# Structural and Mössbauer Spectral Studies of Nanosized Aluminum Doped Manganese Zinc Ferrites

Authors: R. L. Dhiman; S. P. Taneja; V. R. Reddy
Journal: Advances in Condensed Matter Physics
 (2008)

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## Abstract

Nanoparticles of aluminum-substituted manganese zinc ferrites,Mn1.05Zn0.05AlxFe1.9−xO4 with 0.4≤x≤1.0 were synthesized by solid-state reaction route and characterized by XRD, TEM, and Mössbauer spectroscopy measurements. The particle size is found to very from 46 to 28 nm with increase of aluminum ions concentration. The unit cell parameter ‘‘aO’’ is found to decrease linearly with aluminum ions concentration due to smaller ionic radius of aluminum. The cation distributions were estimated from X-ray diffraction intensities of various planes. The theoretical lattice parameter, bulk density, porosity, X-ray density, oxygen positional parameter, ionic radii, jump length, as well as bonds and edges lengths of the tetrahedral (A-) and octahedral (B-) sites were determined. Fe57 Mössbauer spectra recorded at room temperature were fitted with two sextets corresponding to Fe3+ ions at tetrahedral (A-) and octahedral (B-) sites. The magnetic hyperfine fields at A- and B-sites show a gradual decrease with increase of Al3+ ion concentration, which has been explained on the basis of supertransferred hyperfine field. The cation distribution obtained from X-ray intensities and Mössbauer data indicates a decrease in Fe3+(B)/Fe3+(A) ratio with increasing aluminum concentration thereby suggesting a decrease in ferrimagnetic behavior.


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## Body

## 1. Introduction

Nanoparticles of
manganese zinc ferrites exhibit some unusual properties, for example,  enhanced coercivity,
modified saturation magnetization, super paramagnetism, and metastable cation
distributions [1]. These nanoparticles have wide applications in heat transfer
devices, drug delivery systems, and medical diagnostics, that is, especially in
cancer  treatment, and so forth [2, 3]. Mn-Zn
ferrites  possess a
spinel structure in which Mn2+, Zn2+, and Fe3+ cations are distributed among two interstitial tetrahedral (A-) and octahedral (B-) sites. In general, spinels are classified as normal, inverse, and mixed. In most
cases, Mn2+ ions  tend
to occupy the octahedral B-sites and produce an inverse spinel structure. In
normal spinel structure,
Zn2+
ions prefer to hold the tetrahedral A-sites
position. Although the Mn-Zn  ferrites
have mixed spinel structure,  the magnetic and electronic properties of
these ferrites are dependent of their chemical composition, method of
preparation, grain size, and distribution of cation between two interstitial
sites [4–6]. The
structural and the magnetic environments of these two sites are quite
different. The substitutions of nonmagnetic ions, for example, Al3+ ions in simple and mixed ferrites have received a great deal of attention over
the past few years [7–9]. It has been
reported that the presence of nonmagnetic ions reduces the magnetic
interactions between the two interstitial sites, causes decrease in hyperfine
magnetic fields, and hence changes the magnetic and electronic properties. These
effects are particularly apparent in the Mössbauer studies [10, 11]. In recent
years, researches on spinel ferrites have received great interest due to
availability of new and sophisticated techniques for the synthesis and characterization
of nanoparticles. Presently, the ferrites nanoparticles of average diameter 7 nm
and 20 nm were synthesized by doping titanium and gallium ions, respectively,
using solid-state reaction technique [12, 13]. In the
present work, aluminum substituted manganese zinc ferrites nanoparticles have
been synthesized via solid-state reaction route. In order to determine the
cation distribution in the ferrite system understudy XRD, Mössbauer and TEM
measurements have been carried out.


## 2. Experimental

Samples of the mixed spinel ferritesMn1.05Zn0.5AlxFe1.9−xO4 for x=0.4,0.6,0.8, and 1.0 were synthesized by using standard
solid-state reaction technique. The required compositions were weighed and
mixed in a mortar and pestle for 2 hours with acetone. The mixed powders were
calcinated in air by raising temperature up to 200°C at the rate of
about 5°C/min and then up to 600°C in 60 minutes and
allowed to cool to room temperature; finally the temperature was raised to 1000°C in 10 hours and slowly cooled to room temperature at a rate of 2°C/min.
The reacted materials were well grounded in an agate mortar and  pestled by adding a few
drops of acetone again to reduce the particle size. To ensure their
single-phase nature, the powder X-ray diffraction studies were made on Rigaku
X-ray diffractometer by using Cu-Kα radiation of 1.54060Å. The particle morphology was examined by transmission
electron microscope model Hitachi H-7500. Fe57 Mössbauer absorption
spectra were recorded in transmission geometry at room temperature using a
multichannel analyzer with a drive in constant acceleration mode. A Co57(Rh)
source with initial activity of 20 mCi was used. The spectrometer was
periodically calibrated using a natural iron foil as a standard.


