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Far-infrared spectra for copper–zinc mixed ferrites
Physica B 405 (2010) 4476–4479
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Far-infrared spectra for copper–zinc mixed ferrites H.M. Zaki a,b, H.A. Dawoud c a b c
Physics Department, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabia Physics Department, Faculty of Science, Zagazig University, Egypt University of Gaza, P.O. Box 108, Gaza Strip, Palestine Physics Department
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 April 2010 Received in revised form 23 July 2010 Accepted 6 August 2010
Infrared spectra of Zn2 + ions substituted Cu ferrites with the general formula Cu1 xZnxFe2O4 (where x¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1) have been analyzed in the frequency range 200–1000 cm 1. These mixed ferrites were prepared by the standard double sintering ceramic method. Two prominent bands were observed, high-frequency band n1 around 550 cm 1 and low-frequency band n2 around 395 cm 1 and assigned to tetrahedral and octahedral sites for spinel lattice, respectively. On introducing zinc ions IR spectra indicate new shoulders or splitting on tetrahedral absorption bands around 600 and 700 cm 1. A small absorption band n3 was observed around 310 cm 1. This indicates the migration of some Zn2 + ions to octahedral site. Another small weak absorption band was also observed around 265 cm 1; its intensity increased with Zn content. Force constant was calculated for both tetrahedral and octahedral sites. Threshold frequency nth for the electronic transition was determined and found to increase with an increase in Zn ions. The half bandwidth for each site was calculated and the ratio seemed to increase with an increase in zinc content. The cation distribution for these ferrites was estimated in the light of IR spectra. & 2010 Elsevier B.V. All rights reserved.
Keywords: IR spectra Ferrite Threshold frequency Half bandwidth
1. Introduction Polycrystalline soft ferrites are magnetic semiconductors, which cannot be replaced by any other magnetic material because ferrites are stable, relatively inexpensive, easily manufactured and have widespread applications in the electronics and communications industries due to their interesting electrical and magnetic properties. These properties strongly depend on their chemical and physical structures, in determining the precise configuration of the atoms and the ions in the ferromagnetic semiconductor [1]. Introduction of a relatively small amount of foreign ions can change the electrical and magnetic properties of the ferrites [2–4]. These changes can provide us information about the kind and amount of impurity required for obtaining a high-quality ferrite for any particular application. The IR spectra of the ferrite materials are an important tool to describe various ordering problems in the investigation of structural properties of the mixed ferrite. They give information not only about the positions of the ions in the spinel lattice but also about their vibration modes. The vibration spectra can be used to indicate the valance state of the ions and their occupation in the spinel lattice crystal. Furthermore, an increase in the concentration of divalent metal ions in some mixed ferrites may give rise to the structural change within the unit cell without affecting the spinel
structure as a whole. This structural change brought about by metal ions strongly influences the lattice vibrations. The application of IR spectroscopy to the ferrite materials can be used to detect the completion of solid-state reaction, cations distribution and the deformation of spinel structure [5]. The IR spectra absorption bands mainly appear due to the vibrations of the oxygen ions with the cations producing various frequencies in the unit cell. In a certain mixed ferrite materials, as the concentration of the divalent metal ions increases, it gives rise to the structural change or the cations distribution in spinel lattice crystal without affecting the spinel ferrite structure [1]. The structural changes brought by the metal ions that are either lighter or heavier than divalent ions in the ferrites strongly influence the lattice vibration. Also the vibration frequency depends on the cations’ mass, oxygen distance and the bonding force [6]. Many researchers have studied IR spectra for several ferrites. However, this work is carried out to study the effect of Zn2 + substitution on IR absorption bands by analyzing the IR spectra for such ferrite. The aim of this work is to study the compositional structure using IR spectra for ferrites with spinel structure from the series of chemical composition Cu1 xZnxFe2O4.
2. Experimental technique E-mail addresses: [email protected], [email protected] (H.M. Zaki). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.018
Polycrystalline mixed ferrite having the formula Cu1 xZnxFe2O4 (where x stepped by 0.2 according to 0.0rxr1.0) was investigated.
