Download Characterization of the citrate precursor, used for

Cent. Eur. J. Chem. • 7(3) • 2009 • 415-422
DOI: 10.2478/s11532-009-0054-7
Central European Journal of Chemistry
Characterization of the citrate precursor,
used for synthesis of nanosized Mg-Zn ferrites
Research Article
Violeta D.Kassabova-Zhetcheva
Department Technology of Silicates,
University of Chemical Technology and Metallurgy,
Sofia 1756, Bulgaria
Received 22 September 2008; Accepted 07 January 2009
Abstract: The citrate precursor has been used to synthesize nanocrystalline Mg-Zn-ferrites. The nature of the prepared precursor is characterized
and compared with those of the precursors studied earlier, prepared by the same process. The study has been performed by
inorganic and organic elemental analyses, Fourier Transformed Infrared Spectroscopy (FTIR), Mössbauer spectroscopy, X-ray
Photoelectron Spectroscopy (XPS), Electron Paramagnetic Resonance (EPR), Electronic absorption spectrometry in the UV-VIS
region, Differential Thermal analysis/ Thermogravimetry (DTA-TG) analyses, and X-ray diffraction (XRD) analysis. The collected
results determined the precursor as a coordination polymer with monomer unit (NH4)4{M [Fe(C6H5O7)2]2}, where M=Zn or Mg.
Keywords: Iron(III) citrate complexes • Mg-ferrite • Zn- ferrite • Citrate method
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1. Introduction
The citrate precursor technique has been used to
synthesize nanocrystalline Mg-Zn-ferrites with respect
to their application in thermal cancer therapy [1]. The
citrate precursors are widely used to produce a number
of multi-component, phase-homogeneous nanosized
ferrites [2-8]. According to its creators, the technique
comprises in the obtaining of amorphous mixture
retaining all the necessary metallic ions and citric acid in
pure homogeneity [2]. From previous research studies,
the citrate precursors are presented by the formulas:
Zn3Fe6(C6H5O7)8•(12+n)H2O [3]; Ni3Fe6O4(C6H6O7)8•6H2O [4];
Co3Fe6(C6H6O7)8•6H2O [5] which defined them as
mixtures of metal ions and citric acid. The authors in the
following articles determined the precursors for ferrites
of Mg(II), Ca(II) and Zn(II) as citrate complexes:
Mg3[Fe(C6H5O7)2]2•10H2O [6]; Ca3[Fe(C6H5O7)2]2•8H2O [6]
and Zn3[Fe(C6H5O7)2]2•12H2O [7]. The researchers in the
next study described the citrate precursor for Ni2Fe2O4
as a linear-type polymer [(C6H6O7)4NiFe2]n [7]. The
complexes described in [6,7] consist of citrate anions
C6H5O73-, without information about the usage of a base.
In [8], ammonium hydroxide was used, but the possibility
for participation of NH4+ ions in the formed complex was
not discussed.
The possibility for a complex formation in the
studied system: citric acid-Fe(III)-Mg(II)-Zn(II)-NH4+,
was adopted due to the well known chelating properties
of citric acid. The complex compounds that form the
ions of magnesium and zinc, depend on the pH rate and
the amount of citric acid. According to the dissociation
scheme of citric acid [9,10], at a pH rate of about 6, theions
of Zn(II) and Mg(II) may form the following complexes:
[MC6H6O7]0, and [MC6H5O7]-, where M=Zn(II) and Mg(II).
The coordination chemistry of citric acid - Fe(III) system
is studied taking into account the biological importance
only [10-12]. The established results in contributed to
the better elucidation of the complex formation between
Fe(III) and citric acid. It is important to note that two
iron(III) citrate complexes, crystallized from solution at
pH 6 and pH 7, consisted of ammonium ions as outer
coordination sphere [12]. The authors have established
that the complexes corresponded to the following
formulas: (NH 4 ) 4 [Fe(C 6 H 5 O 7 )(C 6 H 6 O 7 )]• 3H 2 O, and
(NH4)5[Fe(C6H5O7)2]•2H2O.