## 3. Results and Discussion

### 3.1. XRD and Tem Analysis

The X-ray diffraction patterns of mixed
spinel ferrites (Mn1.05Zn0.5AlxFe1.9−xO4 for x=0.4,0.6,0.8, and 1.0) are displayed in Figure 1. The indexing
of diffraction lines from various planes  was assigned using JCPDS file no. 742 435. All the
samples were found to be face-centered cubic (FCC) with Fd-3m space group. The
values of the lattice constant ‘‘aO’’
for all the samples determined from X-ray data are listed in Table 1. The
lattice constant ‘‘aO’’ is
found to decrease linearly with aluminum concentration (x) as shown in Figure 2(a), thereby obeying Vegard’s law [14]. The
decrease in lattice constant is attributed to the replacement of Fe3+ ions of larger ionic radius (0.64Å) by smaller Al3+ ions (0.50Å), which
causes shrinking in the unit cell dimensions. The decrease in ‘‘aO’’ and the shift of
reflections toward higher angle with the increasing aluminum concentration  show that aluminum atoms
have been incorporated into the spinel structure [15]. The bulk density of each
sample was determined by Archimedes’ principle and is listed in Table 1. The X-ray
density ‘‘dx’’ of each sample was calculated from the values of
the lattice constant using the formula
(1)dx=ZMNaO3,
where ‘‘Z’’ represents the
number of molecules in a unit cell of the spinel lattice, M the
molecular weight of the ferrite samples, N the Avogadro’s number, and aO the lattice constant of the ferrite. The X-ray density decreases with
increasing aluminum concentration (x)
and hence follows similar trend as that of lattice constant ‘‘aO’’
as shown in Figure 2(b). This is due to the fact that the decrease in mass (due
to replacement of iron by aluminum) overtakes the decrease in volume of the
unit cell. The percentage porosity of each sample was calculated using the relation:
(2)Percentageporosity(p%)=[1−ddx]×100,
where notations have their usual
meanings. The calculated values of the bulk density, X-ray density, and the
percentage porosity of the present ferrite system are listed in Table 1. The
calculated values of the porosity are in good agreement with the values generally
observed in ferrites prepared by solid-state reaction route [12, 16].Table 1
Lattice constant, density, porosity, and grain
size forMn1.05Zn0.05AlxFe1.9−xO4 system.
CompositionLattice constantX-ray densityBulk densityPorosityGrain sizeD (±1 nm)(x)(aO±0.002)Å(at±0.002)Å(dx±0.002)gm/cm3(d±0.002)gm/cm3p%XRDTEM0.48.43748.4734.8534.5166.94446450.68.39438.4264.7994.4986.27237350.88.34728.3794.7494.4695.89632301.08.30418.3324.6904.3856.5032825Figure 1
X-ray diffraction pattern ofMn1.05Zn0.05AlxFe1.9−xO4 system.Figure 2
Variation of (a) lattice constant and (b) density
with aluminum concentration.The average grain size of all the
samples  is evaluated
from the line broadening of the (311) peaks using Debye-Scherrer equation(3)D(311)=0.9λβ1/2cosθ,
where ‘‘D’’ is the average particle size of the crystallites, λ is the
wavelength of the incident X-ray, θ is the
corresponding Bragg angle, and β1/2 is the full width at the half maximum (FWHM) of the (311) peak. The
uncertainties in the particle size determination were estimated from the errors
in the fitting procedures while fitting the 311 peak, which lie in the range ±1 nm.
The average particle sizes of all the samples are listed in Table 1. The
transmission electron micrographs of the ferrite nanoparticles are displayed in
