# Preparation and Characterization of Manganese Ferrite Aluminates

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

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## Abstract

Aluminum doped manganese ferritesMnAlxFe2−xO4
with
0.0≤x≤1.0 have been prepared by the double ceramic route. The formation of mixed spinel phase has been confirmed by X-ray diffraction analysis. The unit cell parameter `aO' is found to decrease linearly with aluminum 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, X-ray density, oxygen positional parameter, ionic radii, jump length, and bonds and edges lengths of the tetrahedral (A) and octahedral (B) sites were determined. 57Fe Mössbauer spectra recorded at room temperature were fitted with two sextets corresponding to Fe3+ ions at A- and B-sites. In the present ferrite system, the area ratio of Fe3+
ions at the A- and B-sites determined from the spectral analysis of Mössbauer spectra gives evidence that Al3+ ions replace iron ions at B-sites.
This change in the site preference reflects an abrupt change in magnetic hyperfine fields at A- and B-sites as aluminum concentration increases, which has been explained on the basis of supertransferred hyperfine field. On the basis of estimated cation distribution, it is concluded that aluminum doped manganese ferrites exhibit a 55% normal spinel structure.


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## Body

## 1. Introduction

Spinel
ferrites have been the subject of great interest for the past five decades, because of their wide
range of applications  in transformers,
inductors, choke
coils, noise filters magnetic recording heads, and so forth [1]. These ferrites
possessing cubic
close-packed structure of oxygen ions, are described by the formula (A)[B]2O4, where (A) and [B] represent tetrahedral and
octahedral sites, respectively. The site occupancy is often depicted in the
chemical formula as (M1−δFeδ)[MδFe2−δ]O4, where round and square brackets denote the A-
and B-sites, respectively, M represents a metal cation, and ‘δ’
is the inversion parameter. The degree of inversion ‘δ’ for spinel
ferrites is defined as the fraction of tetrahedral (A)-sites occupied by
trivalent cations. Accordingly, for a normal spinel δ=0 and for a
completely inverse spinel, δ=1. The magnetic and the electronic
properties of such a ferrite system depend upon the type of metal cations and their distribution
among the two interstitial sites, that is, A- and B-sites. Therefore, the
knowledge of cation distribution is essential to understand the magnetic
behavior of spinel ferrites. Manganese ferrite is early known to be a mixed
inverse spinel, and the degree of inversion mainly depends upon the method of
preparation. The presence of nonmagnetic ions in these spinel ferrites is found
to alter their magnetic and electronic properties. The addition of metal
cations such as trivalent or tetravalent influences the electronic and magnetic
properties of the ferrite system [2–6]. Various studies
showed that heating might change the distribution of metal cations at the A-
and B-sites of MnFe2O4. It has been reported that by
using neutron diffraction technique, the degree of inversion, that is, the
distribution of the cationic ions between the tetrahedral and octahedral sites
of MnFe2O4 prepared by usual ceramic route was determined
81% normal [7]. However, this value reduced to 33%, when MnFe2O4  was prepared by wet chemical method [8]. Thus, the method of preparation may
play a crucial role in order to obtain the desired electronic and magnetic
properties. Extensive investigations regarding the substitution of metal
cations, for example, Cu2+, Zn2+, Ti2+, Co3+,
and  Ni2+ in manganese ferrites
have been reported,  giving useful
information about the influence of such metal cations [9–13]. However, no
systematic results regarding the estimation of cation distribution on
substitution of aluminum ion in MnFe2O4   was available. In the present studies, the
cation distribution in tetrahedral and octahedral sites of aluminum substituted
manganese ferrites synthesized via double ceramic route has been determined by X-ray diffraction
and Mössbauer spectroscopic measurements.


