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Effect of sintering temperature and thermoelectric power studies of the system MgFe2−xCrxO4
Solid State Sciences 11 (2009) 2075–2079
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Effect of sintering temperature and thermoelectric power studies of the system MgFe2xCrxO4 P.P. Hankare a, *, V.T. Vader a, U.B. Sankpal a, L.V. Gavali a, R. Sasikala b, I.S. Mulla c a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India Chemistry Division, Bhabha Atomic Research Centre, Mumbai-411 008, India c National Chemical Laboratory, Pune-411 008, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 April 2009 Received in revised form 26 July 2009 Accepted 1 September 2009 Available online 16 September 2009
Mixed metal oxides showing the spinel structure exhibit interesting structural and electrical properties. Substances with specific compositions in the system MgFe2xCrxO4 were synthesized by the simple coprecipitation method and have been investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM) to study the effect of temperature on the size of particles and grains. The infrared spectrum shows, two strong bands around 600 and 500 cm1. An elemental composition of one of the samples, MgFeCrO4 was found by energy dispersive X-ray spectroscopy (EDS). The thermoelectric power measurements carried out from room temperature to 500 C, show both n-type and p-type behavior. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Synthesis Temperature effect Thermoelectric studies
1. Introduction
2. Experimental
Ferrites have been used by various industries as a catalyst for some industrially important catalytic reactions. The interesting physicochemical properties of ferrospinels arise from their ability to distribute the cations among the available tetrahedral (A) and octahedral [B] sites [1–3]. Magnesium ferrite is a magnetic material having an inverse spinel structure of which the degree of inversion depends upon the heat treatment. At present, several chemical methods including the solid state reaction, co-precipitation, sol–gel, citrate method have been developed to synthesize small particle sized ferrites [4–6]. The conventional ceramic method for the preparation of ferrites has certain limitations such as long heating schedule at high temperatures. Hence, in the present investigation chromium substituted magnesium ferrites have been prepared by the simple co-precipitation technique. The main important topic in the present study concerns the substitution of Cr3þ and the heat treatment on structure and thermoemf properties. The thermoelectric power studies also give the information about the type of conductivity, weather it is n-type or p-type.
For the purpose of this investigation, various compositions of MgFe2xCrxO4 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0) were prepared by the simple co-precipitation method [7,8]. The high purity A.R. grade magnesium chloride, chromium chloride and iron chloride were weighed carefully to have the desired stoichiometric proportion. A mixed solution of the above chlorides was prepared in double distilled water. The precipitation was carried out at a controlled pH of w9.0 using a 10% NaOH solution. It was heated on a water bath (95–100 C) for 5 h. and oxidized by 30% H2O2 with constant stirring at the same temperature. The precipitate was filtered through the whatmann paper no. 41 and washed with double distilled water to remove excess alkali and Cl ions. The compounds were dried and sintered at different temperatures say 500, 600, 700, 800 and 900 C. The samples sintered at 900 C were mixed with 5 wt% polyvinyl alcohol as a binder for granulation. The granulated powders were compacted in a stainless die into pellets of 1.2 cm diameter and 0.15 cm thickness under a pressure of 10 tons. X-ray powder diffractograms were recorded on the Philips 3710 diffractometer with CuKa radiation (1.54056 Å) for phase identification, crystallite size and lattice parameter. The FT-IR spectra (Perkin Elmer-USA) were recorded in the range of 400–900 cm1 with a sample in the form of KBr pellets. The surface morphology was determined by using SEM for the samples sintered at different temperature while the compound MgFeCrO4 was used for the
* Corresponding author. Tel.: þ91 231 2609381. E-mail addresses: [email protected] (P.P. Hankare), vtv_chem@ rediffmail.com (V.T. Vader). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.09.005
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P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079 Table 1 Data on lattice constant (a), crystallite size (D), site radii (rA and rB) and bond length (RA and RB) for MgFe2xCrxO4 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0).