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
The above mixed ferrites were prepared by the standard double ceramic method. Weighted high-purity metal oxides were mixed, and then ground to a very fine powder for 5 h, then pre-sintered at 750 1C for 3 h soaking time. After that, the pre-fired powder was ground well. The mixture was pressed under a constant pressure (3 tons) in the form of a disc shape with diameter 11 mm. All samples were sintered for 5 h at 1100 1C and then cooled gradually to room temperature with a rate of 1 1C/min. For recording IR spectra, the powder was mixed with KBr in the ratio 1:200 and then pressed into a disc of 1 mm thickness. IR measurements were carried out at room temperature in the range from 200 up to 1000 cm 1 by using an infraspectrophotometer Perkin Elmer model 883.
3. Results and discussion The absorption bands of the system Cu1 xZnxFe2O4 are shown in Fig. 1. The band positions are listed in Table 1. The highfrequency band n1 lies in the range between 568 and 536 cm 1 and the lower-frequency band n2 lies around 395 cm 1. Absorption bands observed within this limit revealed the formation of single-phase spinel structure having two sublattices: tetrahedral (A) site and octahedral (B) site [7]. The absorption band n1 that was observed at around 550 cm 1 is attributed to the tetrahedral
th
x = 1.0
1sh. 4
x = 0.8
1 2sh.
3
x = 0.6
Transmittance
2
x = 0.4
x = 0.2
x = 0.0
Wave Number cm-1
1000 900
800
700
600
500
400
300
200
Fig. 1. IR absorption spectra of the mixed Cu–Zn ferrites.
4477
Table 1 Absorption band frequency for the Td and the Oh sites of the mixed Cu–Zn ferrites. x
0.0 0.2 0.4 0.6 0.8 1.0
Td sites
Oh sites
v1sh (cm 1)
v2sh (cm 1)
v1 (cm 1)
v2sh (cm 1)
v3sh (cm 1)
v4sh (cm 1)
– – 700 700 700 700
600 600 600 600 600 600
568 564 556 546 540 536
395 395 395 395 395 395
300 305 310 315 320 325
265 265 267 270 266 266
site whereas n2 that was observed at around 395 cm 1 is assigned to octahedral group complexes. A difference in the band position of n1 and n2 is expected because of the difference in the Fe3 + –O2 distances for the tetrahedral and octahedral complexes. It was ˚ is smaller than found that the Fe O distance of A-site (1.89 A) ˚ [8]. This has been interpreted by more that of the B-site (1.99 A) covalent bonding of Fe3 + ions at the A-sites. From Table 1, it is found that the positions of n1 (tetrahedral bond) are composition dependent and show a close agreement with the previous reports [9,10]. The wave number of band n1 shows a decrease with an increase in zinc concentration (x). This variation in the band positions may be due to variation in the cation oxygen bond (A O) lengths [11], and this is discussed briefly later. A shoulder appearing closer to n2 denoted by n3 at around 310 cm 1, which increases by an increase in Zn content, is observed as shown in Fig. 1. Many researchers [10,12,13] have reported this shoulder in case of zinc rich compositions, assigning it to the divalent octahedral metal ion–oxygen group complexes. This shoulder appears relatively narrow and weak in its intensity starting from x ¼0.2, which increases with an increase in Zn2 + ions. This leads to the narrowing of octahedral band and n3 becomes more intense. Therefore, n3 could be seen very clearly at high concentration of the Zn ions. This appearance of shoulder for x40.0 confirms that a part of Zn ions could be present in B-site. However Amer [14] and many other authors reported that many Zn ions are resident at A-site and this reflects the presence of two shoulders in the tetra-site at around 600 and 700 cm 1. These shoulders begin to appear for x 40.0 and increase while Zn2 + ions increase. A change in the place of two shoulders may attribute to the presence of two covalent states for Zn ions. It has been shown by Potakova