Cr3+ and Al3+ co-substituted zinc ferrite: Structural analysis, magnetic and electrical properties

Polyhedron 70 (2014) 110–118

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Cr3+ and Al3+ co-substituted zinc ferrite: Structural analysis, magnetic and electrical properties Adrian I. Borhan a, Vasile Hulea b, Alexandra R. Iordan a, Mircea N. Palamaru a,⇑ a b

‘‘Alexandru Ioan Cuza’’ University of Iasi, Faculty of Chemistry, 11 Carol 1 Blvd., Iasi, Romania Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Matériaux Avancés pour la Catalyse et la Santé, 8 rue de l’Ecole Normale, 34 296 Cedex 5, France

a r t i c l e

i n f o

Article history: Received 12 August 2013 Accepted 22 December 2013 Available online 31 December 2013 Keywords: Ferrite Chromium and aluminum Sol-gel processes Mössbauer spectroscopy

a b s t r a c t Nanocrystalline powders of chromium and aluminum co-substituted zinc ferrites with general formula ZnFe22xCrxAlxO4 (0 6 x 6 1) have been synthesized for the first time. Using the sol-gel auto-combustion technique and the tartaric acid as combustion-complexion agent, materials with spinel mono-phase cubic spinel structure were successfully prepared. The materials were characterized by IR, XRD, SEM and 57Fe Mössbauer spectroscopy. The crystallite size estimated by Scherrer formula has been found in the range of 16–69 nm. The experimental results combined with those obtained from a mathematical model suggested that all compounds have a mixed ionic distribution. Moreover, relationships between the structure features and the electric and magnetic properties have been established. The magnetic measurements showed that the hysteresis losses and the magnetization at 10 KOE linearly decreased when the Cr–Al content in Zn ferrite increased. The results obtained in dielectric study showed very low values of dielectric loss at frequencies over 1 MHz. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Spinel ferrites, with a general formula of MFe2O4 (M = Zn, Ni, Mn, Cu, Co, Mg, Ca, etc.), are a very important class of materials, displaying a large range of properties and applications [1,2]. Due to their high chemical/thermal stability, these magnetic materials play a major role in many areas of technological applications. Ferrites are used in the fabrication of chemical sensors, catalysts, power transformers, video and audio equipments, telephone and telecommunication systems (microwave devices), data storage systems (discs, disc drives), etc. [1–5]. The properties of spinels mainly depend on their cation distribution and composition, as well as the synthesis method. Bulk magnetic materials exhibited high losses at high frequencies, while ferrite nanoparticles show a flat frequency response with very low loss against all frequency. Therefore, the huge advantage of magnetic nanoparticles includes reduction in total core power losses due to eddy currents. Based on these considerations, our research focused on the preparation of material nanoparticles that provide the advantage of very low power loss on a broad range of frequencies. However, through substitution of a cation with another cation can be produced changes in particle size and structure, which directly influences the magnetic and electrical behavior of materials prepared. ZnFe2O4 with normal spinel structure is one of the most studied ferrite because ⇑ Corresponding author. Tel.: +40 232201341; fax: +40 232201313. E-mail address: [email protected] (M.N. Palamaru). 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.12.022

of their potential applications as magnetic materials [6], (photo)catalysts [3–7], gas sensor [8] and high frequency transformers [9]. When the iron atoms in zinc ferrite have been replaced by other metals, polymetalic oxides with spinel structure and specific properties were obtained. Thus, ZnCr2O4 is a geometrically frustrated antiferromagnet [10] and is expected to have perfect sensing properties and could be possibly used as humidity-sensing material [11]. ZnAl2O4 is well-known wide band gap semiconductor with a normal spinel structure [12]. According to our literature data, no report has been made about the combined effect of Cr–Al substitution for Fe in ZnFe2O4. Note that there are several studies focusing on the effect of Cr–Al cosubstitution for other types of ferrites and its magnetic properties. Mane et al. [13] studied structural and magnetic properties of aluminum and chromium co-substituted cobalt ferrite and reported that the saturation magnetization decreases with Al–Cr content (x), indicating reduction in ferrimagnetic behavior. Chhaya et al. [14] studied the magnetic properties of mixed spinel NiAlxCrxFe22xO4. The Mössbauer results confirmed a collinear ferrimagnetic structure. Additionally, the Curie temperature obtained from a.c. susceptibility decreases nearly linear with increase of Al–Cr concentration from x = 0.1–0.5. More et al. [15] studied the structural properties and the magnetic interactions in Al3+ and Cr3+ co-substituted CoFe2O4 ferrite. They reported that the saturation magnetization decreases when the Al–Cr content increases. These materials can be used for recording media due to the high