## 2. Experimental

Samples of the mixed spinel ferritesMnAlxFe2.0−xO4 for  x=0.0,0.2,0.4,0.6,0.8, and 1.0 were synthesized by usual double ceramic processing technique.
The starting materials were high-purity
analytical reagent grade oxides, Fe2O3, MnO, and Al2O3.
The required compositions were weighed and mixed in a mortar and pestle. The
mixed powders were presintered at 1000°C for 10 hours in the air and allowed to cool to room
temperature at the rate of about 2°C/min. In the final sintering
process, the samples were placed in a furnace at 1300°C for 10 hours
in the air and then
cooled slowly to room temperature at the rate of 2°C/min. The
finally sintered materials were well grounded. To ensure their single-phase
nature, the powder X-ray diffraction studies were made on Regaku X-ray
diffractometer by using Cu-Kα radiation of
wave length 1.54060 Å. Fe57 Mössbauer absorption spectra were
recorded in transmission geometry at room temperature using a multichannel
analyzer with a drive in constant acceleration mode.    A  Co(Rh)57
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. X-Ray Diffraction Analysis

The X-ray diffraction patterns of mixed
spinel ferrites (MnAlxFe2.0−xO4 for x=0.0,0.2,0.4,0.6,0.8, and 1.0) are shown in Figure 1.Figure 1
X-ray diffraction pattern ofMnAlxFe2.0−xO4 system.The positions of diffraction
peaks from various planes were identified using JCPDS file no. 74-2403. It is
evident from Figure1 that each ferrite sample exhibits single-phase cubic
spinel structure with Fd-3m (227) space group. The value of the lattice
constant ‘aO’  for all the
samples was determined from the position of principal (311) peak using (1)aO=dhklh2+k2+l2,  where h, k, and l are the miller indices.The observed values of lattice
constant ‘aO’  listed in
Table 1 are slightly smaller than the JCPDS table value of 8.518 Å. 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].Table 1
Lattice
constant, density, and volume forMnAlxFe2.0−xO4 system.
Composition (x)Lattice constantVolumeX-ray density(aO ± 0.002) Å(at ± 0.002) ÅÅ3(dx ± 0.002) gm/cm30.08.4548.504604.235.070.28.4178.455596.465.010.48.3968.407592.054.920.68.3728.358586.844.830.88.3488.310581.964.741.08.3158.261574.894.64Figure 2
Variation of (a) lattice constant and (b) X-ray density with aluminum ions concentration.The decrease in lattice constant is
attributed to the fact that the Pauling ionic radius of Al3+ (0.50 Å) is smaller than that of Fe3+ (0.64 Å), which causes the shrinking in the unit cell dimensions. The decrease in ‘aO’  and the shift of reflections toward higher angle
with the increasing aluminum concentration  (x)
show that aluminum
atoms have been incorporated into the spinel structure [15].The X-ray density ‘dx’
was calculated using the formula [16] (2)dx=8MNaO3,  where ‘M’ is the molecular weight, ‘N’
is the Avogadro's
number, and ‘aO’ is the lattice constant of the spinel ferrite.
The calculated values of X-ray density are listed in Table 1.  The X-ray density
decreases with increasing aluminum concentration (x), as shown in Figure 2(b). The decrease in X-ray density is due
to the decrease in
mass, which overtakes the decrease in volume of the unit cell.The cation distribution in the
various spinel ferrite systems has been estimated from X-ray diffraction [5, 6], Mössbauer’s
effect [17, 18], and magnetization
measurements  [19, 20]. It has been
reported [21, 22] that the best
information in estimation of cation distribution can be achieved by comparing
the experimental and theoretical intensity ratios for reflections (220), (422),
and (400). However, the intensities of (220), (422), and (400) planes are more
sensitive to cations on A- and B-sites [23, 24]. The X-ray
diffraction intensity of the respective planes was calculated using the formula [25] (3)Ihkl=|Fhkl|2⋅P⋅LP, where  Ihkl  is the relative integral
intensity; Fhkl  is the
structure factor; P is the
multiplicity factor; Lp is the Lorentz factor. The structural factors
were calculated by using the equation suggested by Porta and Furuhashi et al.
[26, 27]. The
multiplicity factor and the Lorentz factors were taken from the literature
[16]. The ionic scattering factor reported in the international tables for
X-ray crystallography [28] is used for the calculation of structural factor. It