Fig. 1. Effect of sintering temperature on MgFeCrO4 sample for different temperature viz. (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C and (e) 900 C.
element analysis by EDS. The thermoelectric power for the samples was measured from room temperature to 773 K using a specially designed sample holder, which provides a temperature gradient of 300 K across the sample faces. 3. Results and discussion 3.1. X-ray diffraction studies X-ray diffraction patterns of MgFeCrO4 at different temperatures are presented in Fig. 1. The peaks are indexed to the characteristic planes of pure spinel cubic structure. The diffraction peaks are found to be sharp and their intensity increases with increase in the sintering temperature. No other phase has been detected and (311) reflections appear to be more intense [9–11] consistent with observations for face centered cubic spinel. Because the (311) diffraction peak at 900 C is sharper than that at lower temperatures, the temperature 900 C has been selected for the sintering of all samples, including those of different compositions. The lattice parameter ‘a’ was calculated using the relation.
1=2 a ¼ d h2 þ k2 þ l2
(1)
Composition Lattice constant Crystallite size (x) (Å) (nm)
Site radii Å rA
rB
RA
RB
0.0 0.5 1.0 1.5 2.0
0.5615 0.5514 0.5536 0.5422 0.5400
0.6996 0.6947 0.6971 0.6849 0.6825
1.9058 1.9013 1.9028 1.8915 1.8892
2.0508 2.0455 2.0479 2.0357 2.0334
8.40 8.38 8.39 8.34 8.33
22 33 32 31 29
Bond length Å
The lattice constant shows a decreasing trend with chromium concentration obeying Vegard’s law. However, for x ¼ 1.0 the lattice parameter slightly increases, because the system is not completely normal or inverse. Chromium has a strong site preference to the Bsite and hence the change in ‘a’ indicates the structural change from inverse to normal. The decreasing trend in lattice parameter is due to smaller ionic radius of Cr3þ (0.63 Å) which replaces the larger crystal radius Fe3þ (0.67 Å). Consequently, there is shrinkage in the unit cell dimension [12]. The crystallite size (D) of the powdered particles was determined from XRD peak broadening of the (311) peak using Scherrer formula,
Dhkl ¼ 0:9l=bcos q
(2)
Fig. 2 shows the variation of crystallite size as a function of Cr ion contents. The X-ray diffraction data was further used to confirm the single phase formation by calculating the tetrahedral and octahedral site radii (rA and rB) . The site radii, rA and rB were calculated using the relation.
pffiffiffi rA ¼ ðm 1=4Þa 3 Ro
(3)
rB ¼ ð5=8 mÞ Ro
(4)
where, m is oxygen parameter of the system and Ro is radius of oxygen ion. For cubic spinel, it is possible to calculate the bond length of tetrahedral and octahedral site using the following equations [13,14].
pffiffiffi RA ¼ a 3ðd þ 1=8Þ
(5)
1=2 2 RB ¼ a 3d þ 1=16 d=2
(6)
where, d ¼ deviation from oxygen parameter (m). The data on lattice constant (a), crystallite size (D), site radii (rA and rB) and the bond lengths (RA and RB) are summarized in the Table 1. The values calculated from the above equations indicate that both bond length and site radii show a decreasing trend with Cr content wherein the site radii rA is smaller than rB. Additionally one can say that the covalent character also decreases with Cr content due to the lower site radii in both tetrahedral and octahedral sites.
Table 2 Infrared spectrum showing Tetrahedral and Octahedral site for the system MgFe2xCrxO4 with different composition.
Fig. 2. Variation of crystallite size (nm) with composition (x) for the system MgFe2xCrxO4.