A.I. Borhan et al. / Polyhedron 70 (2014) 110–118

value and increasing trend of remanent ratio (R) with Al–Cr substitution. The aim of the present work was to prepare and characterize spinels with general formula ZnFe22xCrxAlxO4 (0 6 x 6 1), by cosubstituting Cr3+ (magnetic; 3lB) and Al3+ (diamagnetic) ions for Fe3+ ions (magnetic; 5lB) in ZnFe2O4. Also, the effect of Fe:Cr:Al ratios on the electrical and magnetic properties are investigated. Note that there is no study focusing on dielectric properties of these type of ferrites. As known, one important technique for the ferrites preparation is the sol-gel method because this procedure allows controlling the chemical composition, the solid texture, the size and the shape of the particles. In this study, the materials are obtained by sol-gel auto-combustion method using tartaric acid as chelating and fuel agent [16]. Finally, such research is reduced to the successful preparation of high performance soft magnetic materials that could replace ferrite cores used in production of inductive components. Thereby, the inductive components might be made lighter, smaller, cheaper and more durable, leading to increased performance in high frequency devices. Ternary spinels take relevance because the introduction of a third metal should modify the ion distribution in the spinel ferrites. Therefore, through an adequate selection of a substituting ion and an appropriate chemical composition, the dielectric properties of the ferrites could be improved. Indeed, we showed that the introduction of Cr3+ and Al3+ in zinc ferrite caused a number of changes in the spinel lattice which have helped improve the dielectric properties at higher frequencies. Our results were showing that the increase in Cr3+ and Al3+ content caused a reduction in hysteresis losses, that are in good agreement with the findings on Al and Cr co-substituted CoFe2O4 ferrite [13] and very low values of dielectric loss at frequencies over 1 MHz. However, according to the literature, the mechanism by which a third metal changes the properties of these complex systems is still a matter of controversy. 2. Experimental The samples of the ZnFe22xCrxAlxO4 system (x = 0, 0.125, 0.25, 0.375, 0.50, 0.75, 1.0), hereinafter denoted as ZnFe2O4, ZnFe1.75Cr0.125Al0.125O4, ZnFe1.5Cr0.25Al0.25O4, ZnFe1.25Cr0.375Al0.375O4, ZnFe Cr0.5Al0.5O4, ZnFe0.5Cr0.75Al0.75O4, ZnCrAlO4, were prepared in air by sol-gel auto-combustion method. Mixed oxide powders were produced from aqueous solution containing Zn, Fe, Cr, Al nitrates and tartaric acid as chelating and fuel agent. Analytical grade iron nitrate Fe(NO3)39H2O (99.9%, Aldrich), chromium nitrate Cr(NO3)39H2O (99.9%, Aldrich), aluminum nitrate Al(NO3)39H2O (99.9%, Aldrich) and zinc nitrate solution were mixed in stoichiometric proportions. Zinc nitrate Zn(NO3)2 was obtained in situ from ZnO (99%, Sigma–Aldrich) and nitric acid (Merck) 20% solution. A solution of tartaric acid [C4H6O6] (Merck) was mixed with each sample of metal nitrates mixture in 3:1 molar ratio of tartaric acid to metallic cations. According to our research group experiences, tartaric acid attempt to yield finer-sized powders, possessing desirable powder characteristics, by a more intimate mixing to the starting materials. For ZnFe22xCrxAlxO4 systems (0 6 x 6 1) the following supposed chemical equations could be written:

ZnO þ 2HNO3 ¼ ZnðNO3 Þ2 þ H2 O

converted to a powder. The as burnt powders were then thermally treated in two steps: up to 500 °C/7 h and up to 700 °C/14 h. The optimal crystallization process was achieved by this procedure. Infrared study of the as-prepared powders has been done using a Bruker TENSOR TM27 with ATR cell, with 2 cm1 resolution. IR spectroscopy was used for monitoring solid phase chemical reactions and for the disappearance of the organic phase. The phase formation of the sintered powders was confirmed by X-ray diffraction (XRD), using a Bruker ASX D8 Advance diffractometer with Cu Ka radiation (k = 1.5406 Å), for 2h ranging between 20° and 80°, at a scanning speed of 0.02°/s. The morphology and particles size analysis of the sintered powders at 700 °C were investigated by scanning electron microscopy using a Hitachi S2600 N Microscope. Mössbauer spectra for four representative samples, i.e., x = 0.0, 0.375, 0.50, 0.75, were recorded at room temperature in transmission mode for 57Fe, at a speed of 4 and 12 mm/s (at the source). Scale velocity is different because the relative speed source-absorber is changed in a field suitable to obtain Mössbauer effect in the form of resonance absorption. A standard least-squares minimization routine was used to fit the spectra as a superposition of Lorentzian lines. Magnetic properties of the obtained powders were studied at room temperature using a Vibrating Sample Magnetometer System (VSM CFT). The frequency dependence of capacitance, dielectric permittivity and dielectric losses were studied using a Digital LCR Meter Proteck 9216 A device, ranging between 100 Hz and 1 MHz. The pellets were pressed into a cylindrical disk at 400 kPa/cm2, without subsequent calcination. Silver paste is coated on polished pellets to provide electrical contacts and each sample was inserted between two electrodes. 3. Results and discussion 3.1. Infrared spectra interpretation The IR spectra, in the wavenumber range 900–300 cm1, for ZnFe22xCrxAlxO4 (0 6 x 6 1) powders thermally treated at 700 °C, are shown in Fig. 1. There can be seen the presence of two main 1 and m 2 wavenumbers, characteristic of spiabsorbtion peaks at m nel structure, for all samples. According to Waldron [17], the stron1 , observed in the range 624–527 cm1, is gest absorbtion peak m caused by intrinsic vibration of bonds between metal ions and oxygen ions in tetrahedral positions, whereas the weakest absorbtion 2 , observed in the range 490–360 cm1, is assigned to the peak m stretching vibrations of bonds between octahedral metal ions and

