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Studies on structural and electrical properties of Li0.5−0.5xCoxFe2.5−0.5xO4 (0≤x≤0.6) spinel ferrite
Physica B 474 (2015) 47–52
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Physica B journal homepage: www.elsevier.com/locate/physb
Studies on structural and electrical properties of Li0.5 0.5xCoxFe2.5 0.5xO4 (0 rx r0.6) spinel ferrite V.S. Sawant, A.A. Bagade, K.Y. Rajpure n Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 5 June 2015 Accepted 6 June 2015 Available online 15 June 2015
In the present work, nanocrystalline Li0.5 0.5xCoxFe2.5 0.5xO4 (for x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ferrite systems were synthesized by solution combustion method. The Rietveld analysis of X-ray result confirms the formation of a single phase spinel cubic crystal structure of the ferrite sample. The lattice constant of the material increases from 8.33 Å to 8.36 Å with increasing cobalt content in lithium ferrite. The cation distribution study reveals that the Co–Li ferrite is in the mixed spinel structure of the composition. The DC electrical resistivity result confirms the semiconducting nature and the Curie temperature decreases with increase in Co2 þ content. The dielectric constant, loss tangent and dielectric loss decrease with frequency and remain constant at higher frequencies are observed, showing the usual dielectric dispersion due to space charge polarization. The impedance spectroscopy analysis of samples reveals the grain interior contribution in the conduction process. The AC conductivity as a function of frequency verifies that the small polarons are responsible for conduction process. & 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium ferrite Rietveld analysis Cation distribution DC resistivity Dielectric properties AC conductivity
1. Introduction The polycrystalline lithium ferrites have very good electric and dielectric properties which are associated with various factors such as the sample preparation methods, substitution of ions, sintering temperature, amount of substitution etc. [1–3]. Cobalt substituted lithium ferrites have been found to be very good substitutes for microwave device application due to their low costs, and low eddy current losses. Ferrites are also appropriate in many device applications due to the low electrical conductivity as compared to that of magnetic materials [4]. For this reason the electrical performance is one of the important parameters of the ferrites which give precise information about the conduction mechanism of the material. The electrical as well as dielectric properties of Li ferrites can be modified by substituting them with different metal ions for device applications [5]. The addition of small concentrations of Co2 þ ions in lithium ferrites have been found to be relatively useful. The addition of Co2 þ ions in lithium ferrites is very effective as it opposes the microwave power loss in a device. Gupta et al. [6] has synthesized lithium ferrite by citrate method; they got the lower values of permittivity and permeability in the microwave frequency region. We have reported on the synthesis of high purity, homogeneous and polycrystalline Cobalt substituted Li-ferrite by autocombustion method and n
Corresponding author. Fax: þ 91 231 2691533. E-mail address: [email protected] (K.Y. Rajpure).
http://dx.doi.org/10.1016/j.physb.2015.06.005 0921-4526/& 2015 Elsevier B.V. All rights reserved.
studied there IR absorption spectral analysis as well as physicochemical properties. The cation distribution of Li0.5 0.5x CoxFe2.5 0.5xO4 ferrites system was reported earlier [7, 8]. Several studies have been reported on effect of additions of cobalt ions in lithium ferrites prepared by solution combustion method. However, only few reports in the literature are available on dielectric performance of cobalt substituted lithium ferrite [1,9]. In order to achieve a high degree of molecular mixing, chemical homogeneity and control of stoichiometry, we have used solution auto combustion method for the synthesis of cobalt substituted lithium ferrites samples. We have carried out Reitveld analysis of XRD results of cobalt substituted lithium ferrite samples. The aim of Rietveld analysis is (i) to characterize the samples in terms of microstructural parameters such as unit cell volume, lattice constant and oxygen position parameters etc., and (ii) to estimate the cations distribution among tetrahedral-A and octahedral-B sites in the spinel lattice. In the present study the electric and dielectric properties of cobalt substituted lithium ferrites are studied as a function of composition and frequency. The obtained results have been compared with those prepared by standard ceramic and precursor methods.
2. Experimental Samples of Li0.5 0.5xCoxFe2.5 0.5xO4 system (for x¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were prepared by the solution combustion
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V.S. Sawant et al. / Physica B 474 (2015) 47–52
synthesis method. In this method calculation of stoichiometry is crucial. It is calculated on the basis of total oxidizing and reducing valencies of oxidizer (O) and fuel (F), which serves as a numerical coefficient so that the equivalence ratio, i.e. Φe (O/F), becomes unity and the heat released is at its maximum. A mixture of solution prepared with AR Grade LiNO3 3H2O, Co(NO3)2 6H2O and Fe(NO3)3 9H2O in an appropriate proportion was taken in a beaker. The solution was stirred slowly on magnetic stirrer to obtain a homogeneous mixture and then stoichiometric solution of glycine was added into it to form the redox mixture for the combustion process. The resultant solution was then dehydrated slowly onto a hot plate with continuous stirring until the viscous gel was formed. On further heating, the temperature of the gel increased
and at a certain temperature auto-ignition of the black and fluffy gel took place with the evolution of gases. The resulting powder of all compositions was grinded for 30 min and sintered at the 800 °C for 5 h with a heating rate 2°/min in a programmable furnace. The calcined powder then uniaxially pressed into the pellets by using 2% PVA binder and by applying pressure of about 7 ton for 10 min. These pellets were finally sintered at 1100 °C for 11 h to reduce porosity and increase density. The samples sintered at 1100 °C have further used for studying structural and electrical properties. The structural properties were studied by a Bruker D2-Phaser X-ray powder diffractometer using Cu-Kα radiation (λ ¼1.5406 Å). X-ray diffraction (XRD) patterns were analyzed with the help of FullProf program by employing Rietveld refinement technique. The
Fig. 1. Rietveld refined X-ray diffraction patterns of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
V.S. Sawant et al. / Physica B 474 (2015) 47–52
49
spinel structure irrespective of the composition. As the composition is increased, there is a redistribution of cation within the octahedral site. This change of cation distribution is consistent with the change of the lattice constant.
XRD patterns for all the samples were refined using the F d 3 m space group. The quality of fitting experimental data was assessed by calculating the parameters such as the ‘goodness of fit’ χ2 and the R factors. The DC electrical resistivity of Li0.5 0.5x CoxFe2.5 0.5xO4 system was measured by using the two probe method as a function of temperature with a high sensitive Keithley electrometer model-6514. The frequency dependent dielectric measurements of Li0.5 0.5xCoxFe2.5 0.5xO4 system was carried out using a high precision LCR meter bridge (HP-6284A) in the frequency range 20 Hz to 1 MHz. The AC conductivity of the samples was calculated from the dielectric measurement data.