et al. [15] that the presence of Fe2 + ions in ferrites can produce splitting or shoulders on absorption bands. This is attributed to the Jahn–Teller distortion produced by the Fe2 + ions, which cause local deformations in the lattice owing to the noncubic components of the crystal field potential. Thus the shoulder n4 can be assigned to Fe2 + –O2 octahedral complexes. It can be seen that the shoulder is transferred to a band as zinc concentration increases, which means the increase in Fe2 + . According to Tarte and Preudhommed. [16] and Reddy and Solagram [17] this band may be attributed to some types of lattice vibrations. Its frequency depends on the mass of the foreign atom occupying this site in the ferrite samples and the mass of the divalent cations. Table 1 shows the position of absorption bands as wave numbers. From this table, it is found that the positions of tetrahedral bands are composition dependent and show a close agreement with the previous work reports by Ladgaonkar et al. [9]. The wave number of band n1 shows a decrease with an increase in zinc concentration (x). This variation in the band positions may be due to the variation in the cation oxygen bond length [11]. When zinc ion substitutes its residues on tetrahedral (A) site, it displaces proportional amount of Fe3 + ion from A to B
4478
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
600
820
ν1
800 vth (cm-1)
(Wavenumber) cm-1
550
500
450
780
760 ν2
400
740 0.2
0
0.4 0.6 Composition (x)
0.8
1
350 0
0.2
0.8
0.4 0.6 Composition (x)
Fig. 3. Change in threshold frequency uth with composition x.
1
2.3
Fig. 2. Behavior of the tetrahedral absorption bands u1 and octahedral absorption bands u2 with composition x.
2.2 2.1
Table 2 Calculated values of the force constants FCT and FCO. FCT 105 (dyne cm 1)
FCO 105 (dyne cm 1)
0.0 0.2 0.4 0.6 0.8 1.0
2.361 2.312 2.247 2.182 2.125 2.103
1.142 1.136 1.136 1.136 1.136 1.136
2
(A /B)
x
1.9 1.8 1.7 1.6
site. This leads to an increase in the cation oxygen bond length of tetrahedral site of the spinel [4], which results in a decrease in the wave number of n1 band and this is shown in Fig. 2. It can also be seen from Fig. 2 that the octahedral band has a constant value of around 395 cm 1 and this may be due to the neutralization effect for the migration of both Zn2 + and Fe3 + ions to this site. The force constants for tetrahedral and octahedral sites were calculated employing the following equation [18]: 2 2 2
F e ¼ 4p c n M
ð1Þ
where c is the velocity of light, n the vibration frequency for tetrahedral and octahedral sites and M the reduced mass for Fe3 + and O2 ions. The calculated values for the force constant are tabulated in Table 2. It is observed that the force constants of tetrahedral site (A-site) decrease on increasing zinc concentration. This behavior can be attributed to the variation in cation oxygen bond length [9]. Since the bond length (A O) increased with an increase in zinc concentration, the energy required to break longer bonds is less, and this supports a decrease in the force constant of tetrahedral sites. According to Waldron [7] the threshold frequency for the electronic transition can be determined from the maximum point of the absorption spectra at which it reaches a limiting value. Table 1 shows these threshold values, which are represented in Fig. 3. It is found that the threshold frequency for the electronic transition seems to increase with an increase in Zn2 + content and with a decrease in Cu2 + concentration. From IR spectra, the values of half bandwidth for each site are calculated and the ratio GA/GB is represented in Fig. 4. It can be noticed that the increase in GA/GB is linear with the increase in Zn
1.5 0
0.2
0.4
0.6
0.8
1
Composition (x) Fig. 4. Change in half bandwidth ratio (GA/GB) against composition x.
