Synthesis, magnetic and electrical properties of Fe-containing SiO 2 nanocomposite

Article abstract of DOI:10.1016/j.jallcom.2010.04.135

Nanocrystalline Fe powders were synthesized by transmetallation reaction and embedded in silica to form Fe-SiO₂ nanocomposite. Thermomagnetic study of the as-prepared Fe sample indicates the presence of Fe ₃O₄ and Fe particles. Oxidation studies of Fe and Fe-SiO₂ show an increased thermal stability of Fe-SiO₂ nanocomposite over pure Fe. The Fe-SiO₂ shows an enhanced oxidation temperature (i.e., 780 K) and a maximum saturation magnetization value of (135 emu/g) with 64 wt.% of Fe content in silica. Electrical and dielectric behaviour of the Fe-SiO₂ nanocomposite has been investigated as a function of temperature and frequency. Low frequency ac conductivity and dielectric constants were found to be influenced by desorptions of chemisorbed moisture. High saturation magnetization, thermal stability, frequency-dependent conductivity and low power loss make Fe-silica a promising material for high frequency applications.

Full text of DOI:10.1016/j.jallcom.2010.04.135

Journal of Alloys and Compounds 502 (2010) 169–175
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis, magnetic and electrical properties of Fe-containing SiO2
nanocomposite
E. Thirumala, D. Prabhub, K. Chattopadhyayb, V. Ravichandrana,∗
a Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India
b Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanocrystalline Fe powders were synthesized by transmetallation reaction and embedded in silica to
form Fe–SiO2 nanocomposite. Thermomagnetic study of the as-prepared Fe sample indicates the pres-
ence of Fe3O4 and Fe particles. Oxidation studies of Fe and Fe–SiO2 show an increased thermal stability
of Fe–SiO2 nanocomposite over pure Fe. The Fe–SiO2 shows an enhanced oxidation temperature (i.e.,
780 K) and a maximum saturation magnetization value of (135 emu/g) with 64 wt.% of Fe content in sil-
ica. Electrical and dielectric behaviour of the Fe–SiO2 nanocomposite has been investigated as a function
of temperature and frequency. Low frequency ac conductivity and dielectric constants were found to
be influenced by desorptions of chemisorbed moisture. High saturation magnetization, thermal stabil-
ity, frequency-dependent conductivity and low power loss make Fe–silica a promising material for high
frequency applications.
Received 9 November 2009
Received in revised form 20 April 2010
Accepted 23 April 2010
Available online 18 May 2010
Keywords:
Composite materials
Chemical synthesis
Dielectric response
Oxidation
© 2010 Elsevier B.V. All rights reserved.
Thermal analysis
Electrochemical impedance spectroscopy
1. Introduction
amorphous silica by sol–gel reaction method. Thermal stability
and magnetic properties of native and Fe–silica nanocomposite
Magnetic metal and alloy nanoparticles encapsulated in non-
magnetic metals, insulators, ferrites and polymers have attracted
great interest due to their potential applications in high frequency
transformers, inductive components [1], magneto impedance,
microwave absorber [2], etc. Among various magnetic materials,
nanocrystalline Fe gains attention due to its high saturation mag-
netization, permeability and low coercivity. However, Fe particles
in nano form have a high affinity for oxygen. Despite the reduction
of their total magnetic moment, when dispersed in non-magnetic
matrix or coating, several studies have been conducted to encap-
sulate these particles in silica [1,3], alumina [4], yittria [5,6], gold
[7], carbon [8], boron nitride [9], and Fe3O4 [10,11] in order to
improve their stability, electrical and magnetic properties. Fe–silica
nanocomposites are of particular interest because they have higher
resistivity and this combined with the good magnetic properties of
metallic Fe, makes them a good candidate for dielectric applications
and could possibly be a good replacement material for ferrites.
In this work we have synthesized nanocrystalline Fe particles
by transmetallation chemical method and their composite with
have been studied. The ac electrical and dielectric behaviour of
Fe–SiO2 nanocomposite have been investigated using impedance
spectroscopy.
2. Experimental method
2.1. Materials
Merck products of anhydrous FeCl3 analytical reagent (96%), aluminium fine
metal powder 200 mesh (99.7%), sodium hydroxide (98%), 25% NH3 ammonia
solution, Hong-Yang Chemical product of Ethanol (99.9%) and Aldrich tetraethyl
orthosilicate (TEOS) (98%) were purchased and used without further purification.
