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(2015) 11687–11692

Ceramics International 41 www.elsevier.com/locate/ceramint

Structural and vibrational studies of NiAlxFe2�xO4 ferrites (0rxr1)

N. Bouhadouzaa, A. Raisa,n, S. Kaouab, M. Moreauc, K. Taibib, A. Addoua

aLaboratoire des Sciences et technique de l’Environnement et de la Valorisation, département de Génie des Procédés, Université de Mostaganem, Mostaganem,Algeria

bLaboratoire de Science et Génie des Matériaux, USTHB, Alger, AlgériacLaboratory of LASIR Spectrochemistry, University of Science and Technology, 59650 Villeneuve d’Ascq, France

Received 27 April 2015; received in revised form 22 May 2015; accepted 23 May 2015Available online 5 June 2015

Abstract

Nickel–Aluminium ferrites with the general formula NiAlxFe2�xO4 (0rxr1) were synthesized using the solid-state reaction technique andcharacterized using X-ray diffraction, FT-Infra Red and Raman spectroscopy. XRD diffraction patterns show that all samples have a pure single-phase cubic spinel structure over the whole composition range. From these patterns, the lattice parameter, bonds length, crystallite size, densityand porosity have been calculated. Infra Red spectra showed two significant absorption bands, the high band corresponds to tetrahedral (A) sitesand the lower band to octahedral [B] sites, thus confirming the single phase spinel structure. The force constants Kt and Ko for the two sites havebeen deduced from IR band frequencies and compared with the trend of bond lengths. For all compositions, Raman spectra show the five activemodes A1gþE1gþ3 T2g of the motion of O2� ions and both the A-site and B-site ions. The Raman frequencies trend with aluminium contentshow a blue shift for all modes consistent with the replacement of Fe3þ by lower mass Al3þ .& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Ni�Al ferrites; Spinel structure; XRD; FT-IR; Raman

1. Introduction

Because of their electrical and magnetic properties, softspinel ferrites are widely used for several kinds of industrialapplications such as information storage systems, cores forhigh-frequency transformers, inductors, and antennas for radioreceivers. Various substitutions have been incorporated inthese materials to achieve the desired characteristics andinvestigations are focused on Ni�Al mixed spinel ferritesbecause they have interesting structural, electrical and mag-netic properties [1–8]. Moreover, Ni�Al spinel ferrites arevery popular in microwave applications because of their highsaturation magnetization and very low losses [9].

The application of Infra Red and Raman spectroscopy toferrite materials can be utilized to observe the achievement ofthe solid-state reaction as well as the purity of the spinel

10.1016/j.ceramint.2015.05.13215 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel./fax: þ213 771 244 466.ss: amrais@yahoo.com (A. Rais).

structure phase. IR spectra provide important information onthe deformation of the spinel structure and various orderingproblems [10,11]. Raman spectroscopy is a non-destructivematerial characterization technique and is very sensitive tostructural disorder [12,13]. Furthermore, the concentration ofmetal cations in some mixed ferrites may lead to change ofstructure within the unit cell without affecting the spinelstructure as a whole. These structural changes due to the metalcations that are either lighter or heavier in the ferrites stronglyinfluence the lattice vibrations. The IR and Raman spectraabsorption bands mainly appear to be due to the vibrations ofthe oxygen ions with the cations producing various frequenciesof the unit cell [14].It is essential for industrial applications of aluminium-nickel

ferrites to study their structural and vibrational properties. Tothe best of our knowledge, no vibrational study using both FT-IR and Raman spectroscopy of the Al-substituted Ni ferriteshas been reported. The present work is concerned mainly withthe analysis of experimental results of X-ray diffraction, FT-IR

25 35 45 55 65 75 85 95

220

311

222

400

620

440

511

422 533 553

35,5 35,6 35,7 35,8 35,9 36

Inte

nsity

(a.

u)

--- x=0.5 --- x=0

--- x=1

2 ϴ (degree)

2 ϴ (degree)

Fig. 1. Typical X-ray diffraction pattern for Ni Alx Fe2�xO4 of sample x¼0.5.Inset shows the shifting of peak (311) of samples x¼0; 0.5; 1.