3.2. DC electrical resistivity The DC electrical resistivity of the materials is a significant property related to the materials used in the fabrication of electronic devices. Lithium ferrite is considered to be a high resistive spinel ferrite material [10]. The resistivity could be enhanced further by doping with trivalent or divalent ion and the preference of the occupying ion to fill the A-sites or B-sites. The temperature dependence DC electrical resistivity of Li0.5 0.5xCoxFe2.5 0.5xO4 system is shown in Fig. 2. It follows the exponential relationship i.e. the Arrhenius plot. The inset of Fig. 2 shows the room temperature variation of resistivity with Co content. For all samples, linear decrease in resistivity with increase in temperature shows the semiconducting nature of the samples. The decrease in resistivity with increase in temperature is due to increase in the thermally activated drift mobility of charge carriers, according to the charge hopping conduction mechanism [11]. The activation energy of all the samples has been calculated by using the relation [12],
3. Results and discussion 3.1. Structural analysis The XRD patterns along with Rietveld refined data for the Li0.5 0.5xCoxFe2.5 0.5xO4 system have been shown in Fig. 1. The allowed Braggs positions for the F d 3 m (No.: 227) space group are marked as vertical lines. The existence of the (220), (311), (400), (422), (333) and (440) major lattice planes in the Rietveld refined XRD patterns confirms the formation of single phase mixed cubic spinel structure preserving the F d 3 m space group. Peaks shifting towards the lower angle (2θ) with Co substitution (from 35.696 for x ¼0.1 to 35.578° for x ¼0.6) have been observed. In the refinement, the oxygen positions (x ¼y¼ z) have been taken as free parameters. However, all other atomic fractional positions have been taken as fixed. Other parameters such as lattice constants, isothermal parameters, occupancies, scale factors, and shape parameters have been taken as free parameters. The background was corrected by pseudo-Voigt function. Typical atomic coordinates (x, y, z) and occupancies of different atoms for Li0.5 0.5xCoxFe2.5 0.5xO4 samples are given in Table 1. A low value of χ2 (goodness of fit) have been observed ( 1–2) which supports the goodness of refinement. The various factors for Rietveld profile fitting are specified in Table 2. It is observed that the lattice constant increased with Co2 þ substitution obeying the Vegard's law. The linear dependence of the lattice constant with Co2 þ content is attributed to ionic volume differences of Co2 þ (0.82 Å), Fe3 þ (0.67 Å) and Li þ (0.69 Å). The cation distribution was determined using the Rietveld refinement of the occupancy values. The site occupancy analysis for typical Li0.25Co0.5Fe2.25O4 (x ¼0.5) sample reveals that 5.4% tetrahedral sites have been occupied by the Li ions and the remaining 94.6% tetrahedral sites by Fe ions; 9.8% of the octahedral sites have been occupied by Li, 25% of the octahedral sites by Co ions and the remaining 65.2% octahedral sites by Fe ions. Hence, the chemical formula for this sample is (Fe0.946Li0.054)[Fe1.304Co0.5Li0.196]O4, where cations in round and square bracket occupy the tetrahedral and octahedral sites, respectively. The estimated cation distributions are listed in Table 3. Similar kind of cation distribution is reported in our earlier paper [7]. It is observed that Li2 þ ions are present on both tetrahedral and octahedral sites, which reveals that the samples are in mixed
⎛ ΔE ⎞ ⎟ ρ = ρ0 exp⎜ ⎝ kT ⎠
(1)
where, ΔE is the activation energy, k the Boltzmann constant and ρ0 the resistivity at room temperature. The resistivity graph shows the two regions of resistivity. At lower temperature the first region is observed due to the ordered state of ferromagnetic nature and at higher temperature the second region is observed which is due to electron hopping [13] or disordered paramagnetic nature. The activation energy was estimated in two temperature regions, i.e., ferromagnetic and paramagnetic region. The values of the activation energy for both ferromagnetic and paramagnetic regions has been summarize in Table 4. It is noted from the table that the activation energies of ferromagnetic region are higher than paramagnetic region due to disordered states of the ferromagnetic region and the ordered states of the paramagnetic region [14]. The Curie temperature (Tc) of Li0.5 0.5xCoxFe2.5 0.5xO4 system varies from 567 to 594 °C as shown in Table 4. The observed Curie temperature of lithium ferrite is higher than that reported for the ceramic method [15]. In ferrite the values of Tc depend on the presence of a number and bond strength of the tetrahedral (A)octahedral (B) site. In lithium ferrite the Li2 þ and Fe3 þ ions are replaced by Co2 þ ions therefore some of the Fe ions are migrated in A to B site, hence decrease in the strength of the A–B interaction without change in tetrahedral A-site Fe ions. Therefore the values of Curie temperature decrease with increase in cobalt content in lithium ferrites.
Table 1 Typical atomic coordinates (x, y, z) and occupancy of different cations for Li0.5 0.5xCoxFe2.5 0.5xO4 samples. Atoms
Fe1 Li1 Fe2 Li2 Co1
Coordinate
Occupancy
x
y
z
0
0.1
0.2
0.3
0.4
0.5
0.6
0.125 0.125 0.5 0.5 0.5
0.125 0.125 0.5 0.5 0.5
0.125 0.125 0.5 0.5 0.5
0.972 0.078 1.578 0.422 –
0.941 0.059 1.509 0.391 0.100
0.937 0.063 1.463 0.337 0.200
0.725 0.275 1.625 0.075 0.300
0.795 0.205 1.505 0.095 0.400
0.946 0.054 1.304 0.196 0.500
0.920 0.080 1.280 0.120 0.600
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Table 2 Data on Rietveld agreement factors, lattice constant, unit cell volume, density and oxygen position parameters of Li0.5 0.5xCoxFe2.5 0.5xO4 samples. Composition, x
Reitvield factors
x2 RB (%) RF (%) a (Å) V (Å3) ρ (g/cm3) Oxygen position (x ¼y¼ z)
0
0.1
0.2
0.3
0.4
0.5
0.6
1.04 3.09 5.58 8.3339 578.8 4.719 0.2548
1.13 3.19 4.24 8.3386 579.8 4.808 0.2572
1 3.02 4.83 8.3399 580.0 4.869 0.2568
1.38 3.77 3.31 8.3458 581.3 4.921 0.2680
1.19 5.57 6.2 8.3539 583.0 4.97 0.2632
1 3.87 3.75 8.3602 584.3 5.021 0.2560
1.01 2.35 3.96 8.3615 584.6 5.081 0.2567
Table 3 Estimated cation distribution Li0.5 0.5xCoxFe2.5 0.5xO4 ferrites.