2.2. Methods
For the synthesis of nanocrystalline Fe particles by transmetallation chemical
method, typically, 8.4 g anhydrous FeCl3 was added to 100 ml of distilled water.
After the mixture was stirred for 10 min to form a homogeneous solution, 3 g of fine
aluminium powder was added to the solution with constant stirring and within a
few minutes a vigorous exothermic reaction was observed. After the completion of
the reaction, a black precipitation of Fe powder was obtained and the temperature
of the solution increased from room temperature to 340 K. The precipitates were
repeatedly washed with distilled water in order to remove the byproducts such as
AlCl3, etc. Further, to remove the unreacted aluminium particles, the precipitates
were repeatedly washed with 0.5 M NaOH solution followed by distilled water. The
obtained black powders were washed and stored in ethanol.
∗
Corresponding author.
In order to prepare Fe–silica nanocomposite, certain quantity of Fe powder
was dispersed in 100 ml ethanol. Then 30 ml of 10 vol.% TEOS in ethanol solution
was slowly added to it with mechanical stirring and finally 25 ml of 5 vol.% of
ammonia (25% NH3 in H2O) in ethanol solution was also added slowly. The stir-
Tel.: +91 44 22202800/22351444; fax: +91 44 22570305/44 22352494.
E-mail addresses: es thirumal@yahoo.com (E. Thirumal),
ravichandranvraman@gmail.com (V. Ravichandran).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.04.135

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E. Thirumal et al. / Journal of Alloys and Compounds 502 (2010) 169–175
ring was continued for 12 h and aged for 48 h. The resulting Fe–silica composite
powders was washed with acetone and dried in air for 1 h at 350 K. The phase
purity of Fe and Fe–silica composite particles were studied by X-ray diffraction
(XRD) with Bragg–Brentano configuration using Cu K␣ radiation. Room tempera-
ture hysteresis loops were recorded using a vibrating sample magnetometer (VSM;
EG&G Princeton Applied research 4500 model). Oxidation behaviour and ther-
momagnetic studies were performed by thermogravimetric analysis (TGA) using
PerkinElmer instruments. The microstructures of the materials were studied using
scanning electron microscopy (SEM; with a JEOL JSM 840 model) and transmission
electron microscopy (TEM; of JEOL FX2000). Particle size distribution was mea-
sured using dynamic light scattering (DLS) at room temperature with a Zetasizer
3000HSA (Malvern Instruments, UK). Fourier transform of infra-red (FT-IR) spectra
were recorded in the transmission mode for Fe–SiO2 nanocomposite with an FT-IR
spectrometer (PerkinElmer). Using hydraulic pressure (Lawrence & Mayo (India) Pvt.
Ltd. (Model: LM-17-510)), powder Fe–silica sample was compacted in the form of
a pellet with applied pressure of 4975 kg/cm2. Electrical conductivity and dielectric
properties were measured as a function of frequency in the range of 1 Hz–10 MHz,
using an impedance gain/phase analyzer (Solertron 1260) interfaced to a personal
computer.
3. Results and discussion
Fig. 1. Powder XRD pattern showing high crystallinitiy of the as-prepared Fe and
Fe–silica nanocomposite.
Fig. 1 presents the XRD pattern of the pure and Fe–silica com-
posite. It could be observed that the as-prepared Fe particles have
pure BCC structure with high crystallinity. The intensity ratios as
well as peak positions (Miller indices) which correspond to 44.69◦
(1 1 0), 65.07◦ (2 0 0) and 82.38◦ (2 1 1) match very well with JCPDS
card No.: 87-0721. The average crystallite size of the as-prepared
powder, calculated using Scherrer formula including the correc-
tion for instrumental broadening, was found to be about 50 nm.