Figure 3. It has been observed that the particles prepared with smaller
aluminum substitution displayed in Figure 3(a) are somewhat agglomerated in
nature. The degree of agglomeration decreases with increase of aluminum substitution,
the particles prepared from higher aluminum concentration; x=1.0
are discrete and somewhat
spherical as shown in Figure 3(b). The decrease in agglomeration is due to the
replacement of Fe3+ ions by Al3+ ions that leads to
decrease in cation vacancies, which in turn decreases the grain size. The
average particle sizes estimated from the TEM studies listed in Table 1 are in
good agreement with those values obtained from XRD analysis using Debye-Scherrer equation.Transmission micrographs forMn1.05Zn0.05AlxFe1.9−xO4 (a) for x=0.4 and (b) for x=1.0.
(a)(b)The cation distribution in the various spinel
ferrite systems has been estimated from X-ray diffraction, Mössbauer and Magnetization
measurements [17–19].  It has been reported that the intensities of
(220), (422), and (400) planes are sensitive to cations on tetrahedral (A-) and
octahedral (B-) sites [20, 21].  Therefore, we have used the intensities of
(220), (422), and (400) planes to determine the cation distribution. The
relative X-ray diffraction intensity ‘‘Ihkl’’ of a given diffraction line was calculated using
the formula suggested by Buerger [22]
(4)Ihkl=|Fhkl|2⋅P⋅LP,
where Ihkl is the relative integral
intensity, Fhkl the
structure factor, P the multiplicity
factor, 	LP
the Lorentz factor. The intensity ratios (I220/I400 and I422/I400) have been considered to be
more sensitive to the cation distribution [23]. It is known that Fe3+ and Al3+ ions distribute over A- and B-sites [24], while Mn2+ and Zn2+ ions have strong preference for A-site [25–27]. The
intensities corresponding to (220) and (422) reflections are most sensitive to
cations on  tetrahedral sites [20, 21], while those of (222) reflection to cation on
octahedral sites [20]. Figure 1 shows that the intensities of (220), (422), and
(222) reflections decrease with substitution of aluminum ions indicating that
Al3+ ions enter in both octahedral (B-) and tetrahedral (A-) sites.
Based on the above, intensity ratios: I220/I440,
I422/I440, and I440/I400 for all the samples were calculated for various cation distributions. The calculated values, those
closet to the experimental data, are given in Table 2. On the basis of
estimated cation distribution (Table 2), the value of theoretical lattice
constant ‘‘at’’ for various
compositions has been determined using the relation [28, 29]
(5)at=833(rA+rO)+3(rB+rO),
where rA,rB are the radii of tetrahedral
(A-) and octahedral (B-) sites, respectively, and rO is the radius of the oxygen ion O2− (1.48Å). The
calculated values ‘‘at’’ are
nearly equal to the experimental observed  values ‘‘aO’’
and hence confirming  that the estimated cation distribution is correct. The
site radii rA and rB used above were determined
from the following equation:
(6)rA=(0.627−0.23x)rtetFe3++0.05rtetZn2++0.323rtetMn2++0.23xrtetAl3+,rB=(1.273−0.77x)roctFe3++0.727roctMn2++0.77xroctAl3+.
The calculated
values of rA and rB are listed in Table 3. The
value of rA decreases
slowly; however the value of rB decreases noticeably with increasing aluminum concentration. This is due to the
substitution process and the distribution of cations amongst the tetrahedral