Table 4 Curie temperature and activation energy of Li0.5 0.5xCoxFe2.5 0.5xO4 ferrites.
of
Composition ‘x’ Curie temperature (°C)
Co2 þ content, x Cation distribution 0.0 0.1 0.2 0.3 0.4 0.5 0.6
+ 3+ ⎡Li1 + Fe3 + ⎤ Fe0.922 Li10.078 ⎣ 0.422 1.578⎦
( (Fe (Fe (Fe (Fe (Fe (Fe
3+ 1+ 0.941Li0.059
) )⎡⎣Li )⎡⎣Li )⎡⎣Li )⎡⎣Li )⎡⎣Li
1+ 2+ 3+ ⎤ 0.391Co0.1Fe1.509⎦
3+ 1+ 0.937Li0.063
1+ 2+ 3+ ⎤ 0.337Co0.2Fe1.463⎦
3+ 1+ 0.725Li0.275
1+ 2+ 3+ ⎤ 0.075Co0.3Fe1.625⎦
3+ 1+ 0.795Li0.205
1+ 2+ 3+ ⎤ 0.095Co0.4 Fe1.505⎦
3+ 1+ 0.946Li0.054
1+ 2+ 3+ ⎤ 0.196Co0.5Fe1.304⎦
0.0 0.1 0.2 0.3 0.4 0.5 0.6
⎡Li1 + Co2 + Fe3 + ⎤ ⎣ 0.12 0.6 1.28⎦
)
3+ 1+ 0.92Li0.08
594 591 590 585 579 575 567
Activation energy (eV) Ferromagnetic region
Paramagnetic region
0.68 0.77 0.82 0.69 0.85 0.59 0.62
0.63 0.76 0.79 0.81 0.69 0.88 0.58
permittivity of free space (8.854 10 12 F/m). The room temperature variation of dielectric constant with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 system is shown in Fig. 3. The dielectric constant decreases quickly at lower frequency region and it remains constant at higher frequency region indicating the usual dielectric dispersion. This is also attributed to Maxwell–Wagner type [16,17] interfacial polarization in agreement with Koop's theory [18]. The decrease in dielectric constant at lower frequency side due to the certain frequency electric charge cannot follow the changes of applied electric field. The decrease in polarization of ferrite material is similar to the conduction process mechanism. Due to the presence of Fe3 þ and Fe2 þ ions ferrite is referred as dipolar behavior. The dipoles create rotational displacement results in orientation of polarization. The rotation of Fe2 þ 2Fe3 þ dipoles can be visualized as the exchange of ions so that alignment of dipoles creates response to the applied alternating electric field [19]. The decrease in polarization at lower frequency region is due x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
Fig. 2. Variation of DC electrical resistivity with temperature of Li0.5 0.5xCoxFe2.5 0.5xO4 samples, inset shows the room temperature variation of resistivity with Co content.
3.3. Dielectric performance The dielectric properties such as dielectric constant (ε′) and dissipation factor (tan δ) of the samples were measured in the frequency range 20 Hz to 1 MHz using a precision LCR meter (HP 4284 A). The dielectric constant (ε′) was calculated using the relation,
ε′ =
1400 1200 1000 800 600 400 10
Cpt ε0A
Dielectric constant( ')
1600
(2)
where, Cp is the capacitance of the pellet, t the thickness of the pellet, A the area of cross section of the pellet and ε0 is the
100
1000
10000
100000
1000000
Frequency (Hz) Fig. 3. Room temperature variation of dielectric constant (ε′) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
V.S. Sawant et al. / Physica B 474 (2015) 47–52
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
6 5
Tan
4 3 2
35 30 25
Z" (M )
7
51
20 15
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
10
1
5
0 10
100
1000
10000
100000
1000000
Frequency (Hz) Fig. 4. Variations in loss tangent (tan δ) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
to electron hopping between Fe3 þ and Fe2 þ ions in the ferrite materials [20]. In Li–Co ferrite, the Co2 þ ions present in octahedral B-site are responsible for reductions in concentration of Fe3 þ ions in octahedral B-site which reduces the movement of Fe2 þ to Fe3 þ . Therefore at lower frequency region the dielectric constant decreases and it seems to be constant value at higher frequency region. This kind of decrease in dielectric constant with increasing frequency of lithium ferrite was observed by Reddy et al. [21]. Fig. 4 shows the room temperature variation of loss tangent (tan δ) with frequency and possesses similar nature of dielectric constant. From this graph it is seen that at lower frequencies region the values of loss tangent higher is and then slowly decreases for higher frequencies. The dielectric loss tangent (tan δ) of the ferrite material is observed due to domain wall resonance. At higher frequency region tan δ is found to be lower due to domain wall motion is reversed by rotational motion of dipoles [22]. In ferrite material the electrical conductivity and dielectric dispersion are possible due to the exchange mechanism between charge carriers at A-site and B-site ions situated at spinel lattice [23]. The dielectric constant is roughly inversely proportional to the square root of resistivity and same behavior is observed in the present study. In order to know the conduction mechanism of ferrite, the room temperature complex impedance spectra of Li0.5 0.5xCoxFe2.5 0.5xO4 system are shown in Fig. 5. The plots indicates real (Z′) and imaginary (Z″) part of impedance. The real (Z′) and imaginary (Z″) parts of impedance spectra indicate partial semicircles. These observed incomplete semicircles at room temperature can be attributed due to the fact that samples show high resistance at lower frequencies. The diameter of this incomplete semicircle is varying with increase in Co content in lithium ferrite due to the increase in the grain interior resistance of the material. The Dielectric loss has been calculated using relation, ε″ ¼ ε′ tan δ where, tan δ is the dielectric loss tangent which is a function of energy loss between applied electric field and sample. Fig. 6 shows the variation of dielectric loss (ε″) with frequency of Li–Co ferrite samples. The dielectric loss (ε″) of the material depends on the impurities, structural homogeneity, annealing temperature and deficiency in the crystal structure [24]. The dielectric loss (ε″) in ferrites is a lag in polarization with respect to the applied alternating electric field. The dielectric losses of the ferrite material are encouraged due to domain wall resonance. The dielectric losses are found to be low at higher frequency regions, it may be due to domain wall motion is retained and rotations of dipoles are changed by magnetic force.