is well established that the intensity ratios I220/I440 and I400/I440 are considered to be sensitive
to cation distribution [29]. Therefore in the present ferrite system, intensity
ratios of these planes have been used in estimation of cation distribution. The
intensity ratios of these planes were calculated for various cation distributions
using the following expression suggested by Bertaut [21]: (4)(IhklIhIkIlI)obs=(IhklIhIkIlI)cal.  For all the samples, the calculated values,
those closest to the experimental observed values, are given in Table 2. The
theoretical lattice constant ‘at’
for all composition was calculated on the basis of estimated cation
distribution by using the relation [30] (5)at=833(rA+rO)+3(rB+rO),  where rA and rB are the radii of
the A- and the B-sites, respectively, and rO
is the radius of the oxygen ion O2− (1.48 Å).
The calculated values ‘at’
are nearly equal to the experimental observed value ‘aO’
which confirms the estimated cation distribution (see Table 2). The site radii rA  and  rB used above were determined using the following: (6)rA=0.446rtetFe3++0.554rtetMn2+,rB=(1.554−x)roctFe3++0.446roctMn2++xroctAl3+. 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
replacement of larger ionic radii (Fe3+) with smaller ionic radii
(Al3+) and their distribution amongst the A- and B-sites. The value
of the oxygen positional parameter ‘u’
was calculated by using the following relation:
(7)rA=aO3(u−0.25)−rO. The determined
values of ‘u’ are listed in Table 3.
The values of the tetrahedral (dAL),
octahedral bond length (dBL),
tetrahedral edge length (dAE), and shared (dBE)
and unshared octahedral edge lengths (dBEU) were calculated by using the experimental values of
lattice constant ‘aO’  and
oxygen positional parameter  ‘u’ from
the following [30, 31]: (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).  Various
calculated X-ray parameters are given in Table 3. It is observed that dAL, dBL, dAE, dBE, and dBEU decrease with increasing
aluminum concentration (x). This is due to the substitution process,
that is, replacement of larger ionic radii (Fe3+) by smaller ionic
radii (Al3+) and their distribution among the A- and B-sites. These
results are in consistent with the reported data [32]. 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 [33]. Electrons
those are hopping
between B- and A-sites are less probable compared to that between B- and B-sites,
because the distance between the two metal ions placed in B-sites is smaller
than if they were placed one in B-sites and the other in A-sites [34]. ‘L’ of
the A- and B-sites is determined from the following relations [35]: (9)LA=aO34,LB=aO24. It is observed
that ‘L’ of A- and B-sites decreases
with increasing aluminum concentration (x)
as shown in Figure 3.Table 2
Cation distribution data calculated from XRD
pattern of theMnAlxFe2.0−xO4 system.
X-ray intensityI220/I440I400/I440CompositionExp.Cal.Exp.Cal.Cation distributionFe3+(B)/Fe3+(A)MnFe2.0O41.02471.17960.96220.7643(Fe0.446Mn0.554)A[Fe1.554Mn0446]B3.48MnAl0.2Fe1.8O41.03481.28070.96240.6902(Fe0.446Mn0.554)A[Fe1.354Mn0.446AL0.2]B3.03MnAl0.4Fe1.6O41.05651.37720.94330.6483(Fe0.446Mn0.554)A[Fe1.254Mn0.446AL0.4]B2.58MnAl0.6Fe1.4O41.05791.47720.92570.6112(Fe0.446Mn0.554)A[Fe0.954Mn0.446AL0.6]B2.13MnAl0.8Fe1.2O41.07801.50200.84140.5711(Fe0.446Mn0.554)A[Fe0.754Mn0.446AL0.8]B1.69MnAl1.0Fe1.0O41.09101.57000.59070.5256(Fe0.446Mn0.554)A[Fe0.554Mn0.446AL1.0]B1.24Table 3
X-ray
parameters (error bar ± 0.002 Å):
tetrahedral and octahedral bond lengths (dAL and dBL) and jump lengths (LA and LB),
tetrahedral edge dAE, and shared and unshared octahedral edges (dBE and dBEU).
Composition (x)dAL (Å)dBL (Å)dAE (Å)dBE (Å)dBEU (Å)LA (Å)LB (Å)rA (Å)rB (Å)u (Å)0.02.24771.90313.67042.30753.02823.66072.98890.72861.35140.40350.22.24961.89223.67372.27853.01833.64362.97500.72861.32530.40430.42.24991.88583.74532.26333.01003.63592.96870.72861.29930.40470.62.24911.87763.67272.24723.00233.62522.96000.72861.27320.40510.82.24981.86853.67392.22892.99533.61472.95140.72861.24730.40561.02.24821.85923.67122.20832.98533.60052.93970.72861.22130.4061Figure 3
Variation of jump length ‘L’ with aluminum ions concentration.The decrease in
jump length is due to the decrease in the distance between the magnetic ions by
the substitution of smaller  Al3+ ions at the B-sites and is similar
to those reported earlier [4, 32].