The question of whether mixtures of initial
compounds, complexes of citric acid or coordination
* E-mail: [email protected]
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Characterization of the citrate precursor,
used for synthesis of nanosized Mg-Zn ferrites
polymers are formed needs further clarification.
Moreover, the question regarding the type of citrate
complexes formed and whether the ammonium ions take
part in the citrate precursor for mixed Mg-Zn ferrites has
not yet been answered. Thus, the aim of this paper is to
study the citrate precursor for Mg-Zn ferrites which will
contribute to the better understanding and controlling of
the process of synthesis.
2. Experimental Procedures
The citrate precursor for Mg-Zn-ferrites is prepared by
the same mode described in [1]. In brief, it includes
obtaining a mixed solution of stoichiometric quantities
of Mg(NO3)2•6H2O, Zn(NO3)2•6H2O, and ferric citrate
FeC6H5O7•H2O. The ratio of metal ions to anhydrous
citric acid (C6H8O7) used is 1:3. Ammonia hydroxide is
used for correcting the pH rate up to 6. The obtained
solution is heated at 80°C. Solely, in this study, before
the solution becomes viscous, the evaporation was
stopped and crystalline compound was formed in open
air for a month. The obtained compound was stored in a
desiccator over P2O5. The examined sample was named
“Precursor” and was prepared according to the formula
Mg1-xZnxFe2O4, where: x=0.3. The sample “Precursor”
was heat treated in the dryer at 100°C for 4 hours, after
that the burned material was calcined at 250°C, 350°C,
450°C, and 600°C.
The concentration of Fe(III) is determined by chemical
titration. For determination of Zn and Mg, a method of
Atomic Absorption Spectrometry (AAS) was applied
by using a spectrophotometer Perkin-Elmer 5000. The
content of C, H, and N in the obtained precursor was
determined by the common organic analysis method
using a Karlo Erba analyzer. The DTA-TG of the precursor
was carried out in atmospheric air using a Paulik-PaulikErdey (MOM,Hungary) derivatograph. The heating rate
was 10°C min-1 and amount of the sample was 100 mg.
The X-ray diffraction patterns of studied sample were
taken at room temperature (RT) in the Bragg-Brentano
geometry (2θ from 5° to 80°), with Philips APD 15.
The diffractogram was obtained using Cu Kα radiation
(λ = 1.54178 Å). The FTIR-spectra were collected for
disk specimens mixed with KBr using a Bruker Equinox
55 spectrometer in the range 4000-400 cm-1. Mössbauer
spectroscopy gave additional information about the
valence state of iron ions and their coordination. The
transmission type spectrum was recorded with an
electromechanical spectrometer Wissenschaftliche
Elektronik GMBH, using a 57Co/Cr source in constant
acceleration mode at RT. The velocity was calibrated by
α-Fe standard at room temperature. The experimentally
obtained spectrum was evaluated by a program, which
assumes a Lorentzian shape of the Mössbauer spectral
lines. The following parameters were determined isomer shift (IS), quadrupole splitting (QS), as well as
the line width (FWHM), and the relative weight (G) of
each component. The EPR spectrum was registered
as the first derivative of the absorption signal with an
X-band ERS-220/Q spectrometer at RT. Electronic
absorption spectra were taken at RT with a UV-VIS
spectrometer Cary 100 within the range of 200 to 800
nm. The UV-VIS spectra of the sample “Precursor” were
recorded after dissolving in distilled water in ratio: 1:1
and 1:2, and for ratio 1:1 after a week under sunlight.
The X-ray Photoelectron spectra were obtained using
un-monochromatized Al Kα (1486.6 eV) radiation in
a VG ESCALAB MK II electron spectrometer under
base pressure of 1×10-8 Pa and a total instrumental
resolution of 1 eV. The charging effects were corrected
by using the C1s peak as reference at binding energy
(BE) of 284.6 eV. The photoelectron spectra of C1s,
O1s, N1s, Fe2p, Mg1s, and Zn2p were recorded and
corrected by subtracting a Shirley-type background and
then were quantified using the peak area and Scofield’s
photoionization cross-sections.