0
0
5
10
15
20
25
30
35
Z' (M ) Fig. 5. Variation of room Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
temperature
impedance
spectra
of
Fig. 6. Variation in dielectric loss (ε″) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
3.4. AC conductivity (sac) The AC conductivity (sac) was calculated using the relation [25]
σac = ωε‵εo tan δ
(3)
Where, ω ¼2πf is the angular frequency and tan δ the dielectric loss tangent or dissipation factor. The variation in AC conductivity with respect to angular frequency (ω) of Li0.5 0.5x CoxFe2.5 0.5xO4 system is shown in Fig. 7. From this graph it is seen that, the AC conductivity increases linearly with increase in frequency (ω) which indicate that the conduction mechanism is due to small interaction of electrons and atoms usually known as Polarons. There are two types of polarons i.e. small and large polarons. In case of small polarons, the conductivity increases linearly with increase in angular frequency and in case of large polarons conductivity decreases with an increase in frequency [26]. From this plot it is seen that small polarons are responsible for increasing conductivity [27]. The increase in AC conductivity with frequency can be explained on the basis of the pumping strength of the applied electric field. This helps to the shift charges between the Fe3 þ and Fe2 þ ion and liberates the charges from
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V.S. Sawant et al. / Physica B 474 (2015) 47–52
-2
Acknowledgments
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
-3
log
ac
-4
Authors are very much thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support through its Project no. 03(1284/13/EMR-II). Also the authors are very much thankful to the Department of Science and Technology (DST-SERB), New Delhi, India for the financial support through its Project no. SB/S2/CMP-041/2013.
-5 References
-6
-7 4
6
8
10
12
14
log Fig. 7. Variation in AC conductivity of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
various trapping centers [28]. Hankare et al. [29] showed that in ferrite AC conductivity of the material increases with frequency in case of small polarons. In the present case, variation of AC conductivity with frequency may also be attributed due to conduction by small polarons.
4. Conclusions The pure and polycrystalline cobalt substituted lithium ferrite samples have been successfully prepared by the solution combustion method. The Rietveld refinement of X-ray results confirms pure single phase inverse spinel cubic crystal structure due to the random cations distribution among the two sub-lattice sites inside the spinel structure and the lattice constant increases with increase in cobalt content in lithium ferrite. The dielectric constant (ε′) loss tangent (tan δ) and dielectric loss (ε″) decreases with increase in frequency and it remains constant at higher frequency region. The dielectric measurements for all the ferrite samples show a usual dielectric dispersion due to space charge polarization. The impedance spectroscopy analysis of lithium ferrite suggests grain interior contribution in the conduction process. The AC conductivity study confirms the conduction mechanism is due to the small polarons.
[1] M. Srivastava, J. Singh, R.K. Mishra, M.K. Singh, A.K. Ojha, M. Yashpal, S. Sudhanshu, Curr. Appl. Phys. 14 (2014) 980–990. [2] A.K. Singh, A. Verma, O.P. Thakur, Chandra Prakash, T.C. Goel, R.G. Mendiratta, Mater. Lett. 57 (2003) 1040–1044. [3] S.C. Watawe, B.D. Sarwade, S.S. Bellad, B.D. Sutar, B.K. Chougule, J. Magn. Magn. Mater. 214 (2000) 55–60. [4] S. Kumar, R. Kumar, Alimuddin, P. Thakur, K.H. Chae, B. Angadi, W.K. Choi, J. Phys.: Condens. Matter 19 (2007) 476210. [5] J.M. Song, J.G. Koh, J. Magn. Magn. Mater. 152 (1996) 383–386. [6] N. Gupta, M.C. Dimri, S.C. Kashyap, D.C. Dube, Ceram. Int. 31 (2005) 171–176. [7] V.S. Sawant, K.Y. Rajpure, J. Magn. Magn. Mater. 382 (2015) 152–157. [8] V.S. Sawant, A.A. Bagade, S.V. Mohite, K.Y. Rajpure, Physica B 451 (2014) 39–42. [9] I. Soibam, S. Phanjoubam, H.B. Sharma, H.N.K. Sarma, R. Laishram, C. Prakash, Solid State Commun. 148 (2008) 399–402. [10] M. Maisnam, S. Phanjoubam, H.N.K. Sarma, C. Prakash, L. Radhapiyari Devi, O. P. Thakur, Physica B 370 (2005) 1–5. [11] D.R. Patil, B.K. Chougule, J. Alloy. Compd. 470 (2009) 531–535. [12] A.A. Bagade, V.V. Ganbavle, K.Y. Rajpure, J. Mater. Eng. Perform. 23 (2014) 2787–2794. [13] V.S. Sawant, S.S. Shinde, R.J. Deokate, C.H. Bhosale, B.K. Chougule, K.Y. Rajpure, Appl. Surf. Sci. 255 (2009) 6675–6678. [14] M. El-Shabasy, J. Magn. Magn. Mater. 172 (1997) 188–192. [15] S.C. Watawe, B.D. Sarwade, S.S. Bellad, B.D. Sutar, B.K. Chaugule, Mater. Chem. Phys. 65 (2000) 173–177. [16] J.C. Maxwell, Electricity and Magnetism, vol. 1, Oxford University Press, New York (1973), p. 88. [17] K.W. Wagner, Am. Phys. 40 (1973) 817. [18] C.G. Koops, Phys. Rev. 83 (1951) 121. [19] A.A. Bagade, K.Y. Rajpure, Ceram. Int. 41 (2015) 7394–7401. [20] A.A. Kadam, S.S. Shinde, S.P. Yadav, P.S. Patil, K.Y. Rajpure, J. Magn. Magn. Mater. 329 (2013) 59–64. [21] P.V.B. Reddy, B. Ramesh, Ch. Gopal Reddy, Physica B 405 (2010) 1852–1856. [22] F. Muthafar, A. Hilli, L. Sean, K.S. Kassim, J. Magn. Magn. Mater. 324 (2012) 873–879. [23] S.P. Yadav, S.S. Shinde, A.A. Kadam, K.Y. Rajpure, J. Semicond. 34 (2013) 093002. [24] S.S. Bellad, B.K. Chougle, Mater. Chem. Phys. 66 (2000) 58–63. [25] E. Melagiriyappa, H.S. Jayanna, B.K. Chougule, Mater. Chem. Phys. 112 (2008) 68–73. [26] G. Kumar, S. Sharma, R.K. Kotnala, J. Shah, S.E. Shirsath, K.M. Batoo, M. Singh, J. Mol. Struct. 1051 (2013) 336–344. [27] S.P. Yadav, S.S. Shinde, A.A. Kadam, K.Y. Rajpure, J. Alloy. Compd. 555 (2013) 330–334. [28] M.A. Dar, K.M. Batoo, V. Verma, W.A. Siddiqui, R.K. Kotnala, J. Alloy. Compd. 493 (2010) 553–560. [29] P.P. Hankare, U.B. Sankpal, R.P. Patil, A.V. Jadhav, K.M. Garadkar, B.K. Chougule, J. Magn. Magn. Mater. 323 (2011) 389–393.