Composition (x)
Tetrahedral site (n1) cm1
Octahedral site (n2) cm1
0.0 0.5 1.0 1.5 2.0
583.09 599.11 614.87 629.57 642.30
440.57 454.98 481.30 503.14 516.26
P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079
3.2. FTIR-spectra In normal ferrites both absorption bands depend on the nature of octahedral cations and have no significant dependence on the nature of tetrahedral ions. From Table 2 it is seen that the IR spectra of Cr substituted magnesium ferrite are found to exhibit two bands in the range 400–600 cm1. The high frequency band n1 is observed around 600 cm1 and the lower frequency band n2 is around 500 cm1. These bands are common features of all ferrites. [15]. The absorption band n1 is attributed to the intrinsic vibration of metal oxygen bonds in tetrahedral sites while the lower absorption band n2 is due to metal oxygen vibration in octahedral sites [16]. It was found that Fe–O distance of the A-site (1.90 Å) is smaller than that of the B-site (2.05 Å). This can be interpreted by the more covalent bonding of Fe3þ ions at the A-site than that of the B-site. Because the Fe–O bonding characters in tetrahedral and octahedral complexes differ, a difference in the band positions is expected. Table 2 also shows that on increasing the Cr content in the system
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the bands shift towards higher frequencies and that they becomes sharper. In our present study, the system changes from inverse to normal and the broadening is commonly observed in the inverse spinel ferrite which has been attributed to statistical distribution of Fe3þ ions on A and B site. Hence, the bands are sharpened as one goes from MgFe2O4 to MgCr2O4. 3.3. Effect of temperature on grain size Fig 3 (a–e) shows the SEM images of the sample MgFeCrO4 sintered at different temperatures such as 500, 600, 700, 800 and 900 C. It is evident from the figure that, the number of grains and their size increases with annealing temperature. In the present samples, the grain size increases from 1.8 mm for the sample sintered at 500 C to about 4.2 mm for the sample sintered at 900 C. The calculated values of grain size from SEM image and crystallite size from XRD for the same sample sintered at different temperatures have been presented in Table 3. The crystallite size estimated
Fig. 3. Effect of sintering temperature on grain size for MgFeCrO4 sample (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C and (e) 900 C.
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P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079
Table 3 Average grain size and crystallite size for the same sample MgFeCrO4sintered at different temperature. Sample
Sintering time ( C) Average grain size (mm) Crystallite size (mm)
MgFeCrO4 500 600 700 800 900
1.8 2.6 3.3 3.9 4.2
0.009 0.016 0.021 0.028 0.032
from Scherrer’s formula agrees well with that obtained from the corresponding SEM micrographs. The grains are distributed with uniform size throughout the surface [17]. 3.4. Elemental composition Fig. 4 shows the EDS pattern obtained for the sample MgFeCrO4, which gives the elemental composition in the sample. The compound shows the presence of Mg, Fe, Cr and O without any precipitating cations. Thus cations Naþ of the precursor reaction and most of the undesired chloride ions have been completely washed out. The average atomic ratio was found to be 1:1:1 for Mg:Fe:Cr. 3.5. Thermoelectric power studies The temperature dependence of the Seebeck coefficient for the different compositions were calculated from thermoemf measurement in the temperature range from 300 to 773 K and are presented in Fig. 5. The thermoemf is negative in the beginning indicating a most probable mechanism as electron hopping between Fe2þ and Fe3þ ions. The Mg ions are in þ2 oxidation state, which do not have free electrons. Hence the process is expected to take place between two adjacent octahedral sites in the spinel lattice as
Fe2þ #Fe3þ þ e The Seebeck coefficient calculated by using a ¼ DV/DT (where, DV ¼ thermoemf measured across the sample and DT ¼ the temperature difference across the pellet) is negative up to x ¼ 1.0 hence, till this composition the majority charge carriers are electrons generated from Fe ions, which act as donor centers. From
Fig. 5. Seebeck coefficient (a) measurements for different compositions of ‘x’.
x ¼ 1.5 onwards, the n to p-type conduction is observed due to the decrease in population of Fe3þ ions in B-sites, which are responsible for the reduction in the Seebeck coefficient. The mechanism is due to the hole transfer from Cr3þ to Cr2þ through
Fe3þ þ Cr2þ #Fe2þ þ Cr3þ For x ¼ 1.5, a values change from þve to ve whereas the sample x ¼ 2.0 shows only þve values in the investigated temperature range. This indicates that the majority charge carriers are the holes [18–20]. 4. Conclusions The system MgFe2xCrxO4 is synthesized by the simple coprecipitation method at controlled pH and exhibits the cubic spinel structure. The effect of sintering temperature on the observed structure influences the size of particles and consequently the grains. The calculated site radii and bond lengths show that the octahedral site radius is larger than the tetrahedral site. The FTIR spectrum shows two strong bands n1 and n2 in the expected region and the difference in their observed band position is due to the difference in metal-oxygen bond for octahedral and tetrahedral complexes. The thermoelectric power measurement shows n to ptype transition due to hopping of electrons and transfer of holes respectively. Acknowledgements Authors PPH and VTV wish to acknowledge their sincere gratitude to UGC for Major Research Project [F. No. 32-289/2006 (SR)] New Delhi, India for their financial support. References [1] [2] [3] [4] [5] [6]
Fig. 4. Elemental composition by EDS for the sample MgFeCrO4.