ð1Þ

ZnðNO3 Þ2 þ xCrðNO3 Þ3 þ xAlðNO3 Þ3 þ ð2  2xÞFeðNO3 Þ3 þ ð11=2ÞO2 þ 3C4 H6 O6 ¼ ZnFe22x Crx Alx O4 þ 8NO2 " þ12CO2 " þ9H2 O "

ð2Þ

The mixed solutions are transformed into gel phase on heating for 3 h at 80 °C. The dried gel was gradually heated at 300 °C, when the auto-ignition was clearly observed and all the gel was

111

Fig. 1. IR spectra of ZnFe22xCrxAlxO4 powders heated at 700 °C.

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oxygen ions [18]. A comparison of the observed vibrational frequencies of all samples, indicates an increase of the wavenumber values for both tetrahedral and octahedral vibrations. Increasing 1 and m 2 wavenumbers with increase in Cr3+ and Al3+ content the m can be attributed to elementary cell volume decrease as can be seen from the variation of lattice constants. Cr3+ (radius: 0.615 Å) and Al3+ (radius: 0.51 Å) ions substitute proportional amounts of Fe3+ ions (radius: 0.645 Å) in octahedral sites, which causes a decrease in the metal–oxygen bond length and an increase of wavelength [19]. Cr3+ ion has a strong preference for octahedral sites, influencing competition between Fe3+ and Zn2+ ions for occupying tetrahedral and octahedral sites. Al3+ ions can occupy both tetrahedral A and octahedral B positions [13]. Table 1 includes the force constants corresponding to the tetrahedral and octahedral sites, calculated by using the standard equation given below [20]:

21  102 KT ¼ 7:62  M1  m

ð3Þ

22  102 KO ¼ 10:62  M2 =2  m

ð4Þ

where KT and KO represent the force constants of tetrahedral and octahedral sites, respectively; M1 and M2 are molecular weights of 1 corretetrahedral and octahedral site, respectively; wavenumber m 2 corresponds to octahedral site. M1 sponds to tetrahedral site and m and M2 are calculated from the cation distribution for each sample. It is observed from Table 1 that the force constants of both tetrahedral and octahedral sites increase with increase in Cr3+ and Al3+ content. This is consistent with results obtained from the cation distribution, which we are going to explain below. Force constants are inversely proportional to the bond length. The differ1 and m 2 (as m 1 –m 2 ) decreases with increasing Cr3+ ence between m 3+ and Al content in system. This indicates that super-exchange interaction a–d sites may decrease [17], confirmed also by proposed cation distribution. 3.2. X-ray diffraction study The XRD patterns of as-obtained powders are illustrated in Fig. 2. All reflection peaks identified, corresponding to spinel structure, were indexed in good agreement with the referred database of International Centre for Diffraction Data (ICDD) [21–24]. Spinel monophase completion was observed for all samples. Only for ZnFeCr0.5Al0.5O4 sample has been detected a secondary phase, i.e., hematite a-Fe2O3, with rhombohedral structure [25]. The presence of this secondary phase was also confirmed by Mössbauer spectroscopy analysis. All observed peaks can be indexed as belonging to the Fd3m space group with cubic symmetry. Introduction of both Cr3+ and Al3+ in ferrite lattice produced a shift of the (3 1 1) diffraction peak, because Cr3+ ions (ionic radius: 0.615 Å) and Al3+ ions (ionic radius: 0.51 Å) have smaller ionic radii than Fe3+ ions (radius: 0.645 Å). The lattice was analyzed by means of line broadening of the most intense (3 1 1) diffraction peak. The calculated values of lattice parameter (a311), crystallite size (D), distance (dhkl) and X-ray density (dx) are presented in Table 2.

Fig. 2. X-ray diffraction patterns for ZnFe22xCrxAlxO4 powders heated at 700 °C.

