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RESEARCH PAPER
Raman study of cations’ distribution in ZnxMg12xFe2O4
nanoparticles
S. W. da Silva • F. Nakagomi • M. S. Silva •
A. Franco Jr • V. K. Garg • A. C. Oliveira •
P. C. Morais
Received: 19 September 2011 / Accepted: 25 February 2012
� Springer Science+Business Media B.V. 2012
Abstract In a complementary way, Raman and
Mossbauer spectroscopy were successfully employed
to assess the cations’ distribution among the tetrahe-
dral (A-site) and octahedral (B-site) sites of nonosized
ZnxMg1-xFe2O4 (0 B x B 1) cubic ferrite structure,
synthesized by combustion reaction method. Nano-
particles with little change in size distributions, in the
40 nm (x = 0.0) up to 42 nm (x = 1.0) were
obtained. Mossbauer data indicated that as the Zn-
content (x) increases in the range 0 B x B 1, the Fe3?
ion monotonically increases (decreases) the A-site
(B-site) occupancy up to nearly equal values at the
highest end x value. Analysis of the Raman data,
however, confirms that the three highest energy modes
around 650, 668 and 710 cm-1 are assigned to Zn–O
(B-site), Fe–O (A-site) and Mg–O (A-site) vibrations,
respectively. Additionally, in agreement with the
Mossbauer data, the Raman data show that as
the Zn-content (x) increases in the range 0 B x B 1,
the occupancy of A-sites by Mg2? ions monotonically
reduces with concomitant increase of A- and B-sites
occupancy by Fe3? and Zn2? ions, respectively.
Indeed, combination of the two sets of spectroscopic
data (Raman and Mossbauer) provides an effective
protocol for assessing the cations’ distribution within
the crystal structure of nanosized quaternary cubic
ferrite samples running for instance from Fe3þ0:42
�
Mg2þ0:58�
AZn2þ
0:20Mg2þ0:22Fe3þ
1:58
� �BO2�
4 at x = 0.2 up to
Fe3þ1:0
� �AZn2þ
0:60Mg2þ0:40Fe3þ
1:0
� �BO2�
4 at x = 0.6.
Keywords Raman spectroscopy � Cubic ferrite �Magnetic nanoparticles � Cation’ distribution �Mossbauer spectroscopy
Introduction
The crystal structure of cubic ferrites (MFe2O4;
M = Fe, Co, Ni, Mn, Mg, Zn,���) consist of a face
centered cubic (fcc) lattice of oxygen anions within
which cations (Fe3? and M2?) occupy tetrahedral
(A-sites) and octahedral (B-sites) interstitial sites
arranged in two possible extreme patterns: normal
and inverse. In the normal cubic ferrite structure, the
A-sites are occupied exclusively by M2? cations while
the B-sites are occupied exclusively by Fe3? cations. In
bulk form ZnFe2O4 is a typical normal-like cubic ferrite
with all Fe3? ions on B-sites and all Zn2? ions on
A-sites (O’Neill 1992). In the inverse cubic ferrite
Nanoparticles with little change in size distributions, in the 40
(x = 0.0) up to 42.
S. W. da Silva (&) � F. Nakagomi �V. K. Garg � A. C. Oliveira � P. C. Morais
Instituto de Fısica, Universidade de Brasılia, C.P. 04455,
Brasılia, DF 70919-970, Brazil
e-mail: [email protected]
M. S. Silva � A. Franco Jr
Instituto de Fısica, Universidade Federal de Goias,
Goiania, GO 74001-970, Brazil
123
J Nanopart Res (2012) 14:798
DOI 10.1007/s11051-012-0798-4
structure the A-sites are occupied exclusively by Fe3?
cations while occupation of the B-sites is shared by both
M2? and Fe3? cations, possibly in a random fashion. It
has been shown that in MgFe2O4 the site preference of
the divalent ions leads to a predominantly inverse
structure, with Mg2? ions mainly on B-sites while Fe3?
ions are distributed almost equally among A and B-sites
(McCurrie (1994). In between these two ends there are
cubic ferrites in which M2? ions can occupy both sites
and, therefore, are classified according to their degree
of inversion d (relative occupancy of M2? ions on
B-sites); positioning between normal (d = 0) and
inverse (d = 1). Such changes in cationic order can
be described through the formula [M1-dFed]A[MdFe2-
d]BO4, indicating that some M2? ions in the structure
have switched with some Fe3? ions from the B-sites to
the A-sites at a d degree. Such intermediated structures
are known as mixed ferrites. It is indeed reported that
the vast majority of cubic ferrites at the nanosized scale
are mixed (Soler et al. 2007; Sepelak et al. 2006;
Makovec et al. 2011). Cubic ferrites may also contain a
mixture of divalent metal ions like Zn–Mg ferrites (da
Silva et al. 2010), Co–Zn ferrites (Gul et al. 2007), Mn–
Zn ferrites (Rath et al. 1999). These quaternary
compounds have been prepared by simple substitution
of metal cations using chemical methods and are
extremely rich in terms of magnetic and magneto-
optical properties, which depend upon their degree of
disorder (Antic et al. 2010).