Table 3 Values of vth, GA, GB and GA/GB for the mixed Cu–Zn ferrites. x
vth (cm 1)
GA (cm 1)
GB (cm 1)
GA/GB
0.0 0.2 0.4 0.6 0.8 1.0
760 770 780 790 800 810
140 145 150 155 160 165
90 85 81 78 76 75
1.56 1.71 1.85 1.98 2.1 2.2
content (Table 3). The half bandwidth depends on the statistical distribution of various cations over the two sites [19]. This distribution depends on the replacement process between smaller ionic radii Cu2 + (0.072 nm) and larger ionic radii Zn2 + (0.082 nm). In the light of the above consideration and according to IR spectra, the cation distribution for the mixed Cu–Zn ferrites can be written as follows: (Znx tFe1 x) [Cu1 xZntFe1 + x]O4
(2)
where the parenthesis represents the so-called tetrahedral site (A-site), while the square bracket represents the octahedral site
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
0.848 0.070
0.846
rB
0.068
0.844 0.842 ra
0.840
0.064
0.838 0.062
Theorotical lattice constant, nm
Tetrahedral and octaredral radius
0.850
0.072
0.066
4. Conclusion
0.852
0.074
0.836
ath
0.060
0.834 0.0
0.2
0.4 0.6 Composition (x)
0.8
1.0
Fig. 5. Variation in tetrahedral (rA) and octahedral (rB) radii for each site and theoretical lattice parameter (ath) against composition x.
(B-site), t has the values between 0 rt r0.1 [20–22] and x is the cation distribution parameter (0.0rx r1.0). According to the above cation distribution and reported results in Refs. [21,23], the ionic radius and the lattice constant (a) are correlated as follows: pffiffiffi pffiffiffi a ¼ 8½ðr A þ r O Þ þ 3ðr B þ r O Þ=3 3
4479
ð3Þ
where rA, rB and rO are the ionic radii of the A-site, B-site and the oxygen, respectively and rO is a constant with the value 0.132 nm. The values of rA and rB depend on the cation distribution and are calculated as follows: r A ¼ ðxtÞr Zn þð1xÞr Fe
ð4Þ
r B ¼ ½ð1x þ tr Zn þð1 þ xÞr Fe =2
ð5Þ
where rZn, rCu and rFe are the ionic radii of zinc, copper and ferric ions, respectively. These radii were substituted by 0.074, 0.072 and 0.064 nm, respectively. The theoretical values of the lattice parameters (ath) rA and rB (which depend on the cation distribution) have been calculated and are plotted versus the composition in Fig. 5. It can be seen from this figure that there is an increase in the lattice constant by the increasing value of x; this is in coherence with the bigger ionic radii of Zn ions, which has been replaced by a smaller one for the copper ions.
1. IR spectra indicated two main bands, one for the tetrahedral site at around 550 cm 1 and the other for octahedral site at around 395 cm 1. 2. New shoulders were observed by introducing Zn2 + ions, the first at around 600 cm 1 and the second at around 700 cm 1. 3. Another subsidiary absorption band was detected at around 310 cm 1 indicating the migration of some Zn2 + ions to the octahedral site. 4. Small and weak absorption band was observed at around 265 cm 1; its intensity increased with the increase in Zn content and this is related to Fe2 + ions. 5. The force constants for the two sites were calculated and found to decrease with the increase in Zn concentration. 6. Both threshold frequency and half bandwidth were determined and calculated. 7. On the basis of IR absorption band analysis, the cation distributions of the Cu–Zn ferrites were estimated.