[7] [8]
G. Blasse, Philips Res. 20 (1968) 528. J.B. Goodenough, Mag. Chem. Bond, John Wiley, New York, London, 1963, 120. A. Ahmed, S.G. Oak, V.S. Darshane, Bull. Chem. Soc. Jpn. 68 (1995) 3651–3657. B.A. Verma, T.C. Goel, R.G. Mendiratta, R.G. Gupta, J. Magn. Magn. Mater. 192 (1999) 271–276. P.P. Hankare, P.D. Kamble, M.R. Kadam, K.S. Rane, P.N. Vasambekar, Mater. Lett. 61 (2007) 2769–2771. Cui-Ping Liu, Ming-Wei Li, Zhong Cui, Juan-Ru Huang, Yi-Ling Tian, Tong Lin, Wen-Bo Mi, J. Mater. Sci. 42 (2007) 6133–6138. S.P. Ghorpade, V.S. Darshane, et al., Appl. Catal. A. General 166 (1998) 135–142. K. Nishamol, K.S. Kahna, S. Sugunun, J. Mol. Catal. A: Chemical 209 (2004) 89–96.
P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079 [9] S.R. Murthy, Bull. Mater. Sci. 124 (2001) 379. [10] P.P. Hankare, V.T. Vader, N.M. Patil, S.D. Jadhav, U.B. Sankpal, M.R. Kadam, B.K. Chougule, N.S. Gajbhiye, Mater. Chem. Phys. 113 (2009) 233–238. [11] P. Pramanik, A. Pathak, Bull. Mater. Sci. 17 (1994) 967–975. [12] V.B. Kawade, G.K. Bichile, K.M. Jadhav, Mater. Lett. 42 (2000) 33–37. [13] A.G. Bhosale, B.K. Chougule, Mater. Chem. Phys. 97 (2006) 273–276. [14] M.C. Chhantbar, U.N. Trivedi, P.V. Tanna, H.J. Shah, R.D. Vara, H.H. Joshi, K.B. Modi, Ind. J. Phy. 78A (3) (2004) 321–326.
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[15] R.D. Waldron, Phys. Rev. B99 (1955) 1727. [16] Mansour Al-Haj, J. Magn. Magn. Mater. 311 (2007) 517–522. [17] A.K.M. Akhter Hossain, S.T. Mahamud, M. Selki, T. Kawal, H. Tabata, J. Magn. Magn. Mater. 312 (2007) 210–219. [18] M.G. Patil, V.C. Mahajan, B.V. Bhise, A.K. Ghatage, S.D. Lotke, S.S. Patil, Bull. Mater. Sci. 17 (1994) 399–403. [19] A.G. Bhosale, B.K. Chougule, Mater. Lett. 60 (2006) 3912–3915. [20] C. Ashok, D. Ravinder, J. Alloys Compd. 394 (2005) 5–7.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Effect of sintering temperature and thermoelectric power studies of the system MgFe2xCrxO4 P.P. Hankare a, *, V.T. Vader a, U.B. Sankpal a, L.V. Gavali a, R. Sasikala b, I.S. Mulla c a
Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India Chemistry Division, Bhabha Atomic Research Centre, Mumbai-411 008, India c National Chemical Laboratory, Pune-411 008, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 April 2009 Received in revised form 26 July 2009 Accepted 1 September 2009 Available online 16 September 2009
Mixed metal oxides showing the spinel structure exhibit interesting structural and electrical properties. Substances with specific compositions in the system MgFe2xCrxO4 were synthesized by the simple coprecipitation method and have been investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM) to study the effect of temperature on the size of particles and grains. The infrared spectrum shows, two strong bands around 600 and 500 cm1. An elemental composition of one of the samples, MgFeCrO4 was found by energy dispersive X-ray spectroscopy (EDS). The thermoelectric power measurements carried out from room temperature to 500 C, show both n-type and p-type behavior. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Synthesis Temperature effect Thermoelectric studies
1. Introduction
2. Experimental
Ferrites have been used by various industries as a catalyst for some industrially important catalytic reactions. The interesting physicochemical properties of ferrospinels arise from their ability to distribute the cations among the available tetrahedral (A) and octahedral [B] sites [1–3]. Magnesium ferrite is a magnetic material having an inverse spinel structure of which the degree of inversion depends upon the heat treatment. At present, several chemical methods including the solid state reaction, co-precipitation, sol–gel, citrate method have been developed to synthesize