Table 1Lattice parameters aexp, bond lengths RA and RB at (A) and [B] sites, FTIRvibrational bands, force constants Kt and Ko of Ni Alx Fe2�x O4 ferrites.

x aexp (Å)70.03%

RA

(Å)RB

(Å)v1

(cm�1)v2

(cm�1)v3

(cm�1)Kt� 105

(dyne/cm)

Ko� 105

(dyne/cm)

0 8.331 1.890 1.982 620 420 3.548 1.6280.3 8.319 1.888 1.979 615 410 3.491 1.5510.5 8.313 1.886 1.978 610 410 47075 3.434 1.5510.8 8.296 1.883 1.974 610 405 500 3.434 1.5141 8.280 1.879 1.970 615 405 500 3.491 1.514

8,27

8,29

8,31

8,33

8,35

0 0,2 0,4 0,6 0,8 1

Latti

ce p

aram

eter

(Å)

Al(x)

Fig. 2. Lattice parameter aexp as function of Al content of Ni Alx Fe2�xO4

ferrites. Straight line is a linear least square fit.

N. Bouhadouza et al. / Ceramics International 41 (2015) 11687–1169211688

and Raman spectra in NiAlxFe2�xO4 over the compositionrange from x¼0 to x¼1.

2. Experimental

The conventional double sintering technique [15] wasutilized to prepare samples of NiAlxFe2�xO4 (x¼0; 0.3; 0.5;0.8; 1). The ingredient materials were analytical high puritygrade NiCO3, Fe2O3 and Al2O3 (BDH). These were weighedstoichiometrically as per chemical formula unit and the detailsof samples preparation are described in a previous publication[16]. Sample pellets were first sintered at 1000 1C for 12 h thenthey were sintered a second time at 1100 1C for 24 h. Thesingle-phase spinel structure was confirmed by the X-raydiffraction spectrum of these samples obtained with a Panaly-tical X’Pert Pro diffractometer using CuKα radiation(λ¼1.5406 Å). The scan's range was kept the same for allsamples 2θ¼201–1001 using a step size of 0.011 with sampletime of 10 s. For recording IR spectra, samples were preparedby mixing small quantity of the powdered ferrites with KBr.Then, the samples mixed powder was pressed in a cylindricaldisc at 10 tons/cm2 by hydraulic press. The IR measurementsof the prepared samples were recorded at room temperature inthe range from 400 cm�1 up to 1000 cm�1 using a Perkin-Elmer 783 FT-IR spectrophotometer. The Raman spectra wererecorded using a commercial LABRAM-HR equipped with aCCD detector having 1024� 256 pixels Chip MPP backilluminated and liquid nitrogen cooled. This system has800 mm focal length and is equipped with a grid of 600 t/mm enabling a spectral resolution of 1 cm�1/pixel. Themeasurements were carried out using a laser excitation sourceof 632.8 nm and the optical intensity at the sample surface waskept at 0.1 mW so to avoid damaging. The laser having a100� objective was focused on a spot of approximately0.9 μm2.

3. Results and discussion

3.1. X-ray diffraction analysis

Specimen spectra of X-ray diffraction for the Ni Alx Fe2�x O4

samples at compositions of x¼0; 0.5; and 1 are shown in Fig.1.As can be seen, the XRD patterns present peaks at reflectionplanes indexed (220), (311), (222), (400), (422), (511), (440),(620), (533) and (553) for all samples, thus proving theformation of the single phase cubic spinel structure. The insetof Fig.1 shows the shifting of peak (311) with Al content atx¼0; 0.5 and 1, indicating clearly the lattice parameter variation.The lattice parameters aexp for all samples have been calculatedusing the Nelson�Riley extrapolation method [17]. Values ofaexp are tabulated in Table 1 and their variations with Al contentare shown in Fig. 2. Values of the lattice parameter aexp forNiFe2O4 and NiAlFeO4 samples have been found equal to8.331 Å and 8.280 Å, which agrees reasonably well with theliterature values for NiFe2O4 [18–20] and for NiAlFeO4 [7]respectively. The decreasing trend of aexp with Al content isattributed to the replacement of Fe3þ ions (0.67 Å) by a smaller

ionic radius of the Al3þ (0.56 Å) at the octahedral sites [B].Fig.2 shows a straight line through the experimental data whichis a linear least square fit. Hence, within systematic error

N. Bouhadouza et al. / Ceramics International 41 (2015) 11687–11692 11689

estimated at 70.03%, we may assert that Vegard's law isobeyed in this system. This linear behavior of aexp with Alcontent was reported for other similar ferrites systems passingfrom partially to completely inverse spinel type structure [18].Nickel ferrite, NiFe2O4 has totally inverse spinel structure withhalf of the Fe3þ ions in B sites and the other half in A sites andall the Ni2þ ions in octahedral sites [16]. In the NiAlxFe2�xO4

ferrites, the Ni2þ ions exclusively occupy the octahedral sites,while some of the Al3þ ions prefer to occupy the tetrahedralsites and the rest are stable in the octahedral sites [21].