3. Results and Discussion
3.1 FTIR- study
The FTIR-spectrum of the studied precursor is presented
in Fig. 1. The small shoulder at 3435 cm-1 confirms the
presence of lattice water [13]. The free ammonium ion
has four modes of vibration: a non-degenerate (۷1),
a doubly degenerate (۷2), and two triply degenerate
vibrations (۷3 and ۷4). All four vibrations are Raman
active, whereas only ۷3 and ۷4 are IR-active. The
fundamental frequencies ۷1, ۷2, ۷3, and ۷4 for the free
ammonium ion are 3040, 1680, 3145, and 1400 cm-1
[14]. The infrared spectrum of NH4+ in a symmetrical
environment contains the bands at 3145 and 1400 cm-1
only [14]. Ammonium absorption bands of the sample
“Precursor” correspond to the NH4-bending vibration
(۷4) at 1437 cm-1 and a series of overlapping bands from
2797 to 3213 cm-1. The overlapping bands arise from
the NH4+ stretching vibration ۷3, combination mode ۷2
+ ۷4, and overtones 2۷2 and 2۷4 [14]. The appearance
of IR-inactive bands and overtones corresponds to the
distorted Td symmetry of ammonium ion [13], probably
as a result of interaction with the crystal field of citrate
ligands. Such NH4+ vibrational spectrum is well known
because it corresponds to those in many ammonium salts
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Violeta D.Kassabova-Zhetcheva
[15]. Thus, a formation of ammonium salt in the sample
“Precursor” can be expected. The frequency shift of the
IR-active bands, with respect to the theoretical positions
from 3150 to 3213 cm-1 and from1400 to 1437 cm-1 were
assigned to formation of hydrogen bonds in the sample
“Precursor” with NH4+ participation [15].
The formation of citrate complexes is proven by
the doublets located at 1617 and 1578 cm-1 due to
antisymmetric stretching vibration, and at 1415 and
1399 cm-1 due to symmetrical stretching vibration
of ionized carboxylate groups [5,6,10,13]. The band
positions are close to the ones cited in the analogous
citrate precursors for ferrites, which are listed in Table 1.
The IR-spectrum of the sample “Precursor” differs with a
presence of two pronounced doublets for antisymmetric
and symmetric stretching vibrations which reveals the
occurrence of two non-equivalent carboxylate anions
(Fig. 1). This fact could be explained by the presence
of two types of coordination, which are deduced by
the difference (∆) of antisymmetric and symmetric
wave numbers (Table 1) [13]. As a result, a bidentate
coordination could be assigned to Fe(III) ions. A sharp
and distinct band at 566 cm-1 ascertains the bonding
of Fe(III) to the citrate ligands in octahedral geometry
[5,6]. Probably, in the unidentate type of coordination,
the ions of Zn(II) and Mg(II) are involved. The two
bands at 524 and 480 cm-1 were assigned to Zn-O and
Mg-O bonds [16,17]. The most common coordination
for d10 cations of Zn is tetrahedral and they mainly form
covalent bonds with ligands [18]. For the d° cations
of Mg, interactions with ligands are predominantly
columbic or ionic, however, the tetrahedral geometry is
also characteristic of their complexes [18]. The studied
sample does not show any absorption bands above
1700 cm-1 indicating the absence of an undissociated
-COOH group [18]. The deformation vibrations at 2601,
2527, 2050, 1288, 1262, 1200, 1137, and 1077 cm-1 are
characteristic of the undissociated –OH group [16,19].
For the sample “Precursor”, the sharp and intense band
observed at 1079 cm-1 is characteristic for the metal
hydroxyl deformation modes and may be related to
Fe-OH hydrogen bonding [16,19]. Normally, the metalhydroxyl deformation vibration is located at 1030 cm-1
and shifting the band to higher frequencies can be
obtained by increased strength of hydrogen bonds and
high degree of depolarization of O-H bond due to strong
Lewis acid nature of Fe(III) ion [11].