Oxide phases were not observed by means of XRD and hence the
oxide layer on Fe particles may be in amorphous form. The XRD pat-
tern of Fe particles encapsulated with silica also exhibit the peak
positions of BCC-Fe structure. Absence of peaks corresponding to
silica indicates the formation of amorphous silica. In order to iden-
tify the presence of silica in Fe–SiO2 nanocomposite, the samples
were examined by SEM analysis. Fig. 2(a) is the bright field TEM
micrographs showing typical Fe crystallites about 50 nm in size
and Fig. 2(b) is the bright field image showing such crystallites
Fig. 2. Bright field TEM images showing (a) native Fe crystallites with typical grain sizes of about 50 nm, (b) Fe crystallites dispersed in a silica matrix.

E. Thirumal et al. / Journal of Alloys and Compounds 502 (2010) 169–175
171
Fig. 4. Dynamic light scattering (DLS) plot of Fe particles suspended in ethanol.
bending vibrations at about 1630 cm−1. Another broad band in the
low energy region of the spectrum due to the characteristic vibra-
tions of Si–O–Si is observed in the range of 960–1280 cm−1. An
additional sharp peak around 785 cm−1 suggests the presence of
Si–O–Fe bonds. The band at 694 cm−1 is attributed to the Fe–O–Fe
bond from Fe2O3 as described elsewhere [12]. The FT-IR spectrum
indicates the presence of surface oxidation on Fe particles. Silica
coating is more compatible with iron oxide molecules rather than
with bare Fe atom. The vibration bond around 785 cm−1 indicates
that the silica molecules are bonded with Fe atom through the
Si–O–Fe interface layer.
Fig. 6 shows the thermogravimetric curves of Fe and Fe–SiO2
powders obtained in the temperature range of 320–1073 K with
the heating rate of 10 K/min. The nanocrystalline Fe shows a
small initial weight loss of about 2.5% in the temperature range
of 310–450 K, which can be attributed to the hydroxide impuri-
ties on the Fe particles. Also the curve shows a gain in weight,
which begins at 554 K and increases with temperature up to 1005 K
with maximum percentage of 32.5 wt.%. Hence, according to the
3Fe + 2O2 → Fe3O4 reaction, a weight gain of only 27.6% is expected.
Fig. 3. SEM microstructure of (a) as-prepared Fe particles with hollow spherical
morphology made of network of Fe particles with inset giving the particle size
distribution, (b) Fe particles embedded in a silica matrix.
embedded in an SiO2 matrix. Fig. 3(a) and (b) are SEM micrographs
showing the actual morphology of Fe and Fe–silica composite with
scale bar of 10 ␮m. Pure Fe powder samples have hollow spheri-
cal structures made up of network of particles which have typically
about a micron in size. The particle size distribution measured from
SEM image is shown in the inset of Fig. 3(a). The particle distribution
was fitted with a lognormal function with a mean size of 787 nm.
Fig. 3(b) is the microstructure of the Fe–silica composite, which
shows the presence of thick non-magnetic silica encapsulated with
Fe particles.
The dynamic light scattering (DLS) experiment was carried
out in order to understand the dispersion of Fe particles in EtOH
medium before the silica coating process. Fig. 4 shows the particle
size distribution of the Fe sample dispersed in EtOH. The average
particle size was determined to be 761 nm, in good agreement with
the result obtained using scanning electron microscopy. It is clear
that these large particles (∼750 nm) making up the network of
hollow sphere are typically aggregates of Fe crystallites (∼50 nm).
To confirm the presence of silica on Fe particles, the materials
were also characterized by FT-IR. The IR spectrum of the com-
posite shown in Fig. 5 exhibits a broad envelope in the higher
energy region. This is due to the –OH and C–H stretching vibra-
tions of water and ethanol molecules, which is confirmed by the
Fig. 5. The FT-IR spectrum of the as-prepared Fe–SiO2 nanocomposite showing the
presence of surface oxidation of Fe particles which makes it compatible for SiO2
coating.

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E. Thirumal et al. / Journal of Alloys and Compounds 502 (2010) 169–175
Fig. 8. Complex impedance spectra for Fe–silica nanocomposite measured at indi-
cated temperatures with inset showing the equivalent circuit for the fitted solid
line.
Fig. 6. Thermogravimetric oxidation curves of the as-prepared Fe and Fe–silica
showing the enhanced thermal stability of the Fe–silica nanocomposite.