There are many experimental methods that can be
used to assess the degree of disorder in quaternary
spinel systems and most of them are addressed to the
determination of the d parameter. X-ray absorption
(O’Neill 1992; Oliver et al. 2000), neutron and X-ray
diffraction have been reported among others (Braestrup
et al. 2008). In particular, Mossbauer spectroscopy has
been widely used to determine the cations’ distribution
in cubic ferrites (Sepelak et al. 2006). Nevertheless,
Mossbauer spectroscopy alone cannot be used to
determine the cations’ distribution in quaternary spi-
nels, such as in ZnxMg1-xFe2O4. This is because of the
limitation of the Mossbauer spectroscopy in probing
iron ions only. In this regard it would be very much
interesting to explore the potentialities of employing
Raman spectroscopy as a complementary tool to the
Mossbauer spectroscopy. More specifically, it would be
worth to investigate the outcomes of the analysis while
combining the two set of recorded data, namely data
from Raman and Mossbauer spectroscopy.
Vibrational spectroscopy, both infrared and Raman,
are powerful tools for direct probing of lattice dynamics
in many different compounds. They are both widely
used as qualitative and quantitative analytical tech-
niques and to assess information regarding the chemical
structure and three-dimensional supramolecular struc-
ture. Actually, Raman spectroscopy has been recently
used to determine the cations’ distribution in magne-
sium ferrite (MgFe2O4) (Nakagomi et al. 2009). In this
study Raman and Mossbauer spectroscopy are success-
fully used, in a complementary way, to determine the
cations’ distribution in a series of quaternary cubic
ferrite samples (ZnxMg1-xFe2O4, with 0.0 B x B 1.0).
Experimental
Magnesium-zinc ferrite nanoparticulate samples
(ZnxMg1-xFe2O4, with 0.0 B x B 1.0) were synthesized
by the combustion reaction method without subsequent
calcination steps (Franco et al. 2007). Analytical grade
iron nitrate Fe(NO3)3�9H2O, zinc nitrate Zn(NO3)2�6H2O, magnesium nitrate Mg(NO3)2�6H2O and urea
CO(NH2)2 were used as fuels. Materials manipulation
and chemical reactions were carried out in atmospheric
air. The stoichiometric composition of each mixture was
calculated based on the total oxidizing and reducing
valences of both oxidizer and fuel. The chemical
composition (total zinc and magnesium contents) of all
ferrite samples was assessed by atomic absorption
spectrophotometry using the commercial Perkin-Elmer
5000 system. The nominal compositions were in good
agreement with the core chemical compositions obtained
by atomic spectroscopy (see Table 1).
Information regarding the samples’ crystal struc-
ture, lattice constant and average nanoparticle sizes
were obtained by room temperature X-ray diffraction
(XRD) using a Shimadzu diffractometer (model 6000)
with Cu-Ka (k = 1.54 A) radiation while scanning the
spectra in a wide range of Bragg angles (15� \ 2h\ 80�). Low temperature (77 K) Mossbauer spectra of
all samples were recorded in the transmission geom-
etry using a 57Co source in Rh matrix. The system
velocity was calibrated with a thin natural iron foil
whereas the spectra were least-square fitted to a
combination of Lorentzian-like lines. The Raman
spectra were recorded using a commercial Jobin–
Yvon triple spectrometer (T64000) equipped with a
CCD detector. The 514 nm line of a CW Argon ion
Page 2 of 10 J Nanopart Res (2012) 14:798
123
laser was used to excite the samples whereas the
optical excitation intensity was kept around 0.2 mW.
All Raman measurements were performed at room
temperature.
Results and discussions
The XRD patterns of the as-synthesized samples
(ZnxMg1-xFe2O4, with 0.0 B x B 1.0) are collected
in Fig. 1. Analysis of all XRD patterns revealed the
cubic ferrite structure ðFd�3mÞ; in agreement with the
used synthesis route. The absence of extra reflections
in the diffraction patterns ensures phase purity. The
mean particle sizes (D) were calculated from the X-ray
line broadening of the (311) diffraction peak using the
Scherrer’s equation: D = 0.9 k/b (cosh). From the
analysis of the XRD spectra we found that the mean
particle diameter (D) is nearly the same for all
synthesized specimens, ranging from ca. 40 nm (x =
0.0) up to 42 nm (x = 1.0). This finding means that the
increasing of Zn-content in the range of 0.0 B x B 1.0
does not affect much the average grain size of the as-
synthesized nanoparticulate ferrite samples (ZnxMg1-
xFe2O4). Nevertheless, the XRD data show that the
lattice parameter (a) changes linearly with the Zn-
content (x), increasing from a = 8.365 A to a =
8.430 A with the Zn-content increasing from x = 0.0
to x = 1.0, thus following roughly the Vegard’s law
(Denton and Ashcroft 1991). This behavior can be
attributed to the larger ionic radius of Zn2? (0.74 A—
A-sites and 0.88 A—B-sites) as compared to the ionic
radius of the Mg2? (0.71 A—A-sites and 0.80 A—B-
sites). The incorporation of larger ions into the lattice
of the nanoparticles would expand the lattice and
increase the observed lattice parameter. The values
found for the lattice parameter of the as-synthesized
nanoparticulate samples are close to the values
corresponding to the bulk ferrite (MgFe2O4–JCPDS
card #73-2211, a = 8.366 A and ZnFe2O4–JCPDS
card #22-1012 a = 8.441 A).