small particle sized ferrites [4–6]. The conventional ceramic method for the preparation of ferrites has certain limitations such as long heating schedule at high temperatures. Hence, in the present investigation chromium substituted magnesium ferrites have been prepared by the simple co-precipitation technique. The main important topic in the present study concerns the substitution of Cr3þ and the heat treatment on structure and thermoemf properties. The thermoelectric power studies also give the information about the type of conductivity, weather it is n-type or p-type.
For the purpose of this investigation, various compositions of MgFe2xCrxO4 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0) were prepared by the simple co-precipitation method [7,8]. The high purity A.R. grade magnesium chloride, chromium chloride and iron chloride were weighed carefully to have the desired stoichiometric proportion. A mixed solution of the above chlorides was prepared in double distilled water. The precipitation was carried out at a controlled pH of w9.0 using a 10% NaOH solution. It was heated on a water bath (95–100 C) for 5 h. and oxidized by 30% H2O2 with constant stirring at the same temperature. The precipitate was filtered through the whatmann paper no. 41 and washed with double distilled water to remove excess alkali and Cl ions. The compounds were dried and sintered at different temperatures say 500, 600, 700, 800 and 900 C. The samples sintered at 900 C were mixed with 5 wt% polyvinyl alcohol as a binder for granulation. The granulated powders were compacted in a stainless die into pellets of 1.2 cm diameter and 0.15 cm thickness under a pressure of 10 tons. X-ray powder diffractograms were recorded on the Philips 3710 diffractometer with CuKa radiation (1.54056 Å) for phase identification, crystallite size and lattice parameter. The FT-IR spectra (Perkin Elmer-USA) were recorded in the range of 400–900 cm1 with a sample in the form of KBr pellets. The surface morphology was determined by using SEM for the samples sintered at different temperature while the compound MgFeCrO4 was used for the
* Corresponding author. Tel.: þ91 231 2609381. E-mail addresses: [email protected] (P.P. Hankare), vtv_chem@ rediffmail.com (V.T. Vader). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.09.005
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P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079 Table 1 Data on lattice constant (a), crystallite size (D), site radii (rA and rB) and bond length (RA and RB) for MgFe2xCrxO4 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0).
Fig. 1. Effect of sintering temperature on MgFeCrO4 sample for different temperature viz. (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C and (e) 900 C.
element analysis by EDS. The thermoelectric power for the samples was measured from room temperature to 773 K using a specially designed sample holder, which provides a temperature gradient of 300 K across the sample faces. 3. Results and discussion 3.1. X-ray diffraction studies X-ray diffraction patterns of MgFeCrO4 at different temperatures are presented in Fig. 1. The peaks are indexed to the characteristic planes of pure spinel cubic structure. The diffraction peaks are found to be sharp and their intensity increases with increase in the sintering temperature. No other phase has been detected and (311) reflections appear to be more intense [9–11] consistent with observations for face centered cubic spinel. Because the (311) diffraction peak at 900 C is sharper than that at lower temperatures, the temperature 900 C has been selected for the sintering of all samples, including those of different compositions. The lattice parameter ‘a’ was calculated using the relation.