The FTIR-spectra of the heat treated sample
“Precursor” are shown in Figs. 2a-e. As can be seen,
the bands of carboxylate anions are still present in
the recorded spectra up to 250°C (Figs. 2a, b). Thus,
the citrate complexes were thermally stable up to
temperatures of the ignition of the auto-combustion
process. The thermal decomposition of the citrate
precursor began at 350°C and led to the initial formation
of metal oxycarbonate complex. The spectral evidence
Table 1. Infrared data for citrate precursors, citrate ionic compound, and citrate complex compound
۷as(COO-) ,cm-1
۷ s(COO-) , cm-1
Δ = ۷as -۷s,cm-1
Citrate ionic compound, Na3 Citrate [19]
1609
1402
207, free of coordination
“Precursor” for Mg0.7 Zn0.3Fe2O4
1617
1578
1399
1415
218, unidentate
163, bidentate
Sample
Citrate precursor for MgFe2O4, Mg3[Fe(C6H5O7)2]2•10H2O[6]
1680
1410
270, unidentate
Citrate precursor for ZnFe2O4, Zn3[Fe(C6H5O7)2]2•12H2O [7]
1622
1385
237, unidentate
Citrate precursor for NiFe2O4, [(C6H6O7)4NiFe2]n [8]
Citrate complex compound, (NH4)5 Fe(C6H4O7)2•2H2O [11]
Figure 1. FTIR-spectrum of sample “Precursor”.
1595
1389
206, unidentate
from1626 to1586
from1436 to1372
~200, unidentate
Figure 2.
FTIR-spectra of sample “Precursor” heat treated at:
a) 100°C, b) 250°C, c) 350°C, d) 450°C and e) 600°C.
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Characterization of the citrate precursor,
used for synthesis of nanosized Mg-Zn ferrites
Figure 3. Mössbauer spectrum of sample “Precursor”.
Figure 4. . EPR X-band spectrum of sample “Precursor”.
(Fig. 2c) is revealed as splitting of the doubly degenerate
vibration ν3 into two bands located at 1596 and 1409 cm-1
and appearing of infrared inactive ν1 at 1119 cm-1 [13,20].
With the augmentation of temperature from 450°C to
600°C, the bands assigned to oxycarbonate formation
varied from 1506 to 1517 cm-1 and from 1415 to
1427 cm-1 along with bands between 1106 and
1119 cm-1. The splitting of the degenerate vibration ν3
was lowered with the rising of temperature and it can
be suggested that bidentate oxycarbonate complex
became unidentate [13].
3.3 EPR study
3.2 Mössbauer spectroscopy study
The experimentally obtained Mössbauer spectrum
(Fig. 3) consists of line shoulders and central doublet
(Db) part. The Mössbauer data shown in Table 2
indicate that all iron ions are in high spin (S = 5/2), in
oxidation state +3, and in octahedral coordination. The
coordination number of six for iron(III) satisfied by two
citrate ligands which bind to iron through the oxygen
atoms of the carboxylate groups [5,6]. The values of
Mössbauer parameters for sample “Precursor” are
close to those cited for complexes of magnesium
bis(citrate) ferrate(III) decahydrateand zinc bis(citrate)
ferrate(III) [6].
Table 2.
In Fig. 4 is shown the EPR X-band spectrum of
sample “Precursor”, which was registered as the
first derivative of the absorption signal. It consists of
one wide asymmetric line with width at about 149G
(149.10-4 T) (Fig. 4). This broad single line between
g = 1.89 and g = 1.96 in the spectrum is probably due
to spin-spin interaction of Fe(III)-Fe(III) pairs [21]. It
can be inferred that Fe centers are part of a polymer
structure. The octahedral coordinated Fe(III) ions show
an EPR signal of g-value of about 2 and an octahedral
coordination can be assigned to Fe(III) ions of the
sample “Precursor”. For comparison, a g-factor of citrate
complex (NH4)5[Fe(C6H4O7)2]•H2O is 4.3 and consistent
with the presence of high-spin rhombic Fe(III) species
[10]. The result obtained for sample “Precursor” is in
good agreement with the previously established data
from Mössbauer spectroscopy. The spikes in the low
field part can not be interpreted.
3.4 UV-VIS study
The electronic absorption spectrum (Fig. 5, green
line) of the dissolved sample “Precursor” in ratio 1:1
to distilled water, consisted of a broad, weak maximum
located at 455 and strong, well-defined maximum at
394 nm. After the scanning of the diluted solution
(ratio 1:2), the maxima appeared with a very small
displacement at 453 nm and 380 nm (Fig. 5, brown line).