Table 2 Structural parameter of ZnFe22xCrxAlxO4 powders : lattice constant (a311), crystallite size (D), inter-planar spacing (d311), X-ray density (dx). Composition (x)

a311 (Å) ± 0.2%

D (nm) ± 0.5%

d311 (Å) ± 0.2%

dx (g/cm3)

0.000 0.125 0.250 0.375 0.500 0.750 1.000

8.428 8.397 8.370 8.356 8.305 8.254 8.230

69.3 50.6 29.2 16.2 36.2 23.8 23.4

2.54 2.53 2.52 2.51 2.50 2.488 2.481

5.38 5.31 5.27 5.20 5.21 5.11 4.96

The lattice parameter a311 and the distances between adjacent Miller planes (h k l) were determined according to Laue and Bragg equations for cubic lattice: 2

2

2 1=2

a311 ¼ d311 ðh þ k þ l Þ

ð5Þ

2dhkl sin h ¼ nk

ð6Þ

One can see in Table 2 that the a311 values decrease linearly, from 8.428 Å (x = 0.00) to 8.23 Å (x = 1.00), with increasing chromium–aluminum content. This lattice parameter decrease is because Fe3+ ions, which have a longer ionic radius (0.645 Å), are substituted simultaneously with low ionic radii ions, i.e., 0.615 Å for Cr3+ and 0.51 Å for Al3+ [13]. The crystallite size (D) of the powders was determined by using the Debye-Scherrer formula:

D ¼ 0:9k=b cos h

ð7Þ

where D is the dimension of the crystallite perpendicular to the Miller plane (h k l), k is the wavelength of X-ray source used (Cu Ka = 1.5405 Å), b is full width at half maximum of the diffraction (3 1 1) in radians and h is the Bragg angle. To obtain b and h values,

Table 1 1 and m 2 ), force constants (KT and KO) and (m 1 –m 2 ) difference. IR wavenumber (m Composition (x)

m1 (cm1)

m2 (cm1)

KT  106 (dynes/cm2)

KO  106 (dynes/cm2)

m1 –m2

0.000 0.125 0.250 0.375 0.500 0.750 1.000

527 535 553 582 605 612 624

360 380 406 430 440 475 490

1.33 1.37 1.47 1.64 1.78 1.93 2.03

0.77 0.82 0.90 0.97 0.97 1.02 0.94

167 155 147 152 165 147 134

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there was used Gaussian fitting model with maximum estimated errors ±0.5% for b values and ±0.2% for h values, respectively. The crystallite size decreases with increase in chromium and aluminum content and is observed ranging between 69 and 16 nm (Table 2). Sample with x = 0.50 does not follow this downward trend, probably due to the existence of secondary phase Fe2O3. The values for X-ray density (dx) were calculated for each powder according to the relation:

dx ¼ ZM=Na3

Table 4 Relative bond angles (h1, h2, h3, h4, h5) for effective magnetic interactions (A–O–A, A– O–B, B–O–B) in ZnFe22xCrxAlxO4 system.

ð8Þ

where Z is the number of molecules per primitive unit cell (Z = 8), M is the molecular weight of the each compound, N is the Avogadro’s number (6.0223  1023 particles/mol), and (a) is the lattice constant. It can be seen that the X-ray density (dx) decreases with increasing chromium and aluminum content. The calculated value of average X-ray density of ZnFe2O4 sample is in close agreement with that given by the ICDD card for this compound [21]. It is noted that X-ray density decreases from 5.38 to 4.96 g/cm3. This variation may be due to smaller density and atomic mass of chromium and aluminum cations than those of Fe cations and to the existence of pores in the samples, depending on the synthesis conditions. In agreement with other studies, Zn2+, Fe3+ and Al3+ ions can occupy both octahedral and tetrahedral positions and Cr3+ occupy only octahedral B site, because Cr3+ ions have strong preference for octahedral site [13–15]. In this case, one can propose the following formula for the cation distribution: 3þ

3þ

Composition (x)

h1(°) (A–O–B)

h2(°) (A–O–B)

h3(°) (B–O–B)

h4 (°) (B–O–B)

h5(°) (A–O–A)

0.000 0.125 0.250 0.375 0.500 0.750 1.000

122.32 121.26 120.19 119.08 117.95 115.51 111.91

140.63 136.55 132.70 128.99 125.37 118.06 108.10

94.47 96.24 98.13 100.16 102.37 107.56 116.45

126.27 126.64 127.03 127.43 127.85 128.77 130.15

71.78 69.12 66.56 64.07 61.65 56.84 50.66

3.3. SEM analysis The SEM photographs of the samples with composition x = 0.0, 0.50, 0.75, 1.0, are shown in the Fig. 3(a–d). It is observed that the particles are similarly spherical and show the decreasing trend with increasing chromium and aluminum content in system. The average particles size is smaller than 100 nm for all composition and it is slightly larger than crystallites size determined by XRD. This shows that every particle is formed by a number of crystallites or grains. According to the literature there is a direct connection between the XRD crystallite size and particle size of SEM, meaning that a particle can be formed by agglomeration of crystallites [30]. The particles size decreases significantly with chromium and aluminum content increasing since interionic distances between cations B–B decrease, which leads to agglomeration of the nanoparticles.