The low temperature (77 K) Mossbauer spectra of
all synthesized samples (ZnxMg1-xFe2O4, with 0.0 B
x B 1.0) show a systematic variation as the Zn-content
(x) increases (see Fig. 2). In order to investigate the
effect of the increasing x-content all Mossbauer
spectra were curve-fitted with Lorentzian-like com-
ponents, taking into account the two sublattices
(A- and B-sites). For smaller Zn-content (0.0 B x B
0.4) the Mossbauer spectra were accomplished using
two sextets, corresponding to 57Fe ions located at the
A- and B-sites. At intermediate x values (0.5 B x B
0.7) we observed broad Mossbauer spectra (two
sextets), which can be explained considering the
breakdown of the local magnetic order due to the
increase in Zn-content. At higher Zn-content, say
Table 1 Nominal stoichiometry based on the relative amounts
of the starting materials and chemical composition obtained by
atomic spectroscopy
Nominal (x) Experimental
ZnxMg1-xFe2O4 x (Zn) (1 - x)(Mg)
0.0 0.000 0.999
0.2 0.192 0.808
0.4 0.390 0.610
0.5 0.487 0.513
0.6 0.617 0.383
0.7 0.699 0.300
0.8 0.812 0.188
1.0 0.998 0.000
20 30 40 50 60 70 80
0.5
0.4
0.2
2θ (degree)
0.6
0.7
0.0di
ffrac
tion
Inte
nsity
x = 1.0
0.8
(400
)
(422
) (511
)
(440
)
(533
)
(220
)
(311
) ZnxMg
1-xFe
2O
4
Fig. 1 X-ray diffraction patterns of the as-synthesized
ZnxMg1-xFe2O4 samples, with 0 � x � 1
J Nanopart Res (2012) 14:798 Page 3 of 10
123
x = 0.8 and 1.0, the Mossbauer spectra exhibit a
strong doublet superimposed to a weak sextet, as
expected from a magnetically disordered phase. The
Mossbauer parameters resulting from the least-squares
fitting of the recorded spectra, taking into account the
two sublattices of cubic ferrites, were plotted as
function of the Zn-content (see Fig. 3). The values of
the hyperfine field (HF), isomer shift (IS), quadruple
splitting (QS) and relative Fe-content for 0.0 B
x B 0.8 are shown in Fig. 3a–d, respectively.
As can be seen from the data plotted in Fig. 3a the HF
associated to both crystallographic sites (A- and B-sites)
systematically decrease as the Zn-content (ZnxMg1-
xFe2O4, with 0.0 B x B 1.0) increases. This finding is
partially explained by the reduction of the first nearest
neighbor (A–B) superexchange interaction strength due
to the redistribution of the magnetic Fe3? ions among
A- and B-sites. Additionally, the systematic decrease of
the HF is due to the weakening of the A–B superex-
change interaction owing to the increase in distance
between A- and B-sites as a result of the increase in the
lattice parameter with increasing Zn-content (increasing
x value) (Srivastava et al. 1976; Hamedoun et al. 2010).
Furthermore, Fig. 3b shows the isomer shift behavior
(A- and B-sites) with increasing Zn-content; monoton-
ically increasing (decreasing) for the B-site (A-site) up to
about x = 0.5. For x C 0.5 the IS values reverse their
trends; monotonically decreasing (increasing) for the
B-site (A-site) in the range of 0.5 B x B 0.8, though
keeping the IS values in the vicinity of 0.43 mm/s. The
IS behavior can be explained through the bonding nature
of the Fe3? ion while occupying both sites. With
increasing Zn-content up to about x = 0.6 the Fe3?
population in the A-site increases while decreasing in the
B-site, as shown in Fig. 3d. As the ionic radius of Fe3?
(0.63 A—A-site and 0.78 A—B-site) is smaller than the
ionic radii of Zn2? and Mg2? (see ‘‘Experimental’’
section) there is an expected shrinking of the tetrahedral
coordination (A-site) with consequent increase in the
iron–oxygen orbitals’ overlapping, thus resulting in the
decrease of the IS values associated to the A-site.
Conversely, up to about x = 0.6 the B-site experiences a
systematic reduction of the Fe3? population while
increasing the Zn2? population, thus owing to an expect
expansion of the octahedral coordination with a conse-
quent decrease in the iron–oxygen orbitals’ overlapping.
This trend results in the increase of IS values associated
to the B-site. Notice, however, that above about x = 0.6
Fig. 2 Low temperature (77 K) Mossbauer spectra of the
ZnxMg1-xFe2O4 samples, with 0 � x � 1. Open circles are the
experimental data while the solid lines represent the best fit
using one doublet or two sextet which represent A- and B-sites
(dashed lines)
20
40
60
80
0.36
0.40
0.44
0.48
0.52
350
400
450
500
550Zn
xMg
1-xFe
2O
4
A siteB site
(b)
Z n - content (x)
(c)
(a)
QS
(m
m/s
)
IS (
mm
/s)
HF
(ke
O)
Fe
popu
latio
n (%
)
0.0 0.2 0.4 0.6 0.8 1.00.0 0.2 0.4 0.6 0.8 1.0-0.3
-0.2
-0.1
0.0
0.1 (d)
Fig. 3 Zinc-content dependence of a hyperfine field (HF)
b isomer shift (IS) c quadrupolar splitting and d Fe population
(%) obtained by Mossbauer spectra recorded from the
ZnxMg1-xFe2O4 samples, with 0 � x � 1. The solid and opensymbols represent A- and B-site, respectively. The dashed linesare only guide to the eyes
Page 4 of 10 J Nanopart Res (2012) 14:798
123
little changes were observed in both IS values and
relative Fe3? ion site occupancy. Figure 3c shows that
the cubic symmetry of the oxygen coordination in both
sites (A- and B-sites) is monotonically affected as one
replaces Mg2? by Zn2? ions. Further, deviation from
zero QS values in both sites indicates that the oxygen
cubic symmetry around the Fe3? ions is indeed broken.