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Physica B journal homepage: www.elsevier.com/locate/physb
Studies on structural and electrical properties of Li0.5 0.5xCoxFe2.5 0.5xO4 (0 rx r0.6) spinel ferrite V.S. Sawant, A.A. Bagade, K.Y. Rajpure n Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 5 June 2015 Accepted 6 June 2015 Available online 15 June 2015
In the present work, nanocrystalline Li0.5 0.5xCoxFe2.5 0.5xO4 (for x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ferrite systems were synthesized by solution combustion method. The Rietveld analysis of X-ray result confirms the formation of a single phase spinel cubic crystal structure of the ferrite sample. The lattice constant of the material increases from 8.33 Å to 8.36 Å with increasing cobalt content in lithium ferrite. The cation distribution study reveals that the Co–Li ferrite is in the mixed spinel structure of the composition. The DC electrical resistivity result confirms the semiconducting nature and the Curie temperature decreases with increase in Co2 þ content. The dielectric constant, loss tangent and dielectric loss decrease with frequency and remain constant at higher frequencies are observed, showing the usual dielectric dispersion due to space charge polarization. The impedance spectroscopy analysis of samples reveals the grain interior contribution in the conduction process. The AC conductivity as a function of frequency verifies that the small polarons are responsible for conduction process. & 2015 Elsevier B.V. All rights reserved.
Keywords: Lithium ferrite Rietveld analysis Cation distribution DC resistivity Dielectric properties AC conductivity
1. Introduction The polycrystalline lithium ferrites have very good electric and dielectric properties which are associated with various factors such as the sample preparation methods, substitution of ions, sintering temperature, amount of substitution etc. [1–3]. Cobalt substituted lithium ferrites have been found to be very good substitutes for microwave device application due to their low costs, and low eddy current losses. Ferrites are also appropriate in many device applications due to the low electrical conductivity as compared to that of magnetic materials [4]. For this reason the electrical performance is one of the important parameters of the ferrites which give precise information about the conduction mechanism of the material. The electrical as well as dielectric properties of Li ferrites can be modified by substituting them with different metal ions for device applications [5]. The addition of small concentrations of Co2 þ ions in lithium ferrites have been found to be relatively useful. The addition of Co2 þ ions in lithium ferrites is very effective as it opposes the microwave power loss in a device. Gupta et al. [6] has synthesized lithium ferrite by citrate method; they got the lower values of permittivity and permeability in the microwave frequency region. We have reported on the synthesis of high purity, homogeneous and polycrystalline Cobalt substituted Li-ferrite by autocombustion method and n
Corresponding author. Fax: þ 91 231 2691533. E-mail address: [email protected] (K.Y. Rajpure).
http://dx.doi.org/10.1016/j.physb.2015.06.005 0921-4526/& 2015 Elsevier B.V. All rights reserved.
studied there IR absorption spectral analysis as well as physicochemical properties. The cation distribution of Li0.5 0.5x CoxFe2.5 0.5xO4 ferrites system was reported earlier [7, 8]. Several studies have been reported on effect of additions of cobalt ions in lithium ferrites prepared by solution combustion method. However, only few reports in the literature are available on dielectric performance of cobalt substituted lithium ferrite [1,9]. In order to achieve a high degree of molecular mixing, chemical homogeneity and control of stoichiometry, we have used solution auto combustion method for the synthesis of cobalt substituted lithium ferrites samples. We have carried out Reitveld analysis of XRD results of cobalt substituted lithium ferrite samples. The aim of Rietveld analysis is (i) to characterize the samples in terms of microstructural parameters such as unit cell volume, lattice constant and oxygen position parameters etc., and (ii) to estimate the cations distribution among tetrahedral-A and octahedral-B sites in the spinel lattice. In the present study the electric and dielectric properties of cobalt substituted lithium ferrites are studied as a function of composition and frequency. The obtained results have been compared with those prepared by standard ceramic and precursor methods.
2. Experimental Samples of Li0.5 0.5xCoxFe2.5 0.5xO4 system (for x¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were prepared by the solution combustion
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synthesis method. In this method calculation of stoichiometry is crucial. It is calculated on the basis of total oxidizing and reducing valencies of oxidizer (O) and fuel (F), which serves as a numerical coefficient so that the equivalence ratio, i.e. Φe (O/F), becomes unity and the heat released is at its maximum. A mixture of solution prepared with AR Grade LiNO3 3H2O, Co(NO3)2 6H2O and Fe(NO3)3 9H2O in an appropriate proportion was taken in a beaker. The solution was stirred slowly on magnetic stirrer to obtain a homogeneous mixture and then stoichiometric solution of glycine was added into it to form the redox mixture for the combustion process. The resultant solution was then dehydrated slowly onto a hot plate with continuous stirring until the viscous gel was formed. On further heating, the temperature of the gel increased
and at a certain temperature auto-ignition of the black and fluffy gel took place with the evolution of gases. The resulting powder of all compositions was grinded for 30 min and sintered at the 800 °C for 5 h with a heating rate 2°/min in a programmable furnace. The calcined powder then uniaxially pressed into the pellets by using 2% PVA binder and by applying pressure of about 7 ton for 10 min. These pellets were finally sintered at 1100 °C for 11 h to reduce porosity and increase density. The samples sintered at 1100 °C have further used for studying structural and electrical properties. The structural properties were studied by a Bruker D2-Phaser X-ray powder diffractometer using Cu-Kα radiation (λ ¼1.5406 Å). X-ray diffraction (XRD) patterns were analyzed with the help of FullProf program by employing Rietveld refinement technique. The
Fig. 1. Rietveld refined X-ray diffraction patterns of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
V.S. Sawant et al. / Physica B 474 (2015) 47–52
49
spinel structure irrespective of the composition. As the composition is increased, there is a redistribution of cation within the octahedral site. This change of cation distribution is consistent with the change of the lattice constant.
XRD patterns for all the samples were refined using the F d 3 m space group. The quality of fitting experimental data was assessed by calculating the parameters such as the ‘goodness of fit’ χ2 and the R factors. The DC electrical resistivity of Li0.5 0.5x CoxFe2.5 0.5xO4 system was measured by using the two probe method as a function of temperature with a high sensitive Keithley electrometer model-6514. The frequency dependent dielectric measurements of Li0.5 0.5xCoxFe2.5 0.5xO4 system was carried out using a high precision LCR meter bridge (HP-6284A) in the frequency range 20 Hz to 1 MHz. The AC conductivity of the samples was calculated from the dielectric measurement data.