### 3.2. Mössbauer Analysis

57Fe Mössbauer absorption
spectra of mixed spinel ferrite system MnAlxFe2.0−xO4 for x=0.0,0.2,0.4,0.6,0.8, and 1.0 recorded at room temperature
are displayed in Figure 4. The experimental data were fitted using least
square-fitting (NORMOS/SITE) program [36]. Each spectrum exhibits a
superposition of two Zeeman sextets, one sextet corresponding to a higher
magnetic field is attributed to Fe3+ ions on the B-site, and the
other sextet corresponding to lower magnetic field is attributed to Fe3+ ions on the A-site. The refined values of the hyperfine parameters computed
from the Mössbauer spectra are listed in Table 4. In the present ferrite
system, it is observed that on increasing Al3+ ions concentration,
the values of isomer shift (δ) of tetrahedral A-sites
show almost negligible change, indicating that aluminum ions do not enter in
A-sites. The isomer shift of B-sites is greater than A-site and is in agreement
with the reported data [11]. Furthermore, the observed values of isomer shift (δ) are significantly less than the expected
value, 0.5 mm/s for the Fe2+ ions [20]. 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 decrease with increasing Al3+ concentration (x). This can be explained on the basis of
supertransferred hyperfine field at the central cation that originates from the
magnetic moments of the nearest-neighbor cations, that is, from the intra-sublattice
contributions hAA and hBB and the inter-sublattice contributions hAB and hBA. In the present ferrite
system, the intra-sublattice contributions hAA and hBB are predominant. It has been reported that the intensities corresponding
to (200) and (422) reflections are most sensitive to cations on A-sites
[23, 24]. The X-ray diffraction patterns of the present ferrite system indicate
that the intensity of (220) and (422) reflections remains almost constant as
compared to (311) reflection, suggesting that  Al3+ ions do not enter
in the A-sites. The value of isomer shift (δ)
of A-sites remains invariant on substitution of aluminum ions suggesting that Al3+ ions do not replace
Fe3+ ions from A-sites. The introduction of Al3+ ions
that replaces Fe3+ ions from B-sites decreases intra-sublattice
contributions, which in turn decreases the hyperfine field Hhfs values. As nonmagnetic Al3+ ions replace Fe3+ ions, the correct amount of Fe3+ ions occupying A- and B-sites is estimated by determining the area under the Mössbauer absorption spectra through the least square
fitting program. The Fe3+(B)/Fe3+(A) ratio obtained
from the Mössbauer spectra is
in good agreement with those calculated from X-ray intensities. It is observed
that this ratio decreases with increasing aluminum concentration (x) suggesting a decrease in
ferrimagnetic behavior.Table 4
Room
temperature Mössbauer’s effect parameters for MnAlx Fe2.0−xO4 system as a function of x.
Composition (x)siteIsomershift*Quadrupole splittingHhfsArea (%)Fe3+(B)/Fe3+(A)(δ±0.01) mm/s(Δ±0.01) mm/s(±2.0 T)0.0B0.180.0047.7177.743.49A0.110.0043.4822.260.2B0.170.0047.1675.153.02A0.110.0042.7124.850.4B0.160.0046.4672.012.57A0.100.0042.4127.990.6B0.130.0045.3468.922.20A0.100.0040.5831.080.8B0.120.0042.8262.881.69A0.100.0036.9437.121.0B0.110.0041.8055.901.26A0.100.0036.5444.10Isomer*
shift given relative to α-Fe.Figure 4
Room temperature Mössbauer absorption spectra ofMnAlxFe2.0−xO4, for x=0.0,0.2,0.4,0.6,0.8, and 1.0.