B

3þ 2 3þ 2þ 3þ ðZn2þ y Ala Fe1ya ÞA½Zn1y Feð22xÞð1yÞþa Alxa Crx  O4 ;

3.4. Mössbauer spectroscopy

indicating that Fe ions have migrated from the B sites to A sites. Table 3 includes the proposed cation distribution, theoretical lattice parameter (ath), oxygen positional parameter (u), determined from XRD data [26]. The calculated values for oxygen positional parameter are higher than the ideal values of 0.375 for A-site and of 0.250 for B-site, increasing with Cr–Al content in samples [27,28]. Bond length values of the tetrahedral and octahedral sites, RA and RB were calculated using lattice constant values and oxygen positional parameter u, based on the cation distribution proposed [27,29]. Increasing the occupation degree of the smaller ionic radii of Cr3+ (0.615 Å) and Al3+ (0.51 Å) ions to the B-sites instead of Fe3+ (0.645 Å) ions can explain the decrease of RB. The increase in RA is due to the substitution of Fe3+ ions with Al3+ ions and migration of Zn2+ from A-site to B-site. This variation can be due to the correlation between the lattice parameter and the ionic radius [15]. The relative bond angles (h1, h2, h3, h4, h5) for effective magnetic interactions (A–O–A, A–O–B, B–O–B), determined from XRD data [26] are shown in Table 4. The interionic distances between cations M–M (b, c, d, e, f) and between cations and anions M–O (p, q, r, s) decrease and increase, respectively, with increase in Cr–Al content (Table 5). These results are in agreement with decrease in unit cell volume [27].

Typical Mössbauer spectra recorded at 300 K for samples with x = 0, 0.375, 0.50, 0.75 are displayed in Fig. 4. Mössbauer spectra exhibit a central doublet near zero velocity which has been attributed to Fe3+ located in the octahedral sites of structure, which do not participate in the long-range magnetic ordering due to a large number of nonmagnetic nearest neighbors [31]. In general, with increasing Cr–Al content in system, one can see that isomer shift does not vary significantly, although quadrupole splitting increases monotonously. This indicates that the electron density around the Fe3+ nucleus were unaffected by substitution with Cr and Al [31]. There was no evidence of any magnetically split spectrum characteristic of a-Fe2O3, probably because their spins are not oriented to produce an internal magnetic field, except for the sample with x = 0.50, which is a magnetic component possessing Mössbauer parameters very close to those of hematite [32]. For diamagnetically substituted ferrites, the existence of a central doublet superimposed on well-resolved magnetic sextets was reported earlier [14,33]. The first majority component, corresponding to paramagnetic central doublet, can be attributed to magnetic interactions decrease due to the non-magnetic ions increase [34,35]. The spectra have been fitted using a standard program and the Mössbauer

Table 3 Cation distribution, theoretical lattice parameter (ath), bond length (RA and RB), oxygen positional parameter (u). x 0.000 0.125 0.250 0.375 0.500 0.750 1.000

ath (Å) 8.442 8.405 8.369 8.333 8.297 8.225 8.165

RA (Å) 1.960 1.999 2.038 2.076 2.115 2.195 2.311

RB (Å) 2.033 1.997 1.961 1.925 1.890 1.816 1.727

Cation distribution 2+

3+

2+

3+

(Zn0.825 Fe0.175 ) [Zn0.175 Fe1.825 ] (Zn0.802+Al0.023+ Fe0.183+) [Zn0.202+Fe1.573+Al0.1053+Cr0.1253+] (Zn0.7752+Al0.043+Fe0.1853+) [Zn0.2252+Fe1.3153+Al0.213+Cr0.253+] (Zn0.752+Al0.063+Fe0.193+) [Zn0.252+Fe1.063+Al0.3153+Cr0.3753+] (Zn0.7252+Al0.083+Fe0.1953+) [Zn0.2752+Fe0.8053+Al0.423+Cr0.503+] (Zn0.702+Al0.103+ Fe0.203+) [Zn0.302+Fe0.303+Al0.653+Cr0.753+] (Zn0.882+Al0.123+) [Zn0.122+Al0.883+Cr1.03+]

u43m (Å)

u3m (Å)