(A-) and octahedral (B-) sites. The value of the oxygen positional parameter ‘‘u’’ is calculated by using the following
relation [28, 29]:
(7)rA=aO3(u−0.25)−rO.
The calculated
values of ‘‘u’’ are listed in Table 3. The
values of the tetrahedral (dAL),
octahedral bond  lengths
(dBL), tetrahedral
edge length (dAE),
shared (dBE) and
unshared octahedral edge lengths (dBEU)
are calculated by using the experimental values of lattice constant ‘‘aO’’ and oxygen positional
parameter ‘‘u’’ from the following
equations [28, 29]:
(8)dAL=aO3(u−0.25),dBL=aO(3u2−114u+4364),dAE=aO2(2u−0.5),dBE=aO2(1−2u),dBEU=aO(4u2−3u+1116).
The calculated values
of the above X-ray parameters are listed in Table 3. It is observed that dAL, dBL, dAE, dBE, and dBEU decrease with increasing
aluminum concentration (x). This is attributed to the substitution
process and the cation distribution, especially the replacement of Fe3+ larger ions at B- and A-sites with smaller Al3+ ions. These results are
in good agreement with the results reported earlier [30]. It has been reported
that the jump length ‘‘L’’ (the distance between the magnetic ions) of
electrons influences the physical properties of the ferrite system [31].
Electrons that are hopping between B- and A-sites are less probable compared to
those between B- and B-sites because the  distance between the two metal ions placed at B-sites
is smaller than  the distance if they  are placed one at B-sites and the other at A-sites [32, 33]. The jump
length ‘‘L’’ of the tetrahedral (A-) and octahedral (B-) sites is determined
from the following relation [34, 35]:
(9)LA=aO34,LB=aO24.
The calculated
values of jump length ‘‘L’’ of A- and B-sites are shown in Table 3. It is
evident from Table 3 that the jump length ‘‘L’’  decreases with increasing Al3+ concentration
(x) and  behaves similarly as that of lattice constant. The decrease in jump length is
due to the replacement of Fe3+ ions of larger ionic radius with Al3+ ions of smaller ionic radius from A- and B-sites,  causing a decrease in distance between the magnetic
ions. Pandit et al. [23] and Amer [30] have reported similar results that the
introduction of smaller ions on the A- and B-sites decreases the distance
between the magnetic ions.Table 2
Cation distribution data calculated from XRD
pattern of theMn1.05Zn0.05AlxFe1.9−xO4 system.
CompositionX-ray intensityCation distributionFe3+(B)/
Fe3+(A)I220/I440I422/I440I440/I400Exp.Cal.Exp.Cal.Exp.Cal.Mn1.05Zn0.05Al0.4Fe1.5O41.15721.28230.81910.72361.51981.5278(Fe0.535Zn0.05Mn0.323Al0.092)A[Fe0.965Mn0.727Al0.308]B1.80Mn1.05Zn0.05Al0.6Fe1.3O41.19261.29930.80490.68471.53511.5693(Fe0.489Zn0.05Mn0.323Al0.138)A[Fe0.811Mn0.727Al0.462]B1.65Mn1.05Zn0.05Al0.8Fe1.1O41.29031.34010.68010.58491.68101.6528(Fe0.443Zn0.05Mn0.323Al0.184)A[Fe0.657Mn0.727Al0.616]B1.48Mn1.05Zn0.05Al1.0Fe0.9O41.55231.57000.52720.50911.70461.7501(Fe0.397Zn0.05Mn0.323Al0.230)A[Fe0.503Mn0.727Al0.770]B1.26Table 3
X-ray parameters (errorbar±0.002Å) tetrahedral and octahedral bond  lengths (dAL and dBL),
jump  lengths (LA and LB), tetrahedral edge dAL, and shared
and unshared octahedral  edge lengths (dBE and dBEU).
CompositiondALdBLdAEdBEdBEULALBrArBu(x)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)0.42.16871.93143.54152.42463.00453.65352.98300.68921.35620.39840.62.16201.91783.53042.40512.99383.63482.96700.68331.33630.39870.82.15691.90533.52262.37982.97823.61442.95120.67741.31620.39921.02.15031.89363.51122.36042.96403.59582.93590.67131.29620.3995