Mössbauer parameters of different citrate precursors and citrate complex compound.
Sample
Component / Spectral view
IS mm s-1
QS mm s-1
FWHM mm s-1
G, %
“Precursor”
Fe octa/br sym db
0.41
0.62
0.50
100
Citrate precursor for MgFe2O4 [6]
Fe3+ octa/br sym db
0.38
0.59
-
-
Citrate precursor for ZnFe2O4 [7]
Fe3+ octa/br sym db
0.42
0.58
-
-
Citrate complex compound
(NH4)5[Fe(C6H4O7)2] •H2O
Fe3+ octa/br sym db
0.38
1.80
-
-
3+
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Figure 5.
UV-VIS scans of sample “Precursor diluted in ratio:
1) 1:1, green line; 2) 1:2, brown line; and 3) 1:1, aged
under sunlight, red line.
The maximum at about 450 nm can be assigned
to the 6A1 + 6A1 → 4T1(4G) + 4T1(4G) excitation of an
Fe(III) – Fe(III) pair, which is as result of the magnetic
coupling of electronic spins of next-nearest neighbor
Fe(III) ions and referred to as a pair excitation or double
excitation process [23]. A feature near 390 nm correspond
to the 6A1 → E4 (4D) ligand field transitions of Fe(III)
[25]. Judging by the very pale-yellow color of studied
sample, a weak magnetic coupling can be suggested
[21]. These observations gave rise to the assumption
for the existence of a polymeric structure in the studied
sample “Precursor”, which coordinated well with EPR
results. From the UV-VIS spectrum of the mononuclear
iron citrate complex, the only one ill-defined peak at
450 nm was established without assignment [10].
The last scan (Fig. 5, red line) was recorded after
the aging of the dissolved sample “Precursor” (ratio 1:1)
for a week under sunlight. During aging, the color turned
green which can be explained with the formation of the
Fe(II) citrate complex. The spectral view is a featureless
scan that is characteristic for citrate complexes of Fe(II)
[24]. In this way, the oxidation state of iron ions in the
sample “Precursor” has been ascertained as “3+”.
Figure 6. C1s XPS spectrum of sample “Precursor”.
Figure 7. O1s XPS spectrum of sample “Precursor”.
3.5 XPS study
To further examine the chemical structure of the sample
“Precursor”, the XPS spectra of C1s, O1s, N1s, and
Fe2p core levels were obtained (Figs. 6-9). The C1s
spectrum was fitted with three peaks (Fig. 6). The C1s
peak situated at 284.6 eV is ascribed to the carbon atoms
in the aliphatic chain (C-C) [25]. The peak at 286.6 eV
can be assigned to a C-OH group [26]. The peak
situated at 288.0 eV was assigned to the carboxylate
(-COO-) moiety [27]. No C1s corresponding to carboxylic
carbon (-COOH) appeared in the spectrum, indicating
the absence of free acid in the studied sample [27].
The fitting of O1s spectrum revealed the existence
of three peaks situated at 529.9 eV, 531.4 eV, and
532.9 eV (Fig. 7). The peak at 529.9 eV is consistent with
the data previously obtained for Fe-O bonding in hematite
Figure 8. N1s XPS spectrum of sample “Precursor”.
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Characterization of the citrate precursor,
used for synthesis of nanosized Mg-Zn ferrites
[28]. This assigning of the photoelectron peak suggests
that six oxygen ions were octahedrally coordinated
to Fe(III) in the sample “Precursor”. The single and
symmetric peak at 531.4 eV revealed the presence of
two symmetric oxygen atoms in the carboxylate (-COO-)
moiety and absence of C=O bonds in the sample [29].
The peak at 532.9 eV was assigned to C-OH group [26].
The O1s peaks for Mg-O and Zn-O bindings reported
at about 530.5 eV and 531.2 eV, respectively [30,31].