## 3.1. X-Ray Diffraction Analysis

The X-ray diffraction patterns of mixed
spinel ferrites (MnAlxFe2.0−xO4 for x=0.0,0.2,0.4,0.6,0.8, and 1.0) are shown in Figure 1.Figure 1
X-ray diffraction pattern ofMnAlxFe2.0−xO4 system.The positions of diffraction
peaks from various planes were identified using JCPDS file no. 74-2403. It is
evident from Figure1 that each ferrite sample exhibits single-phase cubic
spinel structure with Fd-3m (227) space group. The value of the lattice
constant ‘aO’  for all the
samples was determined from the position of principal (311) peak using (1)aO=dhklh2+k2+l2,  where h, k, and l are the miller indices.The observed values of lattice
constant ‘aO’  listed in
Table 1 are slightly smaller than the JCPDS table value of 8.518 Å. 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].Table 1
Lattice
constant, density, and volume forMnAlxFe2.0−xO4 system.
Composition (x)Lattice constantVolumeX-ray density(aO ± 0.002) Å(at ± 0.002) ÅÅ3(dx ± 0.002) gm/cm30.08.4548.504604.235.070.28.4178.455596.465.010.48.3968.407592.054.920.68.3728.358586.844.830.88.3488.310581.964.741.08.3158.261574.894.64Figure 2
Variation of (a) lattice constant and (b) X-ray density with aluminum ions concentration.The decrease in lattice constant is
attributed to the fact that the Pauling ionic radius of Al3+ (0.50 Å) is smaller than that of Fe3+ (0.64 Å), which causes the shrinking in the unit cell dimensions. The decrease in ‘aO’  and the shift of reflections toward higher angle
with the increasing aluminum concentration  (x)
show that aluminum
atoms have been incorporated into the spinel structure [15].The X-ray density ‘dx’
was calculated using the formula [16] (2)dx=8MNaO3,  where ‘M’ is the molecular weight, ‘N’
is the Avogadro's
number, and ‘aO’ is the lattice constant of the spinel ferrite.
The calculated values of X-ray density are listed in Table 1.  The X-ray density
decreases with increasing aluminum concentration (x), as shown in Figure 2(b). The decrease in X-ray density is due
to the decrease in
mass, which overtakes the decrease in volume of the unit cell.The cation distribution in the
various spinel ferrite systems has been estimated from X-ray diffraction [5, 6], Mössbauer’s
effect [17, 18], and magnetization
measurements  [19, 20]. It has been
reported [21, 22] that the best
information in estimation of cation distribution can be achieved by comparing
the experimental and theoretical intensity ratios for reflections (220), (422),
and (400). However, the intensities of (220), (422), and (400) planes are more
sensitive to cations on A- and B-sites [23, 24]. The X-ray
diffraction intensity of the respective planes was calculated using the formula [25] (3)Ihkl=|Fhkl|2⋅P⋅LP, where  Ihkl  is the relative integral
intensity; Fhkl  is the
structure factor; P is the
multiplicity factor; Lp is the Lorentz factor. The structural factors
were calculated by using the equation suggested by Porta and Furuhashi et al.
[26, 27]. The
multiplicity factor and the Lorentz factors were taken from the literature
[16]. The ionic scattering factor reported in the international tables for
X-ray crystallography [28] is used for the calculation of structural factor. It
is well established that the intensity ratios I220/I440 and I400/I440 are considered to be sensitive
to cation distribution [29]. Therefore in the present ferrite system, intensity
ratios of these planes have been used in estimation of cation distribution. The
intensity ratios of these planes were calculated for various cation distributions
using the following expression suggested by Bertaut [21]: (4)(IhklIhIkIlI)obs=(IhklIhIkIlI)cal.  For all the samples, the calculated values,
those closest to the experimental observed values, are given in Table 2. The
theoretical lattice constant ‘at’