For the cubic spinel structure, the inter-ioniccations�anions distances (bond lengths) at A-sites, RA andB-sites, RB can be evaluated using the relations [22]:

RA ¼ affiffiffi3

pδþ 1

8

� �

RB ¼ a

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3δ2� δ

2þ 1

16

r

δ¼U�0:375

where U represents the oxygen positional parameter taken as0.381 for Nickel ferrite and δ represents the deviation from theideal value Uideal (¼0.375). Values of RA and RB are shown inTable 1. It is clear that the bond length of A-site RA is less thanthat of B-site RB at any composition. This can be interpreted as

Table 2Bulk and XRD densities dexp, dx, porosity P and crystallite size L of Ni AlxFe2�x O4 ferrites.

x dexp (g/cm3) 70.01 dx (g/cm3) P (%) L (nm) 71

0 4.41 5.387 18.1 1430.3 4.53 5.211 13.1 1240.5 4.42 5.066 12.9 1460.8 4.05 4.918 19.1 1151 3.50 4.812 27.3 150

3

3,5

4

4,5

5

5,5

0 0,2 0,4

Al (

dexp

10

20

30

0

P %

Den

sity

(g/ c

c)

0,5 1

Fig. 3. Bulk and DRX densities as function of Al con

due to more covalent bonding in A-sites than in B-sites [7]. Inaddition, both lengths show a decreasing trend with Al content,this may be attributed to the decrease of the lattice parameterdue to replacement of Fe ions by smaller size Al ions.The X-ray density dx for each sample is calculated according

to the relation [17]:

dx ¼ZM

Na3

where Z, M, N and a3 represent the number of molecules perunit cell (¼8), molecular weight, Avogadro's number andvolume of the unit cell, respectively. The bulk densities dexp(¼mass/volume) of the samples were measured using apycnometer. Values of the calculated dx and measured dexpare shown in Table 2 and plotted in Fig. 3 as function of Alcontent. X-ray density dx decreases linearly while dexpdecreases non-linearly with Al content, these trends supportthe lattice parameter variation. The true density, dx, is higherthan the bulk density dexp for all samples. This is due to theexistence of pores appearing during the preparation. Hence, theporosity P can be calculated using the following relation:

P¼ 1� dexpdx

The values of P (%) are shown in Table 2 and their variationwith Al content is shown in the inset of Fig.3. P appears tohave a minimum of 13% around the aluminium concentrationx¼0.4. Similar values of P and behavior have been reportedfor Ni�Co ferrites [20].The average crystallite size, L, of all samples was evaluated

from the reflected diffraction peaks using Scherrer's equation[17]:

L¼ kλ

β cos θ

where the constant k¼0.89, λ is the wavelength of the X-rayradiation (¼1.5406 Å), θ is the diffraction angle of the most

4

4,5

5

5,5

0,6 0,8 1

x)

dX

tent. Inset shows porosity P (%) with Al content.

400 500 600 700 800 900 1000

Rel

ativ

e Tr

ansm

ittan

ce

x=1

x=0.8

x=0.3

x=0.5

x=0

Wavenumber (cm-1)

Fig. 4. FT-IR spectra of Ni AlxFe2�xO4 (x¼0; 0.3; 0.5; 0.8; 1) ferrite samples.

N. Bouhadouza et al. / Ceramics International 41 (2015) 11687–1169211690

intense peak (311) and β is its full width at half maximum(FWHM) in radian. Values of the crystallite size are given inTable 2. The average value of L was observed to be 130 nm.Comparable results have been reported for Ni�Cr ferrites[23].