### 3.2. Mössbauer Measurements

Fe57 Mössbauer absorption spectra
of mixed spinel ferrite system, Mn1.05Zn0.05AlxFe1.9−xO4; x=0.4,0.6,0.8, and 1.0, recorded at room temperature are displayed
in Figure 4. The experimental data were fitted using NORMOS/SITE program [36]. Each
spectrum exhibits a superposition of two Zeeman sextets. Outer sextet corresponding to a higher
magnetic field is attributed to Fe3+ ions on the octahedral B-site
and the inner sextet corresponding to lower magnetic field is attributed to Fe3+ ions on the tetrahedral A-site. The computed values of the hyperfine parameters
are listed in Table 4. In the present ferrite system, the value of isomer  shift   (δ)
of B-site is greater than
A-site and is in agreement with the reported data. This is due to the large band separation of Fe3+-O2− for the B-site ions compared to A-site ions. It is observed that
on increasing the Al3+ ions concentration (x), the value of
isomer shift of the A- and B-sites do not show any appreciable change,
indicating that the s-electron distribution of Fe3+ ions is hardly
influenced by Al3+ substitution. Furthermore, the observed values
of  isomer shift (δ) are significantly less than the expected
value, 0.5 mm/s for the Fe2+ ions [17]. Hence the presence of Fe2+ ions in the present ferrite system is ruled out. Thus the electron exchange
interaction (Fe2+↔Fe3++e−) does not occur
and hence the oxidation state of Fe3+ remains unchanged during
synthesis process. The hyperfine field Hhfs values at B- and A-sites show a gradual
monotonic decrease with increasing aluminum ion concentration (x). This is
because of supertransferred hyperfine field at the central cation that  originates from the
magnetic moments of the nearest-neighbor cations, that is, from the
intrasublattice contributions hAA and hBB and the
inter sublattice contributions hAB and hBA. In the present ferrite system, under study, the intersulattice contributions hAB and hBA
are  predominant. The
introduction of Al3+ ions replaces Fe3+ ions from the A- and B-sites and decreases inter sublattice contributions, which in turn
decreases the hyperfine field Hhfs at both A- and B-sites. As nonmagnetic Al3+ ions replaces
Fe3+ ions,
the correct amount of Fe3+ present at A- and B-sites is estimated by
determining the area under the Mössbauer absorption. The correct amount of Fe3+ ions occupied by octahedral (B-) and tetrahedral (A-) sites obtained from the
Mössbauer absorption spectra and those calculated on the basis of cation
distribution are in good agreement. The Fe3+(B)/Fe3+(A)
ratio obtained from the Mössbauer spectra and X-ray intensity calculations decreases
with increasing aluminum concentration suggesting the decrease in ferrimagnetic
behavior. With increase of Al3+ ions concentration, intensity
and the line width of the Mössbauer spectra increases indicating a reduction in
particle size and is in accordance with TEM measurements.  These results are in consistent with the
results reported earlier [13].Table 4
Room temperature Mössbauer effect parameters
forMn1.05Zn0.05AlxFe1.9−xO4 system as a function of x.
CompositionSiteIsomershift*Quadrupole splittingHhfsFe3+(B)/Fe3+(A)(x)(δ±0.01)mm/s(Δ±0.01)mm/s(±1.0 T)(%)0.4B0.180.0042.11.79A0.150.0039.50.6B0.170.0040.31.63A0.150.0037.40.8B0.160.0039.01.44A0.140.0035.11.0B0.170.0037.81.21A0.160.0033.1Isomer* shift given relative to α-Fe.Figure 4
Room temperature Mössbauer absorption spectra
ofMn1.05Zn0.05AlxFe1.9−xO4 for x=0.4,0.6,0.8, and 1.0.