Fig. 7 presents the room temperature hysteresis loop of the as-
prepared Fe and Fe–silica powders. The saturation magnetization
(Ms) of pure Fe particles is found to be 210 emu/g, which is very
close to that of the bulk Fe (220 emu/g) and in the case of Fe–silica
particles it is found to be 135 emu/g of Ms. This difference in the
value of the saturation magnetization enables us to estimate the
presence of Fe particles as 64 wt.% in silica matrix regardless of
OH and other removable molecules. In the case of the Fe–SiO2
nanocomposite, the saturation magnetization is found to be four
times higher than the values reported by Zhao et al. [13,14] and
also 27% lower than that reported by Tang et al. [15]. It is evident
that the high concentration of silica (36 wt.%) is the main factor
to reduce the value of the saturation magnetization. The coer-
civity values of both Fe and Fe–SiO2 are found to be 56 Oe. The
absence of change in coercivity for silica-coated Fe particles can be
attributed to several reasons. One of the main reasons is that the
Fe particles may not be well separated in silica-coated Fe. How-
ever, lower coercivity of Fe and Fe–silica indicates a soft magnetic
behaviour.
To find out the Curie transition temperature and identify the
trace oxidation, thermomagnetic measurement was carried out
on the as-prepared Fe powder. Inset in Fig. 7 shows the thermo-
magnetic behaviour of the as-prepared nanocrystalline Fe particles
measured at temperatures ranging from 320 K to 1200 K with a
constant heating rate (10 K/min) and flow of N2 gas. The thermo-
magnetic plot reveals the presence of the Fe3O4 phase with the
Curie transition at 843 K, followed by a Curie transition of metal-
lic BCC-Fe phase at 1053 K. Cao et al. [16] have examined the
thermomagnetic behaviour of amorphous Fe2O3 and have seen
an initial decrease and rise in magnetization due to crystalliza-
tion of these oxides. Our thermomagnetization also shows a similar
behaviour which could be attributed to the formation of thin layer
of oxides around the Fe particles as they are very prone to oxida-
tion.
The observed 32.5% can be due to the formation of Fe2O3 by the
reaction 2Fe + (3/2)O2 → Fe2O3 which would give rise to a weight
gain of 30%.
In the case of as-prepared Fe–silica, the initial weight loss of
the sample from 320 K to 450 K can be attributed to the removal
of water and ethanol present in the sample. With further increase
in temperature there is no significant weight loss, but in the tem-
perature range of 780–1044 K, a drastic weight gain of 17% was
observed. The maximum weight gain of 17% observed for Fe–silica
sample indicates the presence of oxide silica, which will not con-
tribute to weight gain. The onset of weight gain at 780 K clearly
indicates the oxidation temperature of Fe particles in the SiO2
matrix. The observed oxidation temperature of our Fe–SiO2 sample
is much higher than the previously reported oxidation tempera-
ture of 470 K for Fe/silica core–shell structure prepared by high
temperature hydrogen atmosphere reduction method [2]. The high
thermal stability and enhancement in oxidation temperature of our
sample can be attributed to the thick insulating silica shell covering
the Fe particles. A further increase in temperature shows saturation
in the weight of the sample, which indicates the formation of stable
iron oxide phases such as FeO, Fe3O4 and Fe2O3.
Fig. 8 illustrates the typical complex impedance plots of Fe–silica
pellet (with a thickness of 1.1 mm and a diameter of 8 mm) mea-
sured at different temperatures. The Z-plots show the presence of a
single semicircle in the higher frequency region followed by a spike
in the lower frequency region. This semicircle can be attributed to
the grain interior conduction followed by interfacial conduction
in the Fe–silica nanocomposite. It can be seen that the diameter
of the semicircle decreases with temperature, which indicates the
increase of conductivity with temperature. The resistances were
obtained by fitting the data, using the equivalent circuit containing
a series resistance (Rs) which is connected to a parallel combination
of a resistance (Rp) and a constant phase element (CPE) as shown
in inset of Fig. 8. The series resistance (Rs) can be obtained from the
Fig. 7. Room temperature hysteresis loop of native and silica-coated Fe particles
with the latter showing a reduced magnetization due to the presence of silica. (Inset)
Magnetic TG plot of the as-prepared nanocrystalline Fe showing the Curie transition
of Fe3O4 and Fe at 843 K and 1053 K respectively.