for all composition was calculated on the basis of estimated cation
distribution by using the relation [30] (5)at=833(rA+rO)+3(rB+rO),  where rA and rB are the radii of
the A- and the B-sites, respectively, and rO
is the radius of the oxygen ion O2− (1.48 Å).
The calculated values ‘at’
are nearly equal to the experimental observed value ‘aO’
which confirms the estimated cation distribution (see Table 2). The site radii rA  and  rB used above were determined using the following: (6)rA=0.446rtetFe3++0.554rtetMn2+,rB=(1.554−x)roctFe3++0.446roctMn2++xroctAl3+. 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
replacement of larger ionic radii (Fe3+) with smaller ionic radii
(Al3+) and their distribution amongst the A- and B-sites. The value
of the oxygen positional parameter ‘u’
was calculated by using the following relation:
(7)rA=aO3(u−0.25)−rO. The determined
values of ‘u’ are listed in Table 3.
The values of the tetrahedral (dAL),
octahedral bond length (dBL),
tetrahedral edge length (dAE), and shared (dBE)
and unshared octahedral edge lengths (dBEU) were calculated by using the experimental values of
lattice constant ‘aO’  and
oxygen positional parameter  ‘u’ from
the following [30, 31]: (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).  Various
calculated X-ray parameters are given in Table 3. It is observed that dAL, dBL, dAE, dBE, and dBEU decrease with increasing
aluminum concentration (x). This is due to the substitution process,
that is, replacement of larger ionic radii (Fe3+) by smaller ionic
radii (Al3+) and their distribution among the A- and B-sites. These
results are in consistent with the reported data [32]. 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 [33]. Electrons
those are hopping
between B- and A-sites are less probable compared to that between B- and B-sites,
because the distance between the two metal ions placed in B-sites is smaller
than if they were placed one in B-sites and the other in A-sites [34]. ‘L’ of
the A- and B-sites is determined from the following relations [35]: (9)LA=aO34,LB=aO24. It is observed
that ‘L’ of A- and B-sites decreases
with increasing aluminum concentration (x)
as shown in Figure 3.Table 2
Cation distribution data calculated from XRD
pattern of theMnAlxFe2.0−xO4 system.
X-ray intensityI220/I440I400/I440CompositionExp.Cal.Exp.Cal.Cation distributionFe3+(B)/Fe3+(A)MnFe2.0O41.02471.17960.96220.7643(Fe0.446Mn0.554)A[Fe1.554Mn0446]B3.48MnAl0.2Fe1.8O41.03481.28070.96240.6902(Fe0.446Mn0.554)A[Fe1.354Mn0.446AL0.2]B3.03MnAl0.4Fe1.6O41.05651.37720.94330.6483(Fe0.446Mn0.554)A[Fe1.254Mn0.446AL0.4]B2.58MnAl0.6Fe1.4O41.05791.47720.92570.6112(Fe0.446Mn0.554)A[Fe0.954Mn0.446AL0.6]B2.13MnAl0.8Fe1.2O41.07801.50200.84140.5711(Fe0.446Mn0.554)A[Fe0.754Mn0.446AL0.8]B1.69MnAl1.0Fe1.0O41.09101.57000.59070.5256(Fe0.446Mn0.554)A[Fe0.554Mn0.446AL1.0]B1.24Table 3
X-ray
parameters (error bar ± 0.002 Å):
tetrahedral and octahedral bond lengths (dAL and dBL) and jump lengths (LA and LB),
tetrahedral edge dAE, and shared and unshared octahedral edges (dBE and dBEU).
Composition (x)dAL (Å)dBL (Å)dAE (Å)dBE (Å)dBEU (Å)LA (Å)LB (Å)rA (Å)rB (Å)u (Å)0.02.24771.90313.67042.30753.02823.66072.98890.72861.35140.40350.22.24961.89223.67372.27853.01833.64362.97500.72861.32530.40430.42.24991.88583.74532.26333.01003.63592.96870.72861.29930.40470.62.24911.87763.67272.24723.00233.62522.96000.72861.27320.40510.82.24981.86853.67392.22892.99533.61472.95140.72861.24730.40561.02.24821.85923.67122.20832.98533.60052.93970.72861.22130.4061Figure 3
Variation of jump length ‘L’ with aluminum ions concentration.The decrease in
jump length is due to the decrease in the distance between the magnetic ions by
the substitution of smaller  Al3+ ions at the B-sites and is similar
to those reported earlier [4, 32].