## 3.1. XRD and Tem Analysis

The X-ray diffraction patterns of mixed
spinel ferrites (Mn1.05Zn0.5AlxFe1.9−xO4 for x=0.4,0.6,0.8, and 1.0) are displayed in Figure 1. The indexing
of diffraction lines from various planes  was assigned using JCPDS file no. 742 435. All the
samples were found to be face-centered cubic (FCC) with Fd-3m space group. The
values of the lattice constant ‘‘aO’’
for all the samples determined from X-ray data are listed in Table 1. The
lattice constant ‘‘aO’’ is
found to decrease linearly with aluminum concentration (x) as shown in Figure 2(a), thereby obeying Vegard’s law [14]. The
decrease in lattice constant is attributed to the replacement of Fe3+ ions of larger ionic radius (0.64Å) by smaller Al3+ ions (0.50Å), which
causes shrinking in the unit cell dimensions. The decrease in ‘‘aO’’ and the shift of
reflections toward higher angle with the increasing aluminum concentration  show that aluminum atoms
have been incorporated into the spinel structure [15]. The bulk density of each
sample was determined by Archimedes’ principle and is listed in Table 1. The X-ray
density ‘‘dx’’ of each sample was calculated from the values of
the lattice constant using the formula
(1)dx=ZMNaO3,
where ‘‘Z’’ represents the
number of molecules in a unit cell of the spinel lattice, M the
molecular weight of the ferrite samples, N the Avogadro’s number, and aO the lattice constant of the ferrite. The X-ray density decreases with
increasing aluminum concentration (x)
and hence follows similar trend as that of lattice constant ‘‘aO’’
as shown in Figure 2(b). This is due to the fact that the decrease in mass (due
to replacement of iron by aluminum) overtakes the decrease in volume of the
unit cell. The percentage porosity of each sample was calculated using the relation:
(2)Percentageporosity(p%)=[1−ddx]×100,
where notations have their usual
meanings. The calculated values of the bulk density, X-ray density, and the
percentage porosity of the present ferrite system are listed in Table 1. The
calculated values of the porosity are in good agreement with the values generally
observed in ferrites prepared by solid-state reaction route [12, 16].Table 1
Lattice constant, density, porosity, and grain
size forMn1.05Zn0.05AlxFe1.9−xO4 system.
CompositionLattice constantX-ray densityBulk densityPorosityGrain sizeD (±1 nm)(x)(aO±0.002)Å(at±0.002)Å(dx±0.002)gm/cm3(d±0.002)gm/cm3p%XRDTEM0.48.43748.4734.8534.5166.94446450.68.39438.4264.7994.4986.27237350.88.34728.3794.7494.4695.89632301.08.30418.3324.6904.3856.5032825Figure 1
X-ray diffraction pattern ofMn1.05Zn0.05AlxFe1.9−xO4 system.Figure 2
Variation of (a) lattice constant and (b) density
with aluminum concentration.The average grain size of all the
samples  is evaluated
from the line broadening of the (311) peaks using Debye-Scherrer equation(3)D(311)=0.9λβ1/2cosθ,
where ‘‘D’’ is the average particle size of the crystallites, λ is the
wavelength of the incident X-ray, θ is the
corresponding Bragg angle, and β1/2 is the full width at the half maximum (FWHM) of the (311) peak. The
uncertainties in the particle size determination were estimated from the errors
in the fitting procedures while fitting the 311 peak, which lie in the range ±1 nm.
The average particle sizes of all the samples are listed in Table 1. The
transmission electron micrographs of the ferrite nanoparticles are displayed in
Figure 3. It has been observed that the particles prepared with smaller
aluminum substitution displayed in Figure 3(a) are somewhat agglomerated in
nature. The degree of agglomeration decreases with increase of aluminum substitution,
the particles prepared from higher aluminum concentration; x=1.0
are discrete and somewhat
spherical as shown in Figure 3(b). The decrease in agglomeration is due to the
replacement of Fe3+ ions by Al3+ ions that leads to
decrease in cation vacancies, which in turn decreases the grain size. The
average particle sizes estimated from the TEM studies listed in Table 1 are in
good agreement with those values obtained from XRD analysis using Debye-Scherrer equation.Transmission micrographs forMn1.05Zn0.05AlxFe1.9−xO4 (a) for x=0.4 and (b) for x=1.0.
(a)(b)The cation distribution in the various spinel
ferrite systems has been estimated from X-ray diffraction, Mössbauer and Magnetization
measurements [17–19].  It has been reported that the intensities of
(220), (422), and (400) planes are sensitive to cations on tetrahedral (A-) and
octahedral (B-) sites [20, 21].  Therefore, we have used the intensities of
(220), (422), and (400) planes to determine the cation distribution. The
relative X-ray diffraction intensity ‘‘Ihkl’’ of a given diffraction line was calculated using
the formula suggested by Buerger [22]
(4)Ihkl=|Fhkl|2⋅P⋅LP,
where Ihkl is the relative integral
intensity, Fhkl the
structure factor, P the multiplicity
factor, 	LP
the Lorentz factor. The intensity ratios (I220/I400 and I422/I400) have been considered to be
more sensitive to the cation distribution [23]. It is known that Fe3+ and Al3+ ions distribute over A- and B-sites [24], while Mn2+ and Zn2+ ions have strong preference for A-site [25–27]. The