E. Thirumal et al. / Journal of Alloys and Compounds 502 (2010) 169–175
173
Fig. 10. Frequency spectra of ac conductivity (symbols) in the case of Fe–silica
nanocomposite at different temperatures. Solid lines are the best fit for Eq. (4) for
353 K, 373 K and 393 K and Eq. (3) for 413 K and 443 K.
Fig. 9. Plot showing the Arrhenius behaviour of dc conductivity of the Fe–silica
nanocomposite. The activation energy is determined to be 0.59 eV from the linear
fit using the Arrhenius relation (Eq. (1)).
tion of the dc conductivity region and ac conductivity region, with
enhanced critical frequency. According to the Almond and West
dispersive behaviour the ac conductivity can be expressed as,
shift of the semicircle from the origin of plot, which is in the range of
a few hundred Ohm to a few kilo-Ohm. In the case of a parallel Rp–Cp
circuit, in order to analyze the depressed or more flattened semi-
circles, CPE has been used rather than pure capacitance. The CPE
capacitance is defined as C(ω) = A(jωn−1) [17,18], where the param-
eter A is a constant for a given experiment. The power exponent
(n) lies in between 0 and 1. The impedance of CPE behaves as pure
capacitance for n = 1 and pure resistance for n = 0. The solid lines in
Fig. 8 were obtained by the best fitting of non-linear least square
(NLLS) fitting routine. In our case the n values fall in the range of
0.88–0.92, which are directly related to the percentage of depres-
sion of the semicircle, indicating that it is a non-ideal Debye system.
Many reasons (such as rough interface between electrode and sam-
ple, presence of humidity, etc.) were suggested for the non-Debye
CPE behaviour of the sample [19,20]. In our sample the presence
of moisture such as –OH and –C–H molecules can be a main factor
for the depressed semicircle. The obtained resistivity values lie in
between 0.164 Mꢀ m and 1.9 kꢀ m for temperatures in the range
of 320–453 K.
ꢂ
ꢃ
ꢄ ꢅ
n
ω
ꢁ = ꢁdc + Aωn = ꢁdc 1 +
(2)
ωp
which is also called the universal power law [23]. The value of
frequency exponent, n lies between 0 and 1 and the parameter
A is independent of frequency. The above equation is general and
universally found for semiconductors, ceramics and electronic con-
ductors. The equation can be written as,
ꢁac = ꢁdc + [(ωꢂ)n](ꢁdc),
ꢁac = ꢁdc[1 + (ωꢂ)n]
(3)
where ꢂ is the relaxation time.
In the present study for low frequency ac conductivity, the above
equation can be modified as,
ꢁac = ꢁdc[(ωꢂ1)n + (ωꢂ2)n
]
(4)
1
2
where n1 and n2 are the power law exponents for low and high
frequency conducting behaviour respectively; ꢂ1 and ꢂ2 are relax-
ation time for low and high frequency region respectively. Also
ꢂ1 = 1 s for low frequency range, whereas ꢂ2 will vary depend-
ing upon the critical frequency transition from the low frequency
region to the high frequency region. Fig. 10 shows the measured
frequency-dependent ac conductivity plots with the best fitting
with Eq. (4). The n1 value linearly decreases from 0.15 to zero when
the temperature increases from 353 K to 443 K. This indicates that
at lower temperatures intra-grain conduction dominates in the low
frequency region. Above 443 K, the low frequency conducting dis-
persion behaviour disappears, which can be due to the absence of
intra-grain conduction. This is mainly attributed to the presence of
–OH and C–H molecules in our sample, which is also confirmed by
the weight loss in the TGA measurements. Hence, above 443 K, this
moisture was completely de-absorbed from the sample and the ac
conductivity obeys the universal power law (Eq. (3)). At high fre-
quencies, the strong dispersion of conduction behaviour can be due
to grain interior conduction by tunneling of electrons through the
dielectric barrier, a characteristic feature observed in most of the
disordered materials [24,25].
