at 1100 8C.

1002
M.A. Gabal, S.S. Ata-Allah / Journal of Physics and Chemistry of Solids 65 (2004) 995–1003
be an effective method for preparing stoichiometric
manganese ferrite of high chemical purity. Changing the
temperature for the decomposition of the oxalate mixture
has a significant impact on the decomposition and
morphology of the decomposition products during the
decomposition. Evidence obtained from XRD, FT-IR and
¨
Mossbauer data indicates that the ultra-fine particles of a-
Fe2O3 formed at early stages of the decomposition was of a
ferromagnetic and paramagnetic nature, simultaneously,
and begin to grow by raising the calcination temperature up
to 500 8C as revealed from SEM data. The DTA curve
shows an exothermic phase transition at 550 8C attributed
to the formation of Fe2O3–Mn2O3 solid solution which
¨
was also detected using XRD and Mossbauer data. At
1100 8C, single phase manganese ferrite (jacobsite) pre-
dominates over the solid solution. A different cation
distribution ((Fe0.92Mn0.08)[Fe1.08Mn0.92]O4) from that
¨
present in the literature was detected using Mossbauer
spectroscopy and was attributed to the applied method of
preparation.
Acknowledgements
The authors would like to thank Dr A.A. El-Bellihi,
Chemistry Department, Faculty of Science, King Abdul
Aziz University, Jeddah, KSA, for measuring the DTA-TG
experiments.
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