intensities corresponding to (220) and (422) reflections are most sensitive to
cations on  tetrahedral sites [20, 21], while those of (222) reflection to cation on
octahedral sites [20]. Figure 1 shows that the intensities of (220), (422), and
(222) reflections decrease with substitution of aluminum ions indicating that
Al3+ ions enter in both octahedral (B-) and tetrahedral (A-) sites.
Based on the above, intensity ratios: I220/I440,
I422/I440, and I440/I400 for all the samples were calculated for various cation distributions. The calculated values, those
closet to the experimental data, are given in Table 2. On the basis of
estimated cation distribution (Table 2), the value of theoretical lattice
constant ‘‘at’’ for various
compositions has been determined using the relation [28, 29]
(5)at=833(rA+rO)+3(rB+rO),
where rA,rB are the radii of tetrahedral
(A-) and octahedral (B-) sites, respectively, and rO is the radius of the oxygen ion O2− (1.48Å). The
calculated values ‘‘at’’ are
nearly equal to the experimental observed  values ‘‘aO’’
and hence confirming  that the estimated cation distribution is correct. The
site radii rA and rB used above were determined
from the following equation:
(6)rA=(0.627−0.23x)rtetFe3++0.05rtetZn2++0.323rtetMn2++0.23xrtetAl3+,rB=(1.273−0.77x)roctFe3++0.727roctMn2++0.77xroctAl3+.
The calculated
values of rA and rB are listed in Table 3. The
value of rA decreases
slowly; however the value of rB decreases noticeably with increasing aluminum concentration. This is due to the
substitution process and the distribution of cations amongst the tetrahedral
(A-) and octahedral (B-) sites. The value of the oxygen positional parameter ‘‘u’’ is calculated by using the following
relation [28, 29]:
(7)rA=aO3(u−0.25)−rO.
The calculated
values of ‘‘u’’ are listed in Table 3. The
values of the tetrahedral (dAL),
octahedral bond  lengths
(dBL), tetrahedral
edge length (dAE),
shared (dBE) and
unshared octahedral edge lengths (dBEU)
are calculated by using the experimental values of lattice constant ‘‘aO’’ and oxygen positional
parameter ‘‘u’’ from the following
equations [28, 29]:
(8)dAL=aO3(u−0.25),dBL=aO(3u2−114u+4364),dAE=aO2(2u−0.5),dBE=aO2(1−2u),dBEU=aO(4u2−3u+1116).
The calculated values
of the above X-ray parameters are listed in Table 3. It is observed that dAL, dBL, dAE, dBE, and dBEU decrease with increasing
aluminum concentration (x). This is attributed to the substitution
process and the cation distribution, especially the replacement of Fe3+ larger ions at B- and A-sites with smaller Al3+ ions. These results are
in good agreement with the results reported earlier [30]. It has been reported
that the jump length ‘‘L’’ (the distance between the magnetic ions) of
electrons influences the physical properties of the ferrite system [31].
Electrons that are hopping between B- and A-sites are less probable compared to
those between B- and B-sites because the  distance between the two metal ions placed at B-sites
is smaller than  the distance if they  are placed one at B-sites and the other at A-sites [32, 33]. The jump
length ‘‘L’’ of the tetrahedral (A-) and octahedral (B-) sites is determined
from the following relation [34, 35]:
(9)LA=aO34,LB=aO24.
The calculated
values of jump length ‘‘L’’ of A- and B-sites are shown in Table 3. It is
evident from Table 3 that the jump length ‘‘L’’  decreases with increasing Al3+ concentration
(x) and  behaves similarly as that of lattice constant. The decrease in jump length is
due to the replacement of Fe3+ ions of larger ionic radius with Al3+ ions of smaller ionic radius from A- and B-sites,  causing a decrease in distance between the magnetic
ions. Pandit et al. [23] and Amer [30] have reported similar results that the
introduction of smaller ions on the A- and B-sites decreases the distance
between the magnetic ions.Table 2
Cation distribution data calculated from XRD
pattern of theMn1.05Zn0.05AlxFe1.9−xO4 system.
CompositionX-ray intensityCation distributionFe3+(B)/
Fe3+(A)I220/I440I422/I440I440/I400Exp.Cal.Exp.Cal.Exp.Cal.Mn1.05Zn0.05Al0.4Fe1.5O41.15721.28230.81910.72361.51981.5278(Fe0.535Zn0.05Mn0.323Al0.092)A[Fe0.965Mn0.727Al0.308]B1.80Mn1.05Zn0.05Al0.6Fe1.3O41.19261.29930.80490.68471.53511.5693(Fe0.489Zn0.05Mn0.323Al0.138)A[Fe0.811Mn0.727Al0.462]B1.65Mn1.05Zn0.05Al0.8Fe1.1O41.29031.34010.68010.58491.68101.6528(Fe0.443Zn0.05Mn0.323Al0.184)A[Fe0.657Mn0.727Al0.616]B1.48Mn1.05Zn0.05Al1.0Fe0.9O41.55231.57000.52720.50911.70461.7501(Fe0.397Zn0.05Mn0.323Al0.230)A[Fe0.503Mn0.727Al0.770]B1.26Table 3
X-ray parameters (errorbar±0.002Å) tetrahedral and octahedral bond  lengths (dAL and dBL),
jump  lengths (LA and LB), tetrahedral edge dAL, and shared
and unshared octahedral  edge lengths (dBE and dBEU).
CompositiondALdBLdAEdBEdBEULALBrArBu(x)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)(Å)0.42.16871.93143.54152.42463.00453.65352.98300.68921.35620.39840.62.16201.91783.53042.40512.99383.63482.96700.68331.33630.39870.82.15691.90533.52262.37982.97823.61442.95120.67741.31620.39921.02.15031.89363.51122.36042.96403.59582.93590.67131.29620.3995