Fig. 9 shows the temperature dependence of dc conductivity
of as-prepared Fe–SiO2 nanocomposite in the temperature range
from 363 K to 453 K. The conductivity of the materials follows the
Arrhenius behaviour, i.e.,
ꢀ
ꢁ
−Ea
ꢁT = ꢁ0 exp
(1)
kT
where ꢁ0 is the pre-exponential factor, Ea is the activation energy,
k is the Boltzmann constant, and T is the temperature in Kelvin
[21,22]. The activation energy (Ea) of the thermally activated elec-
tronic conduction in Fe–silica nanocomposite is found to be 0.59 eV,
which is higher than that of Fe–Fe3O4–SiO2 gel nanocomposite
reported by Das et al. [11]. They observed that with an increase
in the surface oxidation of the Fe particles, the activation energy of
the nanocomposite decreases. In the present study the high value of
activation energy 0.59 eV indicates the reduced surface oxidation,
which can be attributed to complete coating of amorphous silica
around the Fe particles.
Fig. 10 illustrates the ac conductivity as a function of frequency
for various temperatures. It can be seen that two different ac con-
ductivity dispersions are present in the lower temperature region.
The plateau region is not observed in the temperature range of
323–423 K. Further increase in the temperature leads to a separa-
Fig. 11 demonstrates the ac conductivity as a function of tem-
perature for various frequencies. It can be observed that at lower

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E. Thirumal et al. / Journal of Alloys and Compounds 502 (2010) 169–175
Fig. 12(b) illustrates the dielectric loss factor (tan ı) for Fe–silica
nanocomposite as a function of temperature measured at various
frequencies. There is no significant change in dielectric loss in the
higher frequency region (105 Hz and 106 Hz), whereas larger dielec-
tric loss is observed in the lower frequency region (102 Hz, 103 Hz
and 104 Hz). A sharp rise in tan ı was observed at a thermal des-
orption temperature of 413 K and further increase in temperature
led to a fall of the dielectric loss value below 5. Unlike ferrites,
in which the dielectric loss increases with temperature [29], the
Fe–silica nanocomposite exhibits almost a constant tan ı in the
higher temperature region.
4. Conclusion
We have successfully prepared magnetic metal Fe nanocrys-
talline powders by a simple and cost effective technique and
these particles were encapsulated with silica. Analysis of parti-
cle and crystal size of Fe powder shows that our method is more
suitable for synthesis of submicron size Fe particles with crys-
tallite size of 50 nm. Fe particles dispersed in silica exhibits a
clearly enhanced thermal stability. The material shows a high value
of saturation magnetization and low value of electrical conduc-
tivity. Temperature dependent dielectric constant and dielectric
loss indicate the suitability of this material for high frequency
core materials. The amorphous silica on Fe particles may be
effective in reducing the eddy current losses at high frequen-
cies.
Fig. 11. Variation of ac conductivity of Fe–silica nanocomposite with temperature at
different frequencies showing the strong influence of temperature on conductivity
at low temperatures.
frequencies, the conductivity is strongly influenced by temperature
rather than at higher frequencies. All the plots show a kink around
413 K, which can be attributed to removal of moisture from the
sample.
Fig. 12(a) shows the variation of the dielectric constant of
Fe–silica nanocomposite as a function of temperature at different
frequencies. It is observed that at lower frequencies the value of
the dielectric constant (εꢀ) increases up to 363 K and then slightly
decreases upto 403 K followed by a steep fall at 413 K, which
can be attributed to thermal desorption of removable molecules.
However, in the higher frequency regions (104 Hz, 105 Hz and
106 Hz) almost stable dielectric constant was observed except
for a small change at 413 K, which indicates that the dielec-
tric constant in the low frequency region is strongly influenced
by thermal desorption compared to its behaviour in the high
frequency region, as observed in the conductivity spectra. The
maximum dielectric constant of bulk SiO2 is 3.5 and nano-sized
amorphous SiO2 is 5–35 [26]. The high value of the dielectric
constant in this composite material could be due to a large
volume fraction of (64 wt.%) Fe embedded in silica matrix, as
was observed in epoxy resin [27] and in some polymer matrix
[28].
Acknowledgments
The authors would like to thank Prof. A. Narayanasamy, Prof.
P.R. Subramaniyan, Dr R. Murugaraj, Mr. Ashok Kumar Baral and
Mrs. Kalavathi for fruitful discussions. The work is supported by
UGC-SAP and DST-FIST, Government of India. One of the authors DP
would like to thank the Indian Institute of Science, for the award of
Research Associateship.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jallcom.2010.04.135.
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