1=2 a ¼ d h2 þ k2 þ l2
(1)
Composition Lattice constant Crystallite size (x) (Å) (nm)
Site radii Å rA
rB
RA
RB
0.0 0.5 1.0 1.5 2.0
0.5615 0.5514 0.5536 0.5422 0.5400
0.6996 0.6947 0.6971 0.6849 0.6825
1.9058 1.9013 1.9028 1.8915 1.8892
2.0508 2.0455 2.0479 2.0357 2.0334
8.40 8.38 8.39 8.34 8.33
22 33 32 31 29
Bond length Å
The lattice constant shows a decreasing trend with chromium concentration obeying Vegard’s law. However, for x ¼ 1.0 the lattice parameter slightly increases, because the system is not completely normal or inverse. Chromium has a strong site preference to the Bsite and hence the change in ‘a’ indicates the structural change from inverse to normal. The decreasing trend in lattice parameter is due to smaller ionic radius of Cr3þ (0.63 Å) which replaces the larger crystal radius Fe3þ (0.67 Å). Consequently, there is shrinkage in the unit cell dimension [12]. The crystallite size (D) of the powdered particles was determined from XRD peak broadening of the (311) peak using Scherrer formula,
Dhkl ¼ 0:9l=bcos q
(2)
Fig. 2 shows the variation of crystallite size as a function of Cr ion contents. The X-ray diffraction data was further used to confirm the single phase formation by calculating the tetrahedral and octahedral site radii (rA and rB) . The site radii, rA and rB were calculated using the relation.
pffiffiffi rA ¼ ðm 1=4Þa 3 Ro
(3)
rB ¼ ð5=8 mÞ Ro
(4)
where, m is oxygen parameter of the system and Ro is radius of oxygen ion. For cubic spinel, it is possible to calculate the bond length of tetrahedral and octahedral site using the following equations [13,14].
pffiffiffi RA ¼ a 3ðd þ 1=8Þ
(5)
1=2 2 RB ¼ a 3d þ 1=16 d=2
(6)
where, d ¼ deviation from oxygen parameter (m). The data on lattice constant (a), crystallite size (D), site radii (rA and rB) and the bond lengths (RA and RB) are summarized in the Table 1. The values calculated from the above equations indicate that both bond length and site radii show a decreasing trend with Cr content wherein the site radii rA is smaller than rB. Additionally one can say that the covalent character also decreases with Cr content due to the lower site radii in both tetrahedral and octahedral sites.
Table 2 Infrared spectrum showing Tetrahedral and Octahedral site for the system MgFe2xCrxO4 with different composition.
Fig. 2. Variation of crystallite size (nm) with composition (x) for the system MgFe2xCrxO4.
Composition (x)
Tetrahedral site (n1) cm1
Octahedral site (n2) cm1
0.0 0.5 1.0 1.5 2.0
583.09 599.11 614.87 629.57 642.30
440.57 454.98 481.30 503.14 516.26
P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079
3.2. FTIR-spectra In normal ferrites both absorption bands depend on the nature of octahedral cations and have no significant dependence on the nature of tetrahedral ions. From Table 2 it is seen that the IR spectra of Cr substituted magnesium ferrite are found to exhibit two bands in the range 400–600 cm1. The high frequency band n1 is observed around 600 cm1 and the lower frequency band n2 is around 500 cm1. These bands are common features of all ferrites. [15]. The absorption band n1 is attributed to the intrinsic vibration of metal oxygen bonds in tetrahedral sites while the lower absorption band n2 is due to metal oxygen vibration in octahedral sites [16]. It was found that Fe–O distance of the A-site (1.90 Å) is smaller than that of the B-site (2.05 Å). This can be interpreted by the more covalent bonding of Fe3þ ions at the A-site than that of the B-site. Because the Fe–O bonding characters in tetrahedral and octahedral complexes differ, a difference in the band positions is expected. Table 2 also shows that on increasing the Cr content in the system
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the bands shift towards higher frequencies and that they becomes sharper. In our present study, the system changes from inverse to normal and the broadening is commonly observed in the inverse spinel ferrite which has been attributed to statistical distribution of Fe3þ ions on A and B site. Hence, the bands are sharpened as one goes from MgFe2O4 to MgCr2O4. 3.3. Effect of temperature on grain size Fig 3 (a–e) shows the SEM images of the sample MgFeCrO4 sintered at different temperatures such as 500, 600, 700, 800 and 900 C. It is evident from the figure that, the number of grains and their size increases with annealing temperature. In the present samples, the grain size increases from 1.8 mm for the sample sintered at 500 C to about 4.2 mm for the sample sintered at 900 C. The calculated values of grain size from SEM image and crystallite size from XRD for the same sample sintered at different temperatures have been presented in Table 3. The crystallite size estimated
Fig. 3. Effect of sintering temperature on grain size for MgFeCrO4 sample (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C and (e) 900 C.