Due to overlapping by the neighboring O1s regions,
these peaks are not pronounced. The resolved N1s
spectrum is due to two peaks (Fig. 8). The first peak
situated at 399.7 eV was attributed to existence of NH3
and the second one situated at 401.5 eV was assigned
to ammonium ion NH4+ [26]. The Fe2p peak with the
specific spin-orbit split in two components (2p3/2,1/2)
is shown in Fig. 9. To conclude from the Mössbauer,
EPR, UV-VIS spectral data and assignment of the O1s
core region, the Fe2p line-shape of sample “Precursor”
was analyzed with respect to the fitting of peak positions
for high spin Fe(III) compound. A large amount of XPS
studies on the iron oxidation state in the iron oxides
and reported binding energies value of Fe 2p3/2 peak,
spread over broad superposed ranges: between 709.1
and 710.65 eV for FeO, between 707.9 and 710.7 eV for
Fe2O4, and between 710.6 and 711.5 eV for Fe2O3 [32].
To infer the oxidation state of iron from the Fe 2p XPS
core-level spectrum of sample “Precursor” turned out
to be a difficult task because the cited data correspond
to environment of oxygen ligands. The previous XPS
data about iron citrate complexes are not found. The
BE of Fe2p3/2 and Fe2p1/2 peaks in the analyzed sample
“Precursor” are 709.74 eV and 723 eV respectively, along
with satellites appeared as peak shoulders at 713.4 eV
and 727 eV. It can be seen that BE value of Fe2p3/2 peak
is lower than that cited for “3+” oxidation state in the
Fe2O3 [32]. Since, the Fe(III) ion has a configuration
of d5 in its ground state, probably the lowering of BE
value of Fe2p3/2 peak can be explained by a process,
which occurs in the final state via charge transfer from
ligand to Fe(III) so as to form Fe3+ 2p1/21□2p3/243d6 or
Fe3+ 2p1/22 2p3/23□d6 (□ – hole) and, since such transfer
costs relatively little energy, such process will lower the
binding energy [33]. Moreover, as the electronegativity
of the ligand decreases, the electron density surrounding
the Fe cation increases, meaning that the nucleus is
more shielded, as a result the binding energy associated
with ferric 2p3/2 photoelectron peak also decreased [35].
Identically, in the case of sample “Precursor”, the citrate
ligand is a much weaker crystal field splitting ligand than
that of O2- ligand, therefore the Fe(III) ions in the sample
“Precursor” do not resemble the Fe(III) ions found in the
iron oxides [34].
The sample “Precursor” present a single
photoelectron Mg 1s peak at 1304.2 eV which can be
related to the Mg–O binding and a single photoelectron
Zn 2p3/2 peak at 1021.9 eV which can also be assigned
to the Zn-O binding [31].
Figure 9. XPS spectrum from the Fe2p level of sample “Precursor”.
Figure 10. XRD patterns of the sample “Precursor”.
3.6 XRD Study
The formed precursor is crystalline, contrary to that
previously reported as X-ray amorphous [1,5-7]. The
XRD-patterns of the studied precursor are shown
in Fig. 10. The sample “Precursor” consists of a lot
of initial compounds, which extremely bothers the
phase-identification. Moreover, there is not reference
data concerning similar complicated compositions.
Definitely, the comparison with PDF (Powder Diffraction
Files) base leads to the conclusion that the sample
“Precursor” is not a re-crystallized mixture of initial salts.
Also, the XRD-patterns of the studied sample cannot
be interpreted as phase-formation of the complex
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Figure 11. DTA-TG traces of the sample “Precursor”.
compounds
(NH 4 ) 4 [Fe(C 6 H 5 O 7 )(C 6 H 6 O 7 )]•3H 2 O
or (NH4)5[Fe(C6H5O7)2]•2H2O [11]. The presence of
NH4NO3 was identified with JCPDS 47-0867. The X-ray
diffraction patterns show that the interplanar distances
of one of the crystal phases excising in the studied
sample “Precursor” are very close to ammonium citrate
(NH4)3C6H5O7 (JCPDS 45-1540). This fact could be
interpreted as an indication of ammonium citrate being
present in the system [35].