3.2. Infra Red analysis

The infrared spectra measurements of the NiAlxFe2�xO4 inthe range 400–950 cm�1 indicate the presence of two strongabsorption bands ν1 (610–620 cm�1) and ν2 (405–420 cm�1).Fig.4 shows the IR spectra of NiAlxFe2�xO4 ferrites atcompositions x¼0; 0.3; 0.5 0.8; 1. The band positions arelisted in Table 1 as function of Al content. The observedabsorption bands within this limit reveal and confirm theformation of single-phase spinel structure having two sub-lattices: tetrahedral (A) site and octahedral (B) site [11]. Theabsorption band ν1 observed around 615 cm�1 is attributed tothe stretching vibration mode of metal-oxygen in the tetra-hedral sites, whereas ν2 observed around 410 cm�1 is assignedto octahedral group complexes. The difference in the positionof the two strong bonds can be related to the differences in theFe�O bond lengths at A-sites and B-sites. It is clear thatshorter Fe-O bond length at A-sites (1.890–1.879 Å) than thatof the B-sites (1.982–1.970 Å) would lead to higher frequencyband ν1 than ν2.

In the present system, the IR spectra for x¼0.5; 0.8 and 1,show an additional peak around 500 cm�1. Similar behaviorhas been reported for the same system [7]. The intensity of ν3vanishes completely at x¼0 and 0.3. Thus, appearance of ν3band in IR spectra of Ni�Al ferrite may be due to divalentmetal ion oxygen complexes in B-sites. The presence of Fe2þ

ion induces the splitting of absorption possibly because oflocal lattice deformation known as Jahn�Teller effect [24].

The force constants of the ions at the tetrahedral site (Kt) andoctahedral site (Ko) have been calculated for the IR band

frequencies ν1 and ν2 using the following formula [25]:

Kt=o ¼ 4π2c2ν21=2μ

where c is the light speed (E2.99� 1010 cm/s), ν is thevibration frequency of the A- and B-sites and μ is the reducedmass of the Fe3þ and O2� ions (E2.601� 10�23 g). Table 1shows variation of Kt and Ko with Al content. It can be seenthat the trend of both Kt and Ko is decreasing with Al content.One may interpret this behavior as due to the decrease in bondlengths of both A and B-sites, RA and RB with Al content.Since for a given x, the increase in bond length tends todecrease the force constant, one may attribute this to the factthat oxygen can form under favorable conditions, strong bondswith the metal ions even at large inter-ionic separations.Similar behavior has been reported for other metal oxides likeMg�Zn ferrites [25].

3.3. Raman scattering analysis

Raman spectra performed at room temperature in thefrequency range of 150–850 cm�1 of as synthesized NiAlxFe2�x O4 (0rxr1) ferrites samples are shown in Fig.5.These ferrites have cubic inverse spinel structure of typeAB2O4 belonging to Fd-3m (O7

h) space group with eightformula units per unit cell. Although the full unit cell contains56 atoms (8 molecules per unit cell), only 14 atoms are in theasymmetric unit and therefore 42 vibrational modes areexpected. According to group theory, the irreducible represen-tations for the studied systems are as follows:

Γirred ¼A1gðRÞþEgðRÞþT1gþ3T2gðRÞþ2A2uþ2Eu

þ5T1uðIRÞþ2T2u

The presence of an inversion center in the centro-symmetrical space group Fd3m implies mutual exclusion ofRaman and IR activities for the same vibrational mode. Thereare five first-order Raman active modes A1gþEgþ3T2g and allthese modes were observed at ambient conditions whereasonly the T1u type normal vibrations modes are infrared-active.T1g, A2u, Eu and T2u symmetry vibrations are the silent ones.All the IR-active vibrations are triply degenerated. A is for onedimensional representation, E for two and T for three dimen-sional representations, g denotes the symmetry with respect tothe center of inversion. A1g describes symmetric stretch ofoxygen atoms along Fe–O (or M�O) tetrahedral bonds,T2g (1): translatory movement of the whole tetrahedron(FeO4), T2g (2): asymmetric stretch of Fe(M) �O bond,T2g (3) and Eg: asymmetric and symmetric bends of oxygenwith respect to Fe, respectively. In order to determine thenatural frequency of the Raman active modes of each sample, aleast square fit with Lorentzian line shape was used to fit theRaman spectra. The thick smooth lines are fits to theLorentzian functions. The bands corresponding to these modesare observed at ambient conditions in Raman spectrum ofNiFe2O4 whose values are

T2g 1ð Þ ¼ 213 cm�1;Eg ¼ 337 cm�1;T2g 2ð Þ ¼ 488 cm�1;

T2g 3ð Þ ¼ 574 cm�1;A1g ¼ 702 cm�1

Raman Shift (cm-1)

200 300 400 500 600 700 800

T2g(1) Eg

T2g(2)T2g(3)

A1g

Ni Fe2 O4

Ni Al0.3 Fe1.7 O4

Ni Al0.5 Fe1.5 O4

Ni Al0.8 Fe1.2 O4

Ni Al1 Fe1 O4

Lorentz Fit

Inte

nsity

(a.

u)

T2g(1)

T2g(1)

T2g(1)

T2g(1)

Eg

Eg

Eg

Eg

T2g(2)

T2g(2)

T2g(2)

T2g(2)

T2g(3)

T2g(3)

T2g(3)

T2g(3)

A1g

A1g

A1g

A1g

Fig. 5. Room temperature Raman scattering spectra of NiAlxFe2�xO4 (x¼0.0,0.3, 0.5, 0.8, 1.0) ferrite samples.