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P. P. Hankare / Solid State Sciences 11 (2009) 2075–2079
Table 3 Average grain size and crystallite size for the same sample MgFeCrO4sintered at different temperature. Sample
Sintering time ( C) Average grain size (mm) Crystallite size (mm)
MgFeCrO4 500 600 700 800 900
1.8 2.6 3.3 3.9 4.2
0.009 0.016 0.021 0.028 0.032
from Scherrer’s formula agrees well with that obtained from the corresponding SEM micrographs. The grains are distributed with uniform size throughout the surface [17]. 3.4. Elemental composition Fig. 4 shows the EDS pattern obtained for the sample MgFeCrO4, which gives the elemental composition in the sample. The compound shows the presence of Mg, Fe, Cr and O without any precipitating cations. Thus cations Naþ of the precursor reaction and most of the undesired chloride ions have been completely washed out. The average atomic ratio was found to be 1:1:1 for Mg:Fe:Cr. 3.5. Thermoelectric power studies The temperature dependence of the Seebeck coefficient for the different compositions were calculated from thermoemf measurement in the temperature range from 300 to 773 K and are presented in Fig. 5. The thermoemf is negative in the beginning indicating a most probable mechanism as electron hopping between Fe2þ and Fe3þ ions. The Mg ions are in þ2 oxidation state, which do not have free electrons. Hence the process is expected to take place between two adjacent octahedral sites in the spinel lattice as
Fe2þ #Fe3þ þ e The Seebeck coefficient calculated by using a ¼ DV/DT (where, DV ¼ thermoemf measured across the sample and DT ¼ the temperature difference across the pellet) is negative up to x ¼ 1.0 hence, till this composition the majority charge carriers are electrons generated from Fe ions, which act as donor centers. From
Fig. 5. Seebeck coefficient (a) measurements for different compositions of ‘x’.
x ¼ 1.5 onwards, the n to p-type conduction is observed due to the decrease in population of Fe3þ ions in B-sites, which are responsible for the reduction in the Seebeck coefficient. The mechanism is due to the hole transfer from Cr3þ to Cr2þ through
Fe3þ þ Cr2þ #Fe2þ þ Cr3þ For x ¼ 1.5, a values change from þve to ve whereas the sample x ¼ 2.0 shows only þve values in the investigated temperature range. This indicates that the majority charge carriers are the holes [18–20]. 4. Conclusions The system MgFe2xCrxO4 is synthesized by the simple coprecipitation method at controlled pH and exhibits the cubic spinel structure. The effect of sintering temperature on the observed structure influences the size of particles and consequently the grains. The calculated site radii and bond lengths show that the octahedral site radius is larger than the tetrahedral site. The FTIR spectrum shows two strong bands n1 and n2 in the expected region and the difference in their observed band position is due to the difference in metal-oxygen bond for octahedral and tetrahedral complexes. The thermoelectric power measurement shows n to ptype transition due to hopping of electrons and transfer of holes respectively. Acknowledgements Authors PPH and VTV wish to acknowledge their sincere gratitude to UGC for Major Research Project [F. No. 32-289/2006 (SR)] New Delhi, India for their financial support. References [1] [2] [3] [4] [5] [6]
Fig. 4. Elemental composition by EDS for the sample MgFeCrO4.
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