Nevertheless, introduction of Zn2? ions affects differ-
ently the cubic symmetry of A- and B-sites. In the whole
range of Zn-content (x) investigated small QS values
were found in the A-site, indicating that replacement of
Mg2? by Zn2? ions did not change appreciably the site
symmetry. However, we found a different behavior
associated to the B-site, since the QS value is close to
zero for x = 0 but increases (negative values) system-
atically with increasing Zn-content. This behavior
shows that the B-site is more strongly affected than the
A-site upon replacement of Mg2? by Zn2? ions. Finally,
Fig. 3d shows the evolution of the area under the
Mossbauer subspectra as the Zn-content increases in the
range of 0.0 B x B 0.7. Therefore, as shown in Fig. 3d,
increasing the Zn-content the Fe3? occupancy system-
atically increases at the A-site while decreasing at the
B-site. This finding indicates that lower x values favor
the inverse-like structure with most of the Fe3? ions
occupying B-sites. However, increasing the Zn-content
above about x = 0.6 the Fe3? ions tend to be equally
distributed in both sites. This finding indicates that for
large x values (x C 0.6) the A-site is mostly occupied by
Fe3? ions whereas the B-site is occupied by Fe3?, Zn2?,
and Mg2? ions. Unfortunately, in quaternary cubic
ferrites Mossbauer data alone do not provide enough
information for complete site occupancy determination,
as two non-Mossbauer isotopes are included in the
crystal structure, as for instance Zn2? and Mg2?.
Additionally, at higher Zn-content (x C 0.8), the pres-
ence of the Mossbauer paramagnetic doublet makes it
impossible to assign the cations’ distribution to each
available site. Nevertheless, considering the Mossbauer
data presented in Fig. 3d and taking into account charge
balance requirements it was possible to propose a partial
cations’ distribution for the as-synthesized samples.
Based on the Mossbauer data we included in Table 2 the
partial cations’ distribution for each synthesized
ZnxMg1-xFe2O4 sample (upper row). Notice, however,
that in Table 2, M means the divalent cation, with no
precise assignment to Zn2? or Mg2?.
As mentioned previously, Raman spectroscopy
has shown to be a powerful tool for direct probing
of lattice dynamics of many different compounds.
Table 2 Cations’ distribution
as obtained by Mossbauer
(upper row) and Raman
spectroscopy (lower row).
M means the divalent cation,
with no precise assignment to
Zn2? or Mg2?
(x) Zn Technique Cations’ distribution
0 Mossbauer Mg2þ0:47Fe3þ
0:53
� �AMg2þ
0:53Fe3þ1:47
� �BO2�
4
Raman Mg2þ0:48Fe3þ
0:52
� �AMg2þ
0:52Fe3þ1:48
� �BO2�
4
0.2 Mossbauer M2þ0:60Fe3þ
0:40
� �AM2þ
0:40Fe3þ1:60
� �BO2�
4
Raman Mg2þ0:58Fe3þ
0:42
� �AZn2þ
0:20Mg2þ0:22Fe3þ
1:58
� �BO2�
4
0.4 Mossbauer M2þ0:33Fe3þ
0:67
� �AM2þ
0:67Fe3þ1:33
� �BO2�
4
Raman Mg2þ0:52Fe3þ
0:48
� �AZn2þ
0:40Mg2þ0:08Fe3þ
1:52
� �BO2�
4
0.5 Mossbauer M2þ0:30Fe3þ
0:70
� �AM2þ
0:70Fe3þ1:30
� �BO2�
4
Raman Mg2þ0:28Fe3þ
0:72
� �AZn2þ
0:50Mg2þ0:22Fe3þ
1:28
� �BO2�
4
0.6 Mossbauer M2þ0:06Fe3þ
0:94
� �AM2þ
0:94Fe3þ1:06
� �BO2�
4
Raman Fe3þ1:00
� �AZn2þ
0:60Mg2þ0:40Fe3þ
1:00
� �BO2�
4
0.7 Mossbauer M2þ0:04Fe3þ
0:96
� �AM2þ
0:96Fe3þ1:04
� �BO2�
4
Raman Fe3þ1:00
� �AZn2þ
0:70Mg2þ0:30Fe3þ
1:00
� �BO2�
4
0.8 Mossbauer –
Raman Fe3þ1:00
� �AZn2þ
0:80Mg2þ0:20Fe3þ
1:00
� �BO2�
4
1.0 Mossbauer –
Raman Fe3þ1:00
� �AZn2þ
1:00Fe3þ1:00
� �BO2�
4
J Nanopart Res (2012) 14:798 Page 5 of 10
123
According to the literature (Nakagomi et al. 2009;
Seong et al. 2001), as the Raman integrated intensity is
proportional to the number of the correspondent
oscillators the technique can be successfully used to
determine quantitatively or at least semi-quantita-
tively the content of a particular element in a given
sample. Therefore, in the present study, we selected
Raman spectroscopy as a complementary tool to the
Mossbauer spectroscopy in the task to assess the
cations’ distribution in the as-synthesized ZnxMg1-
xFe2O4 quaternary samples. Actually, Raman spec-
troscopy has been used in the study of cubic ferrites for
more than three decades (Verble 1974). A symmetry
analysis and the assignment of phonon modes to
magnetite, based on the spinel structure, were carried
out by Verble (1974); Shebanova and Lazor (2003);
Degiorgi et al. (1987) and Graves et al. (1988). Results
from these studies vary significantly either in the
number of expected Raman modes or with respect to
their positions and assignments. A major difficulty in
studying this system is related to the order–disorder
phenomena, generally observed in the spinel structure.