Table 3Raman parameters of NiAlx Fe2�x O4 (x¼0; 0.3; 0.5; 0.8; 1) ferrite samples.

Assignment NiAlx Fe2�x O4 Raman shift (cm�1 )

x¼0 x¼0.3 x¼0.5 x¼0.8 x¼1

T2g (1) 213 213 216 218 220Eg 337 337 343 345 347T2g (2) 488 490 489 492 494T2g (3) 574 572 580 584 586A1g 702 702 714 715 717

N. Bouhadouza et al. / Ceramics International 41 (2015) 11687–11692 11691

These results are in good agreement with earlier reported dataon NiFe2O4 spinel [26].

The Raman shifts and their assignment for the five samplesof NiAlx Fe2�xO4 (x¼0; 0.1; 0.3; 0.5; 0.8; 1) are shown inTable 3. In Raman spectrum of any ferrite (cubic spinels),shifts above 600 cm�1 correspond mostly to the motion ofoxygen in tetrahedral AO4 group (A- site) [27,28] and theother low frequency bands represent the characteristics ofoctahedral BO6 group (B-site). It is clear from Table 3 that allmodes of the Ni Alx Fe2�x O4 ferrites are shifting toward thehigher wave number side as the aluminium content increases.This blue shift is attributed to lower atomic mass of Al ion as

compared to Fe ion. Similar behavior has been reported for Mgand Zn substituted Ni-ferrites [29] as well as for Mnsubstituted Zn ferrites [30].Furthermore, it is observed for NiFe2 O4 spectrum that

Raman band at 702 cm�1 shows a shoulder like feature atlower wave number side (665 cm�1). This band is assigned toA1g(1) mode reflecting the stretching vibration of Fe�Obonds in tetrahedral site. According to Singh et al. [31] forinverse spinels such as MFe2O4 (M¼Mg, Ni, Fe), A1g(1)band in the 670–710 cm�1 region in their Raman spectracorresponds to stretching modes of tetrahedral units, whileRaman modes present in the 450–620 cm�1 ferrite spectra aredominated by octahedral groups. However, A1g mode at about615 cm�1 observed in MFe2O4 (M¼Mn, Zn) might beassigned to the motion of an octahedron [29]. Thus, a possibleinterpretation for this shoulder like feature at 665 cm�1

might be due to the bonds distribution among the A-sitesand B-sites since it is outside the 670–710 cm�1 region. Notealso that this feature is present in the spectrum of samplesx¼0.3; 0.5 and is less pronounced in sample x¼0.8. It almostvanishes for sample x¼1, this may be interpreted as a borderline between two different distribution of cations among A andB-sites.

4. Conclusion

The characterization of NiAlxFe2�xO4 ferrites system pre-pared by the conventional solid state reaction with doublesintering around 1050 1C shows that:

1.

The crystalline structure is a pure single phase cubic spinelover the whole composition range from x¼0 to x¼1.

2.

The measured lattice constant aexp decreases with increas-ing Al(x) content and appears to obey Vegard's law.

3.

The IR spectra indicate two main absorption bands, a highband (580–610 cm�1) for tetrahedral (A) sites and a lowerband (400–410 cm�1) for octahedral [B] sites, thus con-firming the single phase spinel structure.

4.

The Raman spectra show the five active modesA1g+E1g+3T2g of the motion of O2- ions and both theA-site and B-site ions for all compositions. The Ramanfrequencies trend with aluminium content show a blue shiftfor all modes consistent with the replacement of Fe3+ bylower mass Al3+.

N. Bouhadouza et al. / Ceramics International 41 (2015) 11687–1169211692

Acknowledgments

This work was supported by a grant from the DirectorateGeneral for Scientific Research and Technological Develop-ment (DG-SRTD), Ministry of Higher Education and ScientificResearch of Algeria.

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