3.2. DC electrical resistivity The DC electrical resistivity of the materials is a significant property related to the materials used in the fabrication of electronic devices. Lithium ferrite is considered to be a high resistive spinel ferrite material [10]. The resistivity could be enhanced further by doping with trivalent or divalent ion and the preference of the occupying ion to fill the A-sites or B-sites. The temperature dependence DC electrical resistivity of Li0.5 0.5xCoxFe2.5 0.5xO4 system is shown in Fig. 2. It follows the exponential relationship i.e. the Arrhenius plot. The inset of Fig. 2 shows the room temperature variation of resistivity with Co content. For all samples, linear decrease in resistivity with increase in temperature shows the semiconducting nature of the samples. The decrease in resistivity with increase in temperature is due to increase in the thermally activated drift mobility of charge carriers, according to the charge hopping conduction mechanism [11]. The activation energy of all the samples has been calculated by using the relation [12],
3. Results and discussion 3.1. Structural analysis The XRD patterns along with Rietveld refined data for the Li0.5 0.5xCoxFe2.5 0.5xO4 system have been shown in Fig. 1. The allowed Braggs positions for the F d 3 m (No.: 227) space group are marked as vertical lines. The existence of the (220), (311), (400), (422), (333) and (440) major lattice planes in the Rietveld refined XRD patterns confirms the formation of single phase mixed cubic spinel structure preserving the F d 3 m space group. Peaks shifting towards the lower angle (2θ) with Co substitution (from 35.696 for x ¼0.1 to 35.578° for x ¼0.6) have been observed. In the refinement, the oxygen positions (x ¼y¼ z) have been taken as free parameters. However, all other atomic fractional positions have been taken as fixed. Other parameters such as lattice constants, isothermal parameters, occupancies, scale factors, and shape parameters have been taken as free parameters. The background was corrected by pseudo-Voigt function. Typical atomic coordinates (x, y, z) and occupancies of different atoms for Li0.5 0.5xCoxFe2.5 0.5xO4 samples are given in Table 1. A low value of χ2 (goodness of fit) have been observed ( 1–2) which supports the goodness of refinement. The various factors for Rietveld profile fitting are specified in Table 2. It is observed that the lattice constant increased with Co2 þ substitution obeying the Vegard's law. The linear dependence of the lattice constant with Co2 þ content is attributed to ionic volume differences of Co2 þ (0.82 Å), Fe3 þ (0.67 Å) and Li þ (0.69 Å). The cation distribution was determined using the Rietveld refinement of the occupancy values. The site occupancy analysis for typical Li0.25Co0.5Fe2.25O4 (x ¼0.5) sample reveals that 5.4% tetrahedral sites have been occupied by the Li ions and the remaining 94.6% tetrahedral sites by Fe ions; 9.8% of the octahedral sites have been occupied by Li, 25% of the octahedral sites by Co ions and the remaining 65.2% octahedral sites by Fe ions. Hence, the chemical formula for this sample is (Fe0.946Li0.054)[Fe1.304Co0.5Li0.196]O4, where cations in round and square bracket occupy the tetrahedral and octahedral sites, respectively. The estimated cation distributions are listed in Table 3. Similar kind of cation distribution is reported in our earlier paper [7]. It is observed that Li2 þ ions are present on both tetrahedral and octahedral sites, which reveals that the samples are in mixed
⎛ ΔE ⎞ ⎟ ρ = ρ0 exp⎜ ⎝ kT ⎠
(1)
where, ΔE is the activation energy, k the Boltzmann constant and ρ0 the resistivity at room temperature. The resistivity graph shows the two regions of resistivity. At lower temperature the first region is observed due to the ordered state of ferromagnetic nature and at higher temperature the second region is observed which is due to electron hopping [13] or disordered paramagnetic nature. The activation energy was estimated in two temperature regions, i.e., ferromagnetic and paramagnetic region. The values of the activation energy for both ferromagnetic and paramagnetic regions has been summarize in Table 4. It is noted from the table that the activation energies of ferromagnetic region are higher than paramagnetic region due to disordered states of the ferromagnetic region and the ordered states of the paramagnetic region [14]. The Curie temperature (Tc) of Li0.5 0.5xCoxFe2.5 0.5xO4 system varies from 567 to 594 °C as shown in Table 4. The observed Curie temperature of lithium ferrite is higher than that reported for the ceramic method [15]. In ferrite the values of Tc depend on the presence of a number and bond strength of the tetrahedral (A)octahedral (B) site. In lithium ferrite the Li2 þ and Fe3 þ ions are replaced by Co2 þ ions therefore some of the Fe ions are migrated in A to B site, hence decrease in the strength of the A–B interaction without change in tetrahedral A-site Fe ions. Therefore the values of Curie temperature decrease with increase in cobalt content in lithium ferrites.
Table 1 Typical atomic coordinates (x, y, z) and occupancy of different cations for Li0.5 0.5xCoxFe2.5 0.5xO4 samples. Atoms
Fe1 Li1 Fe2 Li2 Co1
Coordinate
Occupancy
x
y
z
0
0.1
0.2
0.3
0.4
0.5
0.6
0.125 0.125 0.5 0.5 0.5
0.125 0.125 0.5 0.5 0.5
0.125 0.125 0.5 0.5 0.5
0.972 0.078 1.578 0.422 –
0.941 0.059 1.509 0.391 0.100
0.937 0.063 1.463 0.337 0.200
0.725 0.275 1.625 0.075 0.300
0.795 0.205 1.505 0.095 0.400
0.946 0.054 1.304 0.196 0.500
0.920 0.080 1.280 0.120 0.600
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Table 2 Data on Rietveld agreement factors, lattice constant, unit cell volume, density and oxygen position parameters of Li0.5 0.5xCoxFe2.5 0.5xO4 samples. Composition, x
Reitvield factors
x2 RB (%) RF (%) a (Å) V (Å3) ρ (g/cm3) Oxygen position (x ¼y¼ z)
0
0.1
0.2
0.3
0.4
0.5
0.6
1.04 3.09 5.58 8.3339 578.8 4.719 0.2548
1.13 3.19 4.24 8.3386 579.8 4.808 0.2572
1 3.02 4.83 8.3399 580.0 4.869 0.2568
1.38 3.77 3.31 8.3458 581.3 4.921 0.2680
1.19 5.57 6.2 8.3539 583.0 4.97 0.2632
1 3.87 3.75 8.3602 584.3 5.021 0.2560
1.01 2.35 3.96 8.3615 584.6 5.081 0.2567
Table 3 Estimated cation distribution Li0.5 0.5xCoxFe2.5 0.5xO4 ferrites.