3.7 Elemental analysis
The results of inorganic and organic elemental analyses
are shown in Table 3. The theoretically calculated gross
formula of the studied sample Mg2.35ZnFe6.7C82H156O116N28
and such derived from elemental analyses
Mg2.39ZnFe6.9C83H161O119N26 are in good agreement. The
gross formula of the sample “Precursor” ascertained
the assumption that the compound with high molecular
weight like polymer is formed.
Table 3. The elemental content of the sample “Precursor”.
Elemental content, %
Mg
Zn
Fe
C
H
N
Calculated
1.68
1.47
9.61
25.44
4.00
9.96
Observed
1.66
1.48
9.83
25.19
4.10
9.34
3.8 DTA-TG analysis
The DTA and TG traces of sample “Precursor” (Fig. 11)
revealed that the multi-step decomposition process took
place. Such decomposition path is described in [5-7]
and differs from one-step decomposition process, which
was established in [36]. The DTA trace was started with
a small endothermic effect at 120°C with corresponding
weight loss of 9%. This event was attributed to the
dehydratation process, which ended at about 180°C.
The amount of hydrated water was determined at
about 0.5 mol. The second weight loss of 50% and an
exothermic event at 260°C indicate the beginning of
the rapid decomposition process, which steeply passed
trough the next exothermic event at about 400°C. The
accompanied weight loss reached 73%. Based on the
thermo-gravimetric calculations, it was believed that at
this stage the formation of the intermediate compound,
probably oxycarbonate Mg2.39ZnFe6.9(CO3)O5, took
place. The formation of oxycarbonate compound may
be explained with polymer structure of citrate precursor,
which provides closeness between initial elements and
prevents the fast cleavage leading to the formation of
related metal oxides as it stands in [5,6]. The intermediate
remains stable up to 440°C. At 490°C a decomposition of
oxycarbonate took place, followed by fast exo-process
which was accompanied by a mass loss of 80% and was
attributed to the crystallization of mixed Mg-Zn ferrite
phase at 540°C. The further decomposition of residual
oxycarbonate with weight loss of 4% was characterized
by an endothermic event at 580°C. The weight of the
final residue is 16% of the initial weight, coinciding with
the calculated value of Mg0.7Zn0.3Fe2O4, 16.7%.
4. Conclusions
The collected data indicated that citrate precursor used
for synthesis of Mg-Zn ferrites represents a coordination
polymer. It was believed that monomer unit is (NH4)4{M
[Fe(C6H5O7)2]2}, where M is Zn(II) or Mg(II). Both citrate
ligands are three-ionized, and as such they utilize
one terminal as well and central carboxylate to bind
in a bidentate fashion to Fe(III). In order to satisfy the
coordination requirements of the octahedral Fe(III), the
hydroxyl moiety is bound by hydrogen bond to Fe(III).
The third, terminal, ionized carboxylate group of both
citrate ligands are coordinated to Mg(II) or Zn(II) ions.
In this way, the ions of Mg and Zn bridged neighbor
citrate ligands and form chains. Probably, the NH4+ ions
linked polymeric chains by hydrogen bonds and as a
result a 3D-polymeric network is formed. Additionally,
the NH4+ promotes crystal packing. The elemental and
DTA-TG analyses have ascertained the theoretically
calculated gross formula of the citrate precursor as
Mg2.35ZnFe6.7C82H156O116N28. The polymer structure of
the citrate precursor allowed converting the gel into
corresponding Mg-Zn ferrite powders, avoiding the
stage of a solid state synthesis from corresponding
oxides. This is possible due to structure-based ligand
design. Since the polymer chains are sequences of the
octahedral citrate complexes of Fe(III) and tetrahedral
421
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Characterization of the citrate precursor,
used for synthesis of nanosized Mg-Zn ferrites
citrate complexes of Zn(II) or Mg(II), they served as a
template for the spinel lattice in the precursor stage.
During the calcinations, the cations rested on the
previously set positions as a result of the thermal stability
of the citrate complexes and the formed oxycarbonate
intermediate complex. Thus, by using citrate ligands it
could be possible to tailor the cation distributions which
are crucial for the magnetic properties of the ferrites
Acknowledgements
I am grateful to Prof. Isabelle Gautier-Luneau and Prof.
Athanasios Salifoglou for kindly provided articles.
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