0.384 0.387 0.391 0.394 0.398 0.405 0.417

0.259 0.262 0.266 0.269 0.273 0.280 0.292



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Table 5 Interionic distances (b, c, d, e, f, p, q, r, s) for ZnFe22xCrxAlxO4 system. Composition (x)

p

q

r

s

b

c

d

e

f

0.000 0.125 0.250 0.375 0.500 0.750 1.000

2.0292 1.9936 1.9585 1.9259 1.8841 1.8086 1.7112

1.9593 2.0008 2.0441 2.0915 2.1307 2.2285 2.3815

3.7518 3.8313 3.9142 4.0050 4.0800 4.2672 4.5602

3.6942 3.6969 3.7016 3.7124 3.7070 3.7212 3.7635

2.9797 2.9687 2.9592 2.9542 2.9362 2.9182 2.9097

3.4940 3.4812 3.4700 3.4642 3.4430 3.4219 3.4119

3.6494 3.6360 3.6243 3.6182 3.5961 3.5740 3.5636

5.4741 5.4540 5.4364 5.4273 5.3942 5.3611 5.3455

5.1610 5.1420 5.1255 5.1169 5.0857 5.0545 5.0398

Fig. 3. SEM micrographs of ZnFe22xCrxAlxO4: (a) x = 0, (b) x = 0.5, (c) x = 0.75, (d) x = 1.0.

Fig. 4. Mössbauer spectra of ZnFe22xCrxAlxO4 system (x = 0, 0.375, 0.50, 0.75).

parameters are presented in Table 6. In general, all the parameters show a linearly dependence on the degree substitution of Fe3+ with both Cr3+ and Al3+ in the spinel structure. Isomer shift values (d) are characteristic of the paramagnetic Fe3+ cation in octahedral coordi-

nation. It is observed that isomer shift parameters of hematite, for sample with x = 0.50, are greater when it coexists with the zinc ferrite ZnFe2O4, indicating strong electronic interactions between these phases a-Fe2O3 and ZnFe2O4. Because the electronic

A.I. Borhan et al. / Polyhedron 70 (2014) 110–118 Table 6 Mössbauer parameters of ZnFe22xCrxAlxO4 system (x = 0, 0.375, 0.50, 0.75). x (Cr–Al)

d (mm/s)

D (mm/s)

C (mm/s)

H (T)

A (%)

Attribution

0.000 0.375 0.750 0.500

0.325(1) 0.317(1) 0.329(3) 0.332(2) 0.379(1)

0.364(2) 0.444(2) 0.543(5) 0.483(2) 0.209(2)

0.309(2) 0.316(4) 0.440(1) 0.431(3) 0.434(2)

– – – – 50.9(1)

100 100 100 28(1) 72(1)

Para Fe3+ Para Fe3+ Para Fe3+ Para Fe3+ Magn Fe3+

configuration (3d5) of Fe3+ ion is spherical and symmetric, its compounds do not show the quadrupole splitting. But with increasing covalent character of the Fe–O bond or the influence of chemical vicinity, i.e., decrease of the isomer shift, the quadrupole splitting increases, which can be seen in Table 6 [31]. The isomer shift values increase slightly with increasing Cr–Al content, which means that the degree of octahedral covalent bonds of Fe–O decreases slightly. Furthermore, the quadrupole splitting parameters increase linearly with the value of x. This increase is created by the electric field gradient of neighboring ions [33]. These results consistent with those obtained by XRD, i.e., without microcrystalline phases Cr2O3 and c-Al2O3, because most of the Cr3+ and Al3+ are incorporated into the spinel lattice. Therefore, the electric field gradient around the nucleus of Fe3+ changes and lattice distortion is due to the isomorphic co-substitution of Fe3+ by Cr3+–Al3+ on the neighboring octahedral sites. Therefore, this increase in quadrupole splitting values is caused by lattice distortion, which is associated to the accommodation space of Cr3+ and Al3+ cations in the octahedral sites of spinel structure [31]. Half line-width (C) parameters increase linearly with x values what can be assigned to a different bulk electronic environment of the resonant atom (such as volume and concentration of surface) [31]. From Table 6, it can be seen the doublet area values, which confirm the XRD results. The sample with x = 0.50, indicates a segregation of hematite by 72%. The remaining samples show complete transformation of hematite, a-Fe2O3, in the paramagnetic state of iron, i.e., ZnFe2O4 or ZnFe22xCrxAlxO4, indicating that pure compounds were obtained. This explains the absence of internal magnetic field in the compounds analyzed, except the sample with x = 0.50, that has a hyperfine field of 50.9 T.