Table 4 Curie temperature and activation energy of Li0.5 0.5xCoxFe2.5 0.5xO4 ferrites.
of
Composition ‘x’ Curie temperature (°C)
Co2 þ content, x Cation distribution 0.0 0.1 0.2 0.3 0.4 0.5 0.6
+ 3+ ⎡Li1 + Fe3 + ⎤ Fe0.922 Li10.078 ⎣ 0.422 1.578⎦
( (Fe (Fe (Fe (Fe (Fe (Fe
3+ 1+ 0.941Li0.059
) )⎡⎣Li )⎡⎣Li )⎡⎣Li )⎡⎣Li )⎡⎣Li
1+ 2+ 3+ ⎤ 0.391Co0.1Fe1.509⎦
3+ 1+ 0.937Li0.063
1+ 2+ 3+ ⎤ 0.337Co0.2Fe1.463⎦
3+ 1+ 0.725Li0.275
1+ 2+ 3+ ⎤ 0.075Co0.3Fe1.625⎦
3+ 1+ 0.795Li0.205
1+ 2+ 3+ ⎤ 0.095Co0.4 Fe1.505⎦
3+ 1+ 0.946Li0.054
1+ 2+ 3+ ⎤ 0.196Co0.5Fe1.304⎦
0.0 0.1 0.2 0.3 0.4 0.5 0.6
⎡Li1 + Co2 + Fe3 + ⎤ ⎣ 0.12 0.6 1.28⎦
)
3+ 1+ 0.92Li0.08
594 591 590 585 579 575 567
Activation energy (eV) Ferromagnetic region
Paramagnetic region
0.68 0.77 0.82 0.69 0.85 0.59 0.62
0.63 0.76 0.79 0.81 0.69 0.88 0.58
permittivity of free space (8.854 10 12 F/m). The room temperature variation of dielectric constant with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 system is shown in Fig. 3. The dielectric constant decreases quickly at lower frequency region and it remains constant at higher frequency region indicating the usual dielectric dispersion. This is also attributed to Maxwell–Wagner type [16,17] interfacial polarization in agreement with Koop's theory [18]. The decrease in dielectric constant at lower frequency side due to the certain frequency electric charge cannot follow the changes of applied electric field. The decrease in polarization of ferrite material is similar to the conduction process mechanism. Due to the presence of Fe3 þ and Fe2 þ ions ferrite is referred as dipolar behavior. The dipoles create rotational displacement results in orientation of polarization. The rotation of Fe2 þ 2Fe3 þ dipoles can be visualized as the exchange of ions so that alignment of dipoles creates response to the applied alternating electric field [19]. The decrease in polarization at lower frequency region is due x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
Fig. 2. Variation of DC electrical resistivity with temperature of Li0.5 0.5xCoxFe2.5 0.5xO4 samples, inset shows the room temperature variation of resistivity with Co content.
3.3. Dielectric performance The dielectric properties such as dielectric constant (ε′) and dissipation factor (tan δ) of the samples were measured in the frequency range 20 Hz to 1 MHz using a precision LCR meter (HP 4284 A). The dielectric constant (ε′) was calculated using the relation,
ε′ =
1400 1200 1000 800 600 400 10
Cpt ε0A
Dielectric constant( ')
1600
(2)
where, Cp is the capacitance of the pellet, t the thickness of the pellet, A the area of cross section of the pellet and ε0 is the
100
1000
10000
100000
1000000
Frequency (Hz) Fig. 3. Room temperature variation of dielectric constant (ε′) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
V.S. Sawant et al. / Physica B 474 (2015) 47–52
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
6 5
Tan
4 3 2
35 30 25
Z" (M )
7
51
20 15
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
10
1
5
0 10
100
1000
10000
100000
1000000
Frequency (Hz) Fig. 4. Variations in loss tangent (tan δ) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
to electron hopping between Fe3 þ and Fe2 þ ions in the ferrite materials [20]. In Li–Co ferrite, the Co2 þ ions present in octahedral B-site are responsible for reductions in concentration of Fe3 þ ions in octahedral B-site which reduces the movement of Fe2 þ to Fe3 þ . Therefore at lower frequency region the dielectric constant decreases and it seems to be constant value at higher frequency region. This kind of decrease in dielectric constant with increasing frequency of lithium ferrite was observed by Reddy et al. [21]. Fig. 4 shows the room temperature variation of loss tangent (tan δ) with frequency and possesses similar nature of dielectric constant. From this graph it is seen that at lower frequencies region the values of loss tangent higher is and then slowly decreases for higher frequencies. The dielectric loss tangent (tan δ) of the ferrite material is observed due to domain wall resonance. At higher frequency region tan δ is found to be lower due to domain wall motion is reversed by rotational motion of dipoles [22]. In ferrite material the electrical conductivity and dielectric dispersion are possible due to the exchange mechanism between charge carriers at A-site and B-site ions situated at spinel lattice [23]. The dielectric constant is roughly inversely proportional to the square root of resistivity and same behavior is observed in the present study. In order to know the conduction mechanism of ferrite, the room temperature complex impedance spectra of Li0.5 0.5xCoxFe2.5 0.5xO4 system are shown in Fig. 5. The plots indicates real (Z′) and imaginary (Z″) part of impedance. The real (Z′) and imaginary (Z″) parts of impedance spectra indicate partial semicircles. These observed incomplete semicircles at room temperature can be attributed due to the fact that samples show high resistance at lower frequencies. The diameter of this incomplete semicircle is varying with increase in Co content in lithium ferrite due to the increase in the grain interior resistance of the material. The Dielectric loss has been calculated using relation, ε″ ¼ ε′ tan δ where, tan δ is the dielectric loss tangent which is a function of energy loss between applied electric field and sample. Fig. 6 shows the variation of dielectric loss (ε″) with frequency of Li–Co ferrite samples. The dielectric loss (ε″) of the material depends on the impurities, structural homogeneity, annealing temperature and deficiency in the crystal structure [24]. The dielectric loss (ε″) in ferrites is a lag in polarization with respect to the applied alternating electric field. The dielectric losses of the ferrite material are encouraged due to domain wall resonance. The dielectric losses are found to be low at higher frequency regions, it may be due to domain wall motion is retained and rotations of dipoles are changed by magnetic force.