## 3.2. Mössbauer Analysis

57Fe Mössbauer absorption
spectra of mixed spinel ferrite system MnAlxFe2.0−xO4 for x=0.0,0.2,0.4,0.6,0.8, and 1.0 recorded at room temperature
are displayed in Figure 4. The experimental data were fitted using least
square-fitting (NORMOS/SITE) program [36]. Each spectrum exhibits a
superposition of two Zeeman sextets, one sextet corresponding to a higher
magnetic field is attributed to Fe3+ ions on the B-site, and the
other sextet corresponding to lower magnetic field is attributed to Fe3+ ions on the A-site. The refined values of the hyperfine parameters computed
from the Mössbauer spectra are listed in Table 4. In the present ferrite
system, it is observed that on increasing Al3+ ions concentration,
the values of isomer shift (δ) of tetrahedral A-sites
show almost negligible change, indicating that aluminum ions do not enter in
A-sites. The isomer shift of B-sites is greater than A-site and is in agreement
with the reported data [11]. Furthermore, the observed values of isomer shift (δ) are significantly less than the expected
value, 0.5 mm/s for the Fe2+ ions [20]. 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 decrease with increasing Al3+ concentration (x). This can be explained on the basis of
supertransferred hyperfine field at the central cation that originates from the
magnetic moments of the nearest-neighbor cations, that is, from the intra-sublattice
contributions hAA and hBB and the inter-sublattice contributions hAB and hBA. In the present ferrite
system, the intra-sublattice contributions hAA and hBB are predominant. It has been reported that the intensities corresponding
to (200) and (422) reflections are most sensitive to cations on A-sites
[23, 24]. The X-ray diffraction patterns of the present ferrite system indicate
that the intensity of (220) and (422) reflections remains almost constant as
compared to (311) reflection, suggesting that  Al3+ ions do not enter
in the A-sites. The value of isomer shift (δ)
of A-sites remains invariant on substitution of aluminum ions suggesting that Al3+ ions do not replace
Fe3+ ions from A-sites. The introduction of Al3+ ions
that replaces Fe3+ ions from B-sites decreases intra-sublattice
contributions, which in turn decreases the hyperfine field Hhfs values. As nonmagnetic Al3+ ions replace Fe3+ ions, the correct amount of Fe3+ ions occupying A- and B-sites is estimated by determining the area under the Mössbauer absorption spectra through the least square
fitting program. The Fe3+(B)/Fe3+(A) ratio obtained
from the Mössbauer spectra is
in good agreement with those calculated from X-ray intensities. It is observed
that this ratio decreases with increasing aluminum concentration (x) suggesting a decrease in
ferrimagnetic behavior.Table 4
Room
temperature Mössbauer’s effect parameters for MnAlx Fe2.0−xO4 system as a function of x.
Composition (x)siteIsomershift*Quadrupole splittingHhfsArea (%)Fe3+(B)/Fe3+(A)(δ±0.01) mm/s(Δ±0.01) mm/s(±2.0 T)0.0B0.180.0047.7177.743.49A0.110.0043.4822.260.2B0.170.0047.1675.153.02A0.110.0042.7124.850.4B0.160.0046.4672.012.57A0.100.0042.4127.990.6B0.130.0045.3468.922.20A0.100.0040.5831.080.8B0.120.0042.8262.881.69A0.100.0036.9437.121.0B0.110.0041.8055.901.26A0.100.0036.5444.10Isomer*
shift given relative to α-Fe.Figure 4
Room temperature Mössbauer absorption spectra ofMnAlxFe2.0−xO4, for x=0.0,0.2,0.4,0.6,0.8, and 1.0.


## 4. Conclusion

Aluminum substituted manganese ferritesMnAlxFe2.0−xO4 for  x=0.0,0.2,0.4,0.6,0.8, and 1.0 have been prepared by double ceramic processing
technique. The unit cell parameter decreases linearly with the increase of
aluminum concentration (x) due to its
small ionic radius. The cation distribution estimated from X-ray intensity
ratios has been verified by comparing the theoretical and experimental lattice parameters.  It is observed that the correct amount of Fe3+ ions occupying B-
and A-sites obtained from Mössbauer spectra is in good agreement with those calculated from
X-ray intensity calculations. 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 supertransferred hyperfine field mechanism. The X-ray determined
parameters, for example, lattice constant, X-ray density, ionic radius, bond
length, jump length of the A- and B-sites, oxygen positional parameter, A-site
edge length, and shared and unshared B-site edge lengths were determined and
found affected by Al3+ ions substitution. On the basis of estimated
cation distribution, it is concluded that the present ferrite system exhibits a
55% normal spinel structure.



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*Source: 703479-2009-04-01.xml*