The cations’ exchange among the available A- and
B-sites gives rise to different structural configurations.
As a result, the Raman spectra recorded from different
samples, though presenting the same stoichiometry are
qualitatively different. Therefore, a very important
step while assessing information on cation ion site-
occupancy is to make an appropriate assignment of the
vibrational modes, though this task is an old problem
while related to cubic ferrites.
The cubic ferrite’s family, herein represented by
MFe2O4, has a cubic structure (fcc) belonging to the
space group Fd�3m. The theoretical analysis based on
the factor-group approach predicts five Raman-active
bands, namely A1g;Eg; and three T2g (White and
DeAngeli 1967). Graves et al. (1988) suggested a new
irreducible representation based on a symmetry
brought about lattice defects which leads to a splitting
of one T2g mode into A1g þ Eg representation, thus
justifying the presence of an additional vibrational
mode in the Raman spectra. On the other hand,
considering the treatment of the vibrational modes of
ferrites in terms of a molecular model, as proposed by
Waldron (1955), only sites of symmetry Td and C2v;,
respectively, occupied by Fe3? (A-site) and O2- ions
effectively contribute to the Raman spectrum. There-
fore, as confirmed in the case of inverse spinel ferrite,
such as MgFe2O4, NiFe2O4 and Fe3O4, only the A-site
contribute to the Raman spectrum (Verble 1974;
Shebanova and Lazor 2003; White and DeAngeli
1967). Although the Raman spectra of the inverse
spinels are different, they have a common feature; a
strong A1g band in the 670–710 cm-1 region. This
band is observed regardless the particular bivalent
cation in the sample’s stoichiometry and has been
assigned to the stretching vibrational of the tetrahedral
FeO4 (Kreisel et al. 1998). In the case of normal spinel
ferrites, MFe2O4 (for instance M = Zn or Mn), with
M2? ions occupying A-sites while Fe3? ions fill in
B-sites, it has been observed that the A1g band arises in
the 600–620 cm-1 region (Graves et al. 1988; Kreisel
et al. 1998). No other band has been observed at higher
frequency. Furthermore, this band arises in the
600–620 cm-1 region regardless the particular biva-
lent cation. In analogy with the Raman spectrum of
hematite (highest band at about 600 cm-1), in which
only FeO6 B-sites are found in the crystalline structure,
Kreisel et al. (1998) proposed that this band is
essentially related to the Fe–O vibrational of the
FeO6 octahedron. However, Wang et al. (2003) while
reporting on zinc ferrite considered that the band
observed at 647 cm-1 is due to the oxygen motion in
the A-site. Similar statement was made by Maletin
et al. (2007) while studying ZnxIn1-xFe2O4.
Figure 4 presents the room temperature Raman
spectra of the as-synthesized ZnxMg1-xFe2O4 samples
(0.0 B x B 1.0), recorded in the range of 200–850
cm-1. The Raman modes A1g þ Eg þ 3Tg
� �; charac-
teristic of the cubic spinel Fd�3mð Þ space group, have
been assigned to the features appearing on the
200–850 cm-1 region. The Raman features indicated
in Fig. 4 (dotted lines) are the vibrational modes
typical of the nanoparticle’s crystalline structure.
Raman features other than those related to the spinel
phase (MFe2O4) were not observed, as for instance
from a-Fe2O4, FeO, MgO, or ZnO. Although the
factor-group analysis predicts the presence of only five
Raman modes our fitting procedure, using Lorentzian-
like lines, reveals seven features instead. For the
MgFe2O4 sample (see dotted lines in Fig. 4, for
x = 0), the observed vibrational modes are around
218, 335, 388, 487, 550, 668, and 710 cm-1. The
presence of seven vibrational modes, however, are
consistent with recent data reporting on magnesium
ferrite (Nakagomi et al. 2009; Wang et al. 2002).
Page 6 of 10 J Nanopart Res (2012) 14:798
123
Indeed, for cubic ferrites there is an acceptable
consistency in regard to the strongest Raman modes
peaking around 710, 487 and 335 cm-1. However, the
agreement for the remaining Raman modes is less
satisfactory (Shebanova and Lazor 2003; Gupta et al.
2002). It was shown in the work of Nakagomi et al.
(2009) that the presence of two Raman modes above
670 cm-1 are mainly associated to the motion of
oxygen atoms in the tetrahedral complex (MO4),
where some Fe3? ions are replaced by Mg2? ions.
The Raman mode featuring at 218 cm-1 is no
longer observed in the as-synthesized samples at
x C 0.2. However, for x C 0.2, a new Raman mode
appears around 650 cm-1 (gray) as can be observed in
the spectra included in Fig. 4. Note that as the
Zn-content increases in the range of 0.2 B x B 1.0
the three highest wavelength Raman modes (see Fig. 4
for light gray, gray and black emphasized Raman
lines) change in relative intensity. Additionally, a
systematic shift of the vibrational energy of the highest
wavelength Raman modes is also observed. Changes
on relative intensity and peak position can be
explained considering alloy effects, resulting from
the introduction of a new ion at increasing content
(Pusep et al. 1995). Further, the Raman frequency
depends on the Fe(Mg)–O bond length, which changes
with both the variation of the lattice parameter and
ionic radii of divalent ions. Systematic variation of the
relative integrated intensity and peak position of the
three emphasized Raman modes (see Fig. 4) are
collected in Fig. 5. Figure 5a, b clearly show that
while the intensity of the highest wavelength Raman
mode (initially around 710 cm-1) decreases with
increasing Zn-content the intensity of the Raman
mode peaking around 670 cm-1 increases proportion-
ally. Simultaneously, it was observed that the intensity
of the Raman mode peaking around 650 cm-1
increases steeply as the Zn-content (x) increases. At
this point we claim that due the large mass difference
between the two ions (Fe3? and Mg2?) leads to the
splitting of the A1g mode into two branches. Therefore,
200 300 400 500 600 700 800
0.5
0.4
0.2
Wavenumber (cm-1)
0.6
0.0
Ram
an In
tens
ity
x = 1.0
0.8
ZnxMg
1-xFe
2O
4
Fig. 4 Raman spectra of the ZnxMg1-xFe2O4 samples, with
0 � x � 1. The dashed lines show the Lorentzian fit of the data.