0
0
5
10
15
20
25
30
35
Z' (M ) Fig. 5. Variation of room Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
temperature
impedance
spectra
of
Fig. 6. Variation in dielectric loss (ε″) with frequency of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
3.4. AC conductivity (sac) The AC conductivity (sac) was calculated using the relation [25]
σac = ωε‵εo tan δ
(3)
Where, ω ¼2πf is the angular frequency and tan δ the dielectric loss tangent or dissipation factor. The variation in AC conductivity with respect to angular frequency (ω) of Li0.5 0.5x CoxFe2.5 0.5xO4 system is shown in Fig. 7. From this graph it is seen that, the AC conductivity increases linearly with increase in frequency (ω) which indicate that the conduction mechanism is due to small interaction of electrons and atoms usually known as Polarons. There are two types of polarons i.e. small and large polarons. In case of small polarons, the conductivity increases linearly with increase in angular frequency and in case of large polarons conductivity decreases with an increase in frequency [26]. From this plot it is seen that small polarons are responsible for increasing conductivity [27]. The increase in AC conductivity with frequency can be explained on the basis of the pumping strength of the applied electric field. This helps to the shift charges between the Fe3 þ and Fe2 þ ion and liberates the charges from
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V.S. Sawant et al. / Physica B 474 (2015) 47–52
-2
Acknowledgments
x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6
-3
log
ac
-4
Authors are very much thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support through its Project no. 03(1284/13/EMR-II). Also the authors are very much thankful to the Department of Science and Technology (DST-SERB), New Delhi, India for the financial support through its Project no. SB/S2/CMP-041/2013.
-5 References
-6
-7 4
6
8
10
12
14
log Fig. 7. Variation in AC conductivity of Li0.5 0.5xCoxFe2.5 0.5xO4 samples.
various trapping centers [28]. Hankare et al. [29] showed that in ferrite AC conductivity of the material increases with frequency in case of small polarons. In the present case, variation of AC conductivity with frequency may also be attributed due to conduction by small polarons.
4. Conclusions The pure and polycrystalline cobalt substituted lithium ferrite samples have been successfully prepared by the solution combustion method. The Rietveld refinement of X-ray results confirms pure single phase inverse spinel cubic crystal structure due to the random cations distribution among the two sub-lattice sites inside the spinel structure and the lattice constant increases with increase in cobalt content in lithium ferrite. The dielectric constant (ε′) loss tangent (tan δ) and dielectric loss (ε″) decreases with increase in frequency and it remains constant at higher frequency region. The dielectric measurements for all the ferrite samples show a usual dielectric dispersion due to space charge polarization. The impedance spectroscopy analysis of lithium ferrite suggests grain interior contribution in the conduction process. The AC conductivity study confirms the conduction mechanism is due to the small polarons.
[1] M. Srivastava, J. Singh, R.K. Mishra, M.K. Singh, A.K. Ojha, M. Yashpal, S. Sudhanshu, Curr. Appl. Phys. 14 (2014) 980–990. [2] A.K. Singh, A. Verma, O.P. Thakur, Chandra Prakash, T.C. Goel, R.G. Mendiratta, Mater. Lett. 57 (2003) 1040–1044. [3] S.C. Watawe, B.D. Sarwade, S.S. Bellad, B.D. Sutar, B.K. Chougule, J. Magn. Magn. Mater. 214 (2000) 55–60. [4] S. Kumar, R. Kumar, Alimuddin, P. Thakur, K.H. Chae, B. Angadi, W.K. Choi, J. Phys.: Condens. Matter 19 (2007) 476210. [5] J.M. Song, J.G. Koh, J. Magn. Magn. Mater. 152 (1996) 383–386. [6] N. Gupta, M.C. Dimri, S.C. Kashyap, D.C. Dube, Ceram. Int. 31 (2005) 171–176. [7] V.S. Sawant, K.Y. Rajpure, J. Magn. Magn. Mater. 382 (2015) 152–157. [8] V.S. Sawant, A.A. Bagade, S.V. Mohite, K.Y. Rajpure, Physica B 451 (2014) 39–42. [9] I. Soibam, S. Phanjoubam, H.B. Sharma, H.N.K. Sarma, R. Laishram, C. Prakash, Solid State Commun. 148 (2008) 399–402. [10] M. Maisnam, S. Phanjoubam, H.N.K. Sarma, C. Prakash, L. Radhapiyari Devi, O. P. Thakur, Physica B 370 (2005) 1–5. [11] D.R. Patil, B.K. Chougule, J. Alloy. Compd. 470 (2009) 531–535. [12] A.A. Bagade, V.V. Ganbavle, K.Y. Rajpure, J. Mater. Eng. Perform. 23 (2014) 2787–2794. [13] V.S. Sawant, S.S. Shinde, R.J. Deokate, C.H. Bhosale, B.K. Chougule, K.Y. Rajpure, Appl. Surf. Sci. 255 (2009) 6675–6678. [14] M. El-Shabasy, J. Magn. Magn. Mater. 172 (1997) 188–192. [15] S.C. Watawe, B.D. Sarwade, S.S. Bellad, B.D. Sutar, B.K. Chaugule, Mater. Chem. Phys. 65 (2000) 173–177. [16] J.C. Maxwell, Electricity and Magnetism, vol. 1, Oxford University Press, New York (1973), p. 88. [17] K.W. Wagner, Am. Phys. 40 (1973) 817. [18] C.G. Koops, Phys. Rev. 83 (1951) 121. [19] A.A. Bagade, K.Y. Rajpure, Ceram. Int. 41 (2015) 7394–7401. [20] A.A. Kadam, S.S. Shinde, S.P. Yadav, P.S. Patil, K.Y. Rajpure, J. Magn. Magn. Mater. 329 (2013) 59–64. [21] P.V.B. Reddy, B. Ramesh, Ch. Gopal Reddy, Physica B 405 (2010) 1852–1856. [22] F. Muthafar, A. Hilli, L. Sean, K.S. Kassim, J. Magn. Magn. Mater. 324 (2012) 873–879. [23] S.P. Yadav, S.S. Shinde, A.A. Kadam, K.Y. Rajpure, J. Semicond. 34 (2013) 093002. [24] S.S. Bellad, B.K. Chougle, Mater. Chem. Phys. 66 (2000) 58–63. [25] E. Melagiriyappa, H.S. Jayanna, B.K. Chougule, Mater. Chem. Phys. 112 (2008) 68–73. [26] G. Kumar, S. Sharma, R.K. Kotnala, J. Shah, S.E. Shirsath, K.M. Batoo, M. Singh, J. Mol. Struct. 1051 (2013) 336–344. [27] S.P. Yadav, S.S. Shinde, A.A. Kadam, K.Y. Rajpure, J. Alloy. Compd. 555 (2013) 330–334. [28] M.A. Dar, K.M. Batoo, V. Verma, W.A. Siddiqui, R.K. Kotnala, J. Alloy. Compd. 493 (2010) 553–560. [29] P.P. Hankare, U.B. Sankpal, R.P. Patil, A.V. Jadhav, K.M. Garadkar, B.K. Chougule, J. Magn. Magn. Mater. 323 (2011) 389–393.