3.5. Magnetic properties Fig. 5 shows typical plots for the hysteresis loops of ZnFe22xCrxAlxO4 system, measured at room temperature, for a maximum applied field of 10 KOE. There can be observed that all compounds produce a very narrow hysteresis cycle, indicating a behavior characteristic of soft magnetic materials. Shape and width of hysteresis cycle depend on several factors such as synthesis technique, chemical composition and redistribution of cations between the tetrahedral and the octahedral sites, crystallite size and spin canting [36]. It can be seen from Fig. 5 that all investigated powders does not saturate even at the maximum applied field. The main magnetic properties of investigated samples are presented in Table 7. This behavior is because the saturation magnetization depends on particle size [37]. Furthermore, in spinel ferrite the saturation magnetization is dominated by the superexchange interactions between the tetrahedral A-sites and octahedral B-sites cations. One can see that the magnetization decreases linearly with increasing Cr– Al content. This is due to the fact that magnetization is influenced by extrinsic factors like microstructure and bulk density of ferrites [38] and intrinsic factors such as preferential occupation of the cubic lattice sites [1]. Trivalent ions Cr3+ have a strong preference to occupy octahedral positions B, while diamagnetic Al3+ ions can occupy both octahedral B-sites and tetrahedral A-sites. Cr3+ ions that

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have an ionic magnetic moment of 3lB, and diamagnetic Al3+ ions substitute proportional amounts of Fe3+ ions with a magnetic moment of 5lB, which leads to the magnetization values decrease in the two sublattices [13]. The magnetization decrease in octahedral sublattice is higher than that of tetrahedral sublattice, leading to reduction of net magnetization. Also, the magnetization decrease may be due to exchange interactions decrease with increasing Cr–Al content [39]. A surprising feature was the appearance of a hyperfine field of 50.9 T for the composition with x = 0.50 content in Cr–Al, that could also contribute to the variation in Ms and consequently magnetic moment, nB. This variation can be explained more specifically based on the values of the magnetic moment (l) (experimental and calculated) obtained from the Eqs. (7), (8) and canting angle values based on Yafet–Kittle model of triangular spin (Table 7). The values of Néel’s magnetic moment were calculated considering that the ionic magnetic moments for Fe3+, Cr3+, Al3+ and Zn2+ are 5lB, 3lB, 0lB and 0lB, respectively [29,35]:

gB ðxÞ ¼ MB ðxÞ  MA ðxÞ

ð9Þ

gB ¼ MW  Ms =5585

ð10Þ

where MB(x) and MA(x) are the magnetic moments for the B and A sites, Ms is the saturation magnetization for each sample, MW is the molar weight of each compound and 5585 number is magnetic factor. The Yafet–Kittle angles (aY–K) are calculated by the relation (9) [40]:

cos a ¼

gB þ 5ð1  xÞ 6þx

ð11Þ

where x represents the Cr–Al content. It can be noticed that the magnetic moment decreases with increase in Cr–Al content in zinc ferrite and the Néel magnetic moment is greater than the experimental magnetic moment. This difference can be due to the effect of canted or triangular lattice arrangement of magnetic moments [30]. According to the Yafet–Kittle model, the octahedral lattice of spinel is formed by two B0 and B’’ – sublattice with antiparallel spins to each of them. The canting angle (aY–K) appears between these two sublattices, when in octahedral sites were introduced trivalent cations such as Cr3+ and Al3+ [35]. Canting angle values increase with increasing Cr–Al content, leading to a decrease of octahedral site interactions, which is confirmed by the cation distribution, proposed using a mathematical model. Table 4 shows that the relative bond angles h1, h2, h5 decrease, while the angle values h3, h4 increase with chromium-aluminum content. The observed decrease of h1, h2, h5 suggests that A–O–A and A–O–B effective magnetic interactions weaken, while the increase of h3 and h4 indicates that B–O–B exchange interaction increases [27]. The magnetic interactions increase depends on the bonds length and the relative bond angles. Interactions strength is inversely proportional to the bonds length but proportionaly to the canting angles. The coercive field (Hc) shows a nonlinearly variation with increasing Cr–Al content, which is due to effective anisotropy increased caused by surface spin disorder [41]. The lowest values of remanent magnetization Mr can be due to the low magnetization and nanosize crystallites. The calculated values of Mr/Ms loop squareness ratio are less than 0.50. These values indicate that the particles can interact by magnetostatic interactions [42] and the materials can be used to manufacture cores and coils with low inductance [43]. 3.6. Dielectric study Variation of real (e0 ) and imaginary (e00 ) parts of dielectric permittivity in the frequency range 100 Hz–1 MHz for ZnFe22xCrxAlx-

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Fig. 5. M–H hysteresis loop of the ZnFe22xCrxAlxO4 powders.

Table 7 Magnetic parameters measured at room temperature and calculated for ZnFe22xCrxAlxO4 powders. Composition (x)

Ms (emu/g)

Mr (emu/g)

Mr/Ms

Hc (Oe)

lexp (Bohr magneton)

lcalc (Bohr magneton)

Canting angle (°)