The structure in black (light gray) and gray represent the bonds
at the Mg(Fe)O4 at A-site and ZnO6 at B-site, respectively
(a) (d)
(b) (e)
(c) (f)
Fig. 5 The left panel shows the integrated intensity (%) of
Raman modes associated to the bonds a Mg�O4�Mð1;2Þ12 and
b Fe�O4�Mð1;2Þ12 at A-sites and c Zn�O6�M
ð1;2Þ6 at B-sites. The
right panel shows the Raman shifts associated to the bonds
d Mg�O4�Mð1;2Þ12 (5 cm-1) e Fe�O4�M
ð1;2Þ12 (23 cm-1), and
f Zn�O6�Mð1;2Þ6 (11 cm-1), where Mð1Þ= Mg and Mð2Þ = Fe.
The dashed lines are only guide to the eyes
J Nanopart Res (2012) 14:798 Page 7 of 10
123
the modes around 710 (black) and 668 cm-1 (light
gray) can be attributed to the Mg–O and Fe–O
vibrations. Furthermore, the onset of the Raman
feature around 650 cm-1 at x C 0.2, with intensity
increasing as the Zn-content (x) increases, is a strong
evidence that this mode is related to Zn–O bond
vibration. However, the task of assigning the sublat-
tice (A- or B-site) to this particular Raman mode
(650 cm-1) is not straightforward. The first hypothesis
would be related to the well-know strong A-site
preference of Zn2? ions in bulk ferrites. In this case it
would be reasonable to expect that with increasing Zn-
content the Fe3? ion tends to migrate from A- to B-site.
However, the Mossbauer data show that the Fe3? ion
population increases in the A-site with increasing x, as
represented by the solid circles in Fig. 3d. Addition-
ally, the Mossbauer data suggest that at x C 0.6 the A-
site is mainly occupied by Fe3? ions. Therefore, the
hypothesis of Zn2? ions entering the A-site of the as-
synthesized samples has no experimental support.
Alternatively, a second hypothesis for the onset of the
Raman mode around 650 cm-1 would be based on the
proposal put forward by Kreisel et al. (1998), in which
this Raman band is associated to the Fe–O vibration in
B-sites (FeO6 complex). Again, as can be observed in
Fig. 3d (see open circles), the Mossbauer data show
that the Fe3? population decreases at B-sites as the Zn-
content increases. Additionally, the Raman data show
that the Raman peak around 650 cm-1 increases with
the increasing of the Zn-content. Therefore, like the
first hypothesis, the second one has no experimental
support. Finally, we claim that the Raman band
appearing around 650 cm-1 is associated with the
presence of Zn2? ions at B-sites. This hypothesis,
generally unusual for bulk ferrites, has been recently
reported for nanosized particles (Upadhyay et al.
2004; Choi et al. 2006).
According to the above discussion, we came to the
conclusion that the Raman mode around 650 cm-1 is due
to the Zn–O vibration with Zn2? ions populating B-sites
whereas the two modes at higher energy (*710 and
*668 cm-1) present A1g symmetry and can be
assigned to stretching modes of the A-site. In short,
the Raman mode around 710 cm-1, A1g Mgð Þ; is
associated with Mg2? ions located on A-sites
which are surrounded by twelve nearest metal ions
located on B-sites (Mg�O4�Mð1;2Þ12 ; with M(1) = Mg
and M(2) = Fe). Likewise, the A1g Feð Þ Raman mode
(*668 cm-1) is claimed to be associated with Fe3?
ions located on A-site (Fe�O4�Mð1;2Þ12 ). Once the
integrated intensity of the A1g Mgð Þ and A1g Feð ÞRaman modes are, respectively, proportional to the
number of Mg�O4�Mð1;2Þ12 and Fe�O4�M
ð1;2Þ12 bonds
we use the corresponding integrated intensities (IMg)
and (IFe) to assess the Mg- and Fe-content in the A-site.
Therefore, we can obtain both the Fe-content
xARaman Feð Þ
� �and the Mg-content xA
Raman Mgð Þ ¼�
1 � xARaman Feð ÞÞxA
Raman in the A-site from the Raman
data as follows:
xARaman Feð Þ ¼ aIFe
aIFe þ bIMg
; ð1Þ
where a and b represent the oscillator strength of the
Fe�O4�Mð1;2Þ12 and Mg�O4�M
ð1;2Þ12 bonds, respec-
tively. Nakagomi et al. (2009), while using Raman
spectroscopy to study the cations’ distribution within
the system MgxFe3-xO4, showed that the oscillator
strength involving Fe�O4�Mð1;2Þ12 bonds is twice the
oscillator strength of Mg�O4�Mð1;2Þ12 bonds. There-
fore, the relative oscillator strength is given by R =
b/a = 0.5. Finally, using Eq. 1 with R = 0.5 and
taking into account charge balance requirements it was
possible to assess the total cations’ distribution of all
synthesized samples, as shown in Table 2 (lower
rows). Note form the data displayed in Table 2 that the
Fe3? ion distribution obtained by both Mossbauer and
Raman spectroscopy agree quite well. The calculated
values of xARaman Feð Þ versus the obtained values using
Mossbauer spectroscopy xAMossbauer Feð Þ
� �are plotted in
Fig. 6. Note the excellent correlation factor (1.1)
between the two sets of data presented in Fig. 6,
indicating the robustness of the approach of using the
combination of Raman and Mossbauer data to assess
the cation site distribution in quaternary cubic ferrites.