## 3.2. Mössbauer Measurements

Fe57 Mössbauer absorption spectra
of mixed spinel ferrite system, Mn1.05Zn0.05AlxFe1.9−xO4; x=0.4,0.6,0.8, and 1.0, recorded at room temperature are displayed
in Figure 4. The experimental data were fitted using NORMOS/SITE program [36]. Each
spectrum exhibits a superposition of two Zeeman sextets. Outer sextet corresponding to a higher
magnetic field is attributed to Fe3+ ions on the octahedral B-site
and the inner sextet corresponding to lower magnetic field is attributed to Fe3+ ions on the tetrahedral A-site. The computed values of the hyperfine parameters
are listed in Table 4. In the present ferrite system, the value of isomer  shift   (δ)
of B-site is greater than
A-site and is in agreement with the reported data. This is due to the large band separation of Fe3+-O2− for the B-site ions compared to A-site ions. It is observed that
on increasing the Al3+ ions concentration (x), the value of
isomer shift of the A- and B-sites do not show any appreciable change,
indicating that the s-electron distribution of Fe3+ ions is hardly
influenced by Al3+ substitution. Furthermore, the observed values
of  isomer shift (δ) are significantly less than the expected
value, 0.5 mm/s for the Fe2+ ions [17]. Hence the presence of Fe2+ ions in the present ferrite system is ruled out. Thus the electron exchange
interaction (Fe2+↔Fe3++e−) does not occur
and hence the oxidation state of Fe3+ remains unchanged during
synthesis process. The hyperfine field Hhfs values at B- and A-sites show a gradual
monotonic decrease with increasing aluminum ion concentration (x). This is
because of supertransferred hyperfine field at the central cation that  originates from the
magnetic moments of the nearest-neighbor cations, that is, from the
intrasublattice contributions hAA and hBB and the
inter sublattice contributions hAB and hBA. In the present ferrite system, under study, the intersulattice contributions hAB and hBA
are  predominant. The
introduction of Al3+ ions replaces Fe3+ ions from the A- and B-sites and decreases inter sublattice contributions, which in turn
decreases the hyperfine field Hhfs at both A- and B-sites. As nonmagnetic Al3+ ions replaces
Fe3+ ions,
the correct amount of Fe3+ present at A- and B-sites is estimated by
determining the area under the Mössbauer absorption. The correct amount of Fe3+ ions occupied by octahedral (B-) and tetrahedral (A-) sites obtained from the
Mössbauer absorption spectra and those calculated on the basis of cation
distribution are in good agreement. The Fe3+(B)/Fe3+(A)
ratio obtained from the Mössbauer spectra and X-ray intensity calculations decreases
with increasing aluminum concentration suggesting the decrease in ferrimagnetic
behavior. With increase of Al3+ ions concentration, intensity
and the line width of the Mössbauer spectra increases indicating a reduction in
particle size and is in accordance with TEM measurements.  These results are in consistent with the
results reported earlier [13].Table 4
Room temperature Mössbauer effect parameters
forMn1.05Zn0.05AlxFe1.9−xO4 system as a function of x.
CompositionSiteIsomershift*Quadrupole splittingHhfsFe3+(B)/Fe3+(A)(x)(δ±0.01)mm/s(Δ±0.01)mm/s(±1.0 T)(%)0.4B0.180.0042.11.79A0.150.0039.50.6B0.170.0040.31.63A0.150.0037.40.8B0.160.0039.01.44A0.140.0035.11.0B0.170.0037.81.21A0.160.0033.1Isomer* shift given relative to α-Fe.Figure 4
Room temperature Mössbauer absorption spectra
ofMn1.05Zn0.05AlxFe1.9−xO4 for x=0.4,0.6,0.8, and 1.0.


## 4. Conclusion

The substitution ofAl3+ ions in
Mn1.05Zn0.5AlxFe1.9−xO4 system leads to the formation of nanosized mixed spinel ferrites with particle
size 46 nm to 28 nm. The unit cell parameter decreases linearly with the
increase of aluminum concentration due to its small ionic radius. The cation
distribution has been determined from X-ray intensity ratios of various planes
and hence verified by comparing the theoretical lattice parameter with
experimental observed lattice parameter.  It is observed that the correct amount of Fe3+ ions occupied by B- and A-sites obtained from Mössbauer spectra are in good agreement with
the values determined from X-ray intensity calculations for various Al3+ ions concentration. The Hyperfine magnetic field obtained from the Mössbauer
absorption spectra decreases with increasing aluminum concentration suggesting
the decrease in ferrimagnetic behavior and has been explained on the basis of
super transferred hyperfine fields. The X-ray parameters, for example, lattice
constant, X-ray density, bulk density, ionic radius, bond length, jump length
of the A- and B-sites, oxygen positional parameter, A-site edge length, shared
and unshared B-site edge lengths were determined and found affected by the Al3+ ions substitution.



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*Source: 839536-2009-02-04.xml*