0.000 0.125 0.250 0.375 0.500 0.750 1.000

3.10 3.00 2.75 2.75 2.50 2.50 1.90

0.50 0.60 0.60 0.60 0.50 0.50 0.50

0.16 0.20 0.21 0.21 0.20 0.20 0.26

50 90 60 50 110 50 50

1.33 1.27 1.14 1.12 1.00 0.97 0.70

8.250 7.325 6.400 5.475 4.545 2.750 3.000

– 22.83 38.51 48.25 57.42 70.80 84.25

O4 system is shown in Fig. 6(a–b). The dielectric constant dependence of the frequency for all samples has been studied in vacuum. Variation of dielectric constant with frequency indicates a normal dielectric dispersion due to Maxwell–Wagner type interfacial polarization in accordance with Koop’s phenomenological theory [44,45]. According to the Maxwell–Wagner model, the polarization decreases with increasing frequency and then reached constant

values, because the electronic exchange Fe2+ M Fe3+ cannot follow the alternating field due to the predominance of species such as Fe2+ ions, oxygen vacations, lattice defects, goals, etc. [35]. It is noted that the values of dielectric constant decrease with increasing frequency and then reached constant values at high frequencies. Maximum dielectric constant values are obtained for the two ends of the series, i.e., ZnCrAlO4 and ZnFe2O4, respectively. In

Fig. 6. Variation of real (a) and complex (b) dielectric constant with frequency.

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Fig. 7. Variation of loss tangent, tan d, with frequency.

the range 0.125 6 x 6 0.75, the dielectric constant increases linearly with increasing chromium and aluminum content. This behavior can be explained on the basis of available free charges for the conduction mechanism, which is due to the charge transfer of electrons between cations on B sites of different valencies [46]. In this work, Fe3+ ions at octahedral site are substituted by Cr3+ and Al3+ ions. It is known that Cr3+ ions have strong B-site preferences. Chromium ions do not participate in the conduction process but decrease the number of electron hopping (Fe3+ M Fe2+) between adjacent B sublattices [20]. Al3+ ions are well-known to have B-site preferences, but can also occupy A-site. When the Al3+ ions are introduced in the system, it occupies the octahedral sites B in spinel lattice, decreasing the number of Fe ions, which produce the polarization in ferrites [47]. However at low frequencies is observed the dielectric constant increase in the range 0.125 6 x 6 0.75, which means that there are a large number of polarizable Fe2+ ions in the octahedral sites, so polarization increases. Fig. 7 shows the variation of dielectric loss tangent with frequency, measured in vacuum. One can see that all materials investigated show the normal dielectric behavior. Dielectric loss tangent decreases as the frequency of the alternating field increases. Maximum dispersions in loss tangent for all the samples are obtained at low frequencies and decrease linearly with frequency increasing. The decrease of tan d with increasing frequency can be explained on basis of Koop’s phenomenological theory [47]. All materials show the low dielectric constant for all frequencies in the range 100 Hz–1 MHz and very low loss factors. For frequencies above 1 MHz, the dielectric loss values are very low, ranging between 0.03 and 0.08. Dielectric constant and dielectric loss tangent depend upon combined factors such as the stoichiometry, porosity, structural homogeneity and Fe2+ content. Indeed, the introduction of Cr3+ and Al3+ in zinc ferrite caused a number of changes in the spinel lattice (canted angle increases) which have helped improve the dielectric properties at higher frequencies. Domination of any factors varies with samples composition and sintering temperature [19,47]. These features are very important for the possible manufacture of electronic devices that could be operated successfully in broad frequency range [48].

4. Conclusions ZnFe22xCrxAlxO4 spinel ferrites (0 6 x 6 1) have been successfully synthesized, for the first time by sol-gel auto-combustion

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method, using tartaric acid as fuel agent. XRD patterns revealed spinel cubic structure for all powders and the lattice constant linearly decreases with increasing Cr–Al content. SEM micrographs show that the particle size decreases significantly with Cr–Al content increase since interionic distances between cations B–B decrease. The Mössbauer spectra illustrated complete transformation of hematite a-Fe2O3 in the paramagnetic state of Fe3+, i.e., ZnFe2O4 or ZnFe22xCrxAlxO4, indicating that pure compounds were obtained. The sample with x = 0.50, indicates a segregation of hematite by 72%. The relationship between the structure features and the electric and magnetic properties have been established. The magnetization and coercive field were found to decrease linearly with increasing Cr–Al content, which has been explained by effective magnetic interaction (A–O–A, A–O–B and B–O–B). Introduction of Cr3+ and Al3+ in zinc ferrite caused a number of changes in the spinel lattice (canted angle increases) which have helped improve the dielectric properties at higher frequencies. For high frequencies above 1 MHz, the dielectric loss values are very low, ranging between 0.03 and 0.08. Variation of dielectric constant with frequency indicates a normal dielectric dispersion due to Maxwell–Wagner type interfacial polarization. These features are very important for many electronic devices that could be operated successfully in broad frequency range. Acknowledgments This work was supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectorial Operational Programme for Human Resources Development 2007–2013 [grant POSDRU/107/1.5/S/78342]. We thank Dr. Moulay Tahar Sougrati (Institut Charles Gerhardt Montpellier) for the Mössbauer measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

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