Conclusion
Nanosized ZnxMg1-xFe2O4 (0 B x B 1) quaternary
cubic ferrite particles were synthesized by the com-
bustion reaction method whereas the cations’ distri-
bution was successfully assessed by combining
Raman and Mossbauer spectroscopy while taking
into account charge balance requirements. Due to the
Page 8 of 10 J Nanopart Res (2012) 14:798
123
large mass difference between Fe3?, Zn2? and Mg2?
ions the A1g Raman mode splits into three branches,
allowing us to quantify the cations’ distribution for the
as-synthesized ZnxMg1-xFe2O4-based samples using
the Raman data. We found the cations’ distribution
obtained by Raman spectroscopy in very good agree-
ment with the partial cations’ distribution obtained by
Mossbauer spectroscopy. Analyses of the Mossbauer
data show an increase in the orbital overlapping of the
Fe3? ions in A-sites, while decreasing in B-sites,
resulting in a systematic change of the isomer shift.
Also, it was verified that the hyperfine field decreases
monotonically with increasing Zn-content (x), which
is claimed to be related to the progressive redistribu-
tion of the Fe3? ions among A- and B-sites as well as
due to the increase of the lattice parameter. The Raman
data revealed the presence of Fe3? and Mg2? ions in
the A- and B-sites, whereas Zn2? ions were found only
in the B-site. We finally came to the conclusion that the
combination of the two sets of data (Raman and
Mossbauer) provides a robust protocol for assessing
the cations’ distribution within the crystal structure of
nanosized quaternary cubic ferrite samples.
Acknowledgments The authors acknowledge the financial
support from the Brazilian agencies CNPq, CAPES, and FINEP.
References
Antic B, Jovic N, Pavlovic MB, Kremenovic A, Manojlovic D,
Vucinic-Vasic M, Nikolic AS (2010) Magnetization
enhancement in nanostructured random type MgFe(2)O(4)
spinel prepared by soft mechanochemical route. J Appl
Phys 107:043525. doi:10.1063/1.3319563
Braestrup F, Hauback BC, Hansen KK (2008) Temperature
dependence of the cation distribution in ZnFe2O4 measured
with high temperature neutron diffraction. J Solid State
Chem 181:2364–2369. doi:10.1016/j.jssc.2008.05.028
Choi EJ, Ahn Y, Song KC (2006) Mossbauer study in zinc
ferrite nanoparticles. J Magn Magn Mater 301:171–174.
doi:10.1016/j.jmmm.2005.06.016
da Silva SW, Nakagomi F, Silva MS, Franco A, Garg VK,
Oliveira AC, Morais PC, Lima ECD (2010) Effect of the
Zn content in the structural and magnetic properties of
ZnxMg1 - xFe2O4 mixed ferrites monitored by Raman and
Mossbauer spectroscopies. J Appl Phys 107:09B503. doi:
10.1063/1.3350903
Degiorgi l, Blattermorke I, Wachter P (1987) Magnetite–pho-
non modes and the Verwey transition. Phys Rev B 35:
5421–5424. doi:10.1103/PhysRevB.35.5421
Denton AR, Ashcroft NW (1991) Vegard law. Phys Rev A
43:3161–3164. doi:10.1103/PhysRevA.43.3161
Franco A, Lima ECD, Novak MA, Wells PR (2007) Synthesis of
nanoparticles of CoxFe(3-x)O4 by combustion reaction
method. J Magn Magn Mater 308:198–202. doi:10.1016/
j.jmmm.2006.05.022
Graves PR, Johnston C, Campaniello JJ (1988) Raman-scat-
tering in spinel structure ferrites. Mater Res Bull
23:1651–1660. doi:10.1016/0025-5408(88)90255-3
Gul IH, Abbasi AZ, Amin F, Anis-ur-Rehman M, Maqsood A
(2007) Structural, magnetic and electrical properties of
Co1-xZnxFe2O4 synthesized by co-precipitation method.
J Magn Magn Mater 311:494–499. doi:10.1016/j.jmmm.
2006.08.005
Gupta R, Sood AK, Metcalf P, Honig JM (2002) Raman study of
stoichiometric and Zn-doped Fe3O4. Phys Rev B 65:104
430. doi:10.1103/PhysRevB.65.104430
Hamedoun M, Benyoussef A, Bousmina M (2010) Magnetic
properties and phase diagram of Zn(x)Ni(1-x)Fe(2)O(4):
high-temperature series expansions. J Magn Magn Mater
322:3227–3235. doi:10.1016/j.jmmm.2010.05.030
Kreisel J, Lucazeau G, Vincent H (1998) Raman spectra and
vibrational analysis of BaFe12O19 hexagonal ferrite. J Solid
State Chem 137:127–137. doi:10.1006/jssc.1997.7737
Makovec D, Kodre A, Arcon I, Drofenik M (2011) The structure
of compositionally, constrained zinc-ferrite. J Nanopart
Res 13:1781–1790. doi:10.1007/s11051-010-9929-y
Maletin M, Moshopoulou EG, Kontos AG, Devlin E, Delimitis
A, Zaspalis V, Nalbandian L, Srdic VV (2007) Synthesis
and structural characterization of In-doped ZnFe2O4
nanoparticles. J Eur Ceram Soc 27:4391–4394. doi:
10.1016/j.jeurceramsoc.2007.02.165
McCurrie RA (1994) Ferromagnetic materials–structure and
properties. Academic Press Inc, New York
Nakagomi F, da Silva SW, Garg VK, Oliveira AC, Morais PC,
Franco A (2009) Influence of the Mg-content on the cation
distribution in cubic MgxFe(3-x)O(4) nanoparticles. J Solid
State Chem 182:2423–2429. doi:10.1016/j.jssc.2009.
06.036
O0Neill HS (1992) Temperature dependence of the cation dis-
tribution in zinc ferrite (ZnFe2O4) from powder XRD
structural refinements. Eur J Minerol 4:571–580
0.2
0.4
0.6
0.8
1.0
1.2
0.2 0.4 0.6 0.8 1.0 1.2
xA
Raman = 1.1 xA
Mossbauer
xA
Raman (Fe)= aI
Fe/(aI
Fe+bI
Mg)
R = b/a = 0.5
xA
Mössbauer(Fe)
xA R
aman
(Fe)
Fig. 6 Fe-content xARaman Feð Þ
� �calculated using Eq. 1 versus
Fe-content obtained from the Mossbauer xAMossbauer Feð Þ
� �data.
The solid line represents the linear fit xARaman ¼ 1:1xA
Mossbauer
J Nanopart Res (2012) 14:798 Page 9 of 10
123
Oliver SA, Harris VG, Hamdeh HH, Ho JC (2000) Large zinc
cation occupancy of octahedral sites in mechanically
activated zinc ferrite powders. Appl Phys Lett 76:2761–
2763. doi:10.1063/1.126467
Pusep YA, da Silva SW, Galzerani JC, Milekhin AG, Preo-
brazhenskii VV, Semyagin BR, Marahovka II (1995)
Spectroscopy of the optical vibrational-modes in GaAs/
AlxGa1-xAs heterostructures with monolayer-wide Alx-
Ga1-xAs barriers. Phys Rev B 52:610–2618. doi:10.1103/
PhysRevB.52.261
Rath C, Sahu KK, Anand S, Date SK, Mishra NC, Das RP
(1999) Preparation and characterization of nanosize Mn–
Zn ferrite. J Magn Magn Mater 202:77–84. doi:10.1016/
S0304-8853(99)00217-6
Seong MJ, Hanna MC, Mascarenhas A (2001) Composition
dependence of Raman intensity of the nitrogen localized
vibrational mode in GaAs1 - xNx. Appl Phys Lett
79:3974–3976. doi:10.1063/1.1424469
Sepelak V, Feldhoff A, Heitjans P, Krumeich F, Menzel D,
Litterst FJ, Bergmann I, Becker KD (2006) Nonequilibri-
um cation distribution, canted spin arrangement, and
enhanced magnetization in nanosized MgFe2O4 prepared
by a one-step mechanochemical route. Chem Mater
18:3057–3067. doi:10.1021/cm0514894
Shebanova ON, Lazor P (2003) Raman spectroscopic study of
magnetite (FeFe2O4): a new assignment for the vibrational
spectrum. J Solid State Chem 174:424–430. doi:10.1016/
S0022-4596(03)00294-9
Soler MAG, Lima ECD, da Silva SW, Melo TFO, Pimenta
ACM, Sinnecker JP, Azevedo RB, Garg VK, Oliveira AC,
Novak MA, Morais PC (2007) Aging investigation of
cobalt ferrite nanoparticles in low pH magnetic fluid.
Langmuir 23:9611–9617. doi:10.1021/la701358g
Srivastava CM, Shringi SN, Srivastava RG (1976) Mossbauer
study of relaxation phenomena in zinc–ferrous ferrites.
Phys Rev B 14:2041–2050. doi:10.1103/PhysRevB.14.
2041
Upadhyay C, Verma HC, Anand S (2004) Cation distribution in
nanosized Ni–Zn ferrites. J Appl Phys 95:5746–5751. doi:
10.1063/1.1699501
Verble JL (1974) Temperature-dependent light-scattering
studies of Verwey transition and electronic disorder in
magnetite. Phys Rev B 9:5236–5248. doi:10.1103/Phys
RevB.9.5236
Waldron RD (1955) Infrared spectra of ferrites. Phys Rev
99:1727–1735. doi:10.1103/PhysRev.99.1727
Wang ZW, Lazor P, Saxena SK, O’Neill HS (2002) High
pressure Raman spectroscopy of ferrite MgFe2O4. Mater
Res Bull 37:1589–1602
Wang ZW, Schiferl D, Zhao YS, O’Neill HSC (2003) High
pressure Raman spectroscopy of spinel-type ferrite
ZnFe2O4. J Phys Chem Solids 64:2517–2523. doi:10.1016/
j.jpcs.2003.08.005
White WB, DeAngeli BA (1967) Interpretation of vibrational
spectra of spinels. Spectrochim Acta 23A:985–995. doi:
10.1016/0584-8539(67)80023-0
Page 10 of 10 J Nanopart Res (2012) 14:798
123