References [1] P.V. Treddy, M. Salagram, Phys. Status Solidi (a) 100 (1987) 639. [2] A.Y. Lipare, P.N. Vasambekar, A.S. Vaingankar, Phys. Status Solidi (a) 2 (2003) 372. [3] P. Brahma, A.K. Giri, D. Chakravorty, M. Tiwari, J. Bahadur, Magn. Magn. Mater. 102 (1991) 109. [4] B.P. Ladgonkar, P.N. Vasambekar, A.S. Vaingankar, J. Mater. Sci. Lett. 19 (2000) 1375. [5] A.M. Shaikh, S.A. Jadhav, S.C. Watawe, B.K. Chongnle, Mater. Sci. Lett. 44 (2000) 192. [6] S.A. Patil, S.M. Otarl, V.C. Mahajan, M.G. Patil, A.B. Patil, N.K. Soudagav, B.L. Patil, S.R. Sawant, Solid State Commun. 78 (1991) 139. [7] R.D. Waldron, Phys. Rev. 99 (1955) 1727. [8] B.J. Evans, S. Hafner, J. Phys. Chem. Solids 29 (1968) 1573. [9] B.P. Ladgaonkar, C.B. Kolekar, A.S. Vaingankar, Bull. Mater. Sci. 25 (2002) 351. [10] M.A. El Hitti, A.J. El Shora, A.S. Seoud, S.M. Hammad, Phys. Trans 65 (1996) 35. [11] C.B. Kolekar, P.N. Kambel, A.S. Vaingankar, Indian J. Phys. 68(A) (1994) 529. [12] B.F. Liene, Phys. Rev. 37 (1973) 2591. [13] O.S. Josyulu, J. Sobhanadri, Phys. Status Solidi. (a) 65 (1981) 479. [14] M.A. Amer, Phys. Status Solidi (a) 181 (2000) 539. [15] V.P. Potakova, N.D. Zverv, V.P. Romanov, Phys. Status Solidi (a) 12 (1972) 623. [16] J.P. Tarte, J. Preudhommed, Spectro. Acta 27A (1971) 961. [17] P.V. Reddy, M. Solagram, Phys. Status Solidi (a) 100 (1987) 639. [18] S.A. Mazen, M.H. Abdallah, R.I. Nakhla, H.M. Zaki, Mater. Chem. Phys. 34 (1993) 35. [19] P. Tarte, Acta Crystallogr. 16 (1963) 228. [20] Xiao-Xia Tang, A. Manthiram, J.B. Goodenough, J. Solid State Chem. 79 (1989) 250. [21] S.A. Mazen, M.H. Abdallah, B.A. Sabrah, H.A.M. Hashem, Phys. Status Solidi (a) 134 (1993) 263. [22] A. Elfalaky, Appl. Phys. A 59 (1994) 389. [23] J.C. Dwcius, O.G. Main, A.W. Thomson, Proc. R. Soc. London, Ser. A 275 (1963) 295.
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Physica B journal homepage: www.elsevier.com/locate/physb
Far-infrared spectra for copper–zinc mixed ferrites H.M. Zaki a,b, H.A. Dawoud c a b c
Physics Department, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabia Physics Department, Faculty of Science, Zagazig University, Egypt University of Gaza, P.O. Box 108, Gaza Strip, Palestine Physics Department
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 April 2010 Received in revised form 23 July 2010 Accepted 6 August 2010
Infrared spectra of Zn2 + ions substituted Cu ferrites with the general formula Cu1 xZnxFe2O4 (where x¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1) have been analyzed in the frequency range 200–1000 cm 1. These mixed ferrites were prepared by the standard double sintering ceramic method. Two prominent bands were observed, high-frequency band n1 around 550 cm 1 and low-frequency band n2 around 395 cm 1 and assigned to tetrahedral and octahedral sites for spinel lattice, respectively. On introducing zinc ions IR spectra indicate new shoulders or splitting on tetrahedral absorption bands around 600 and 700 cm 1. A small absorption band n3 was observed around 310 cm 1. This indicates the migration of some Zn2 + ions to octahedral site. Another small weak absorption band was also observed around 265 cm 1; its intensity increased with Zn content. Force constant was calculated for both tetrahedral and octahedral sites. Threshold frequency nth for the electronic transition was determined and found to increase with an increase in Zn ions. The half bandwidth for each site was calculated and the ratio seemed to increase with an increase in zinc content. The cation distribution for these ferrites was estimated in the light of IR spectra. & 2010 Elsevier B.V. All rights reserved.
Keywords: IR spectra Ferrite Threshold frequency Half bandwidth
1. Introduction Polycrystalline soft ferrites are magnetic semiconductors, which cannot be replaced by any other magnetic material because ferrites are stable, relatively inexpensive, easily manufactured and have widespread applications in the electronics and communications industries due to their interesting electrical and magnetic properties. These properties strongly depend on their chemical and physical structures, in determining the precise configuration of the atoms and the ions in the ferromagnetic semiconductor [1]. Introduction of a relatively small amount of foreign ions can change the electrical and magnetic properties of the ferrites [2–4]. These changes can provide us information about the kind and amount of impurity required for obtaining a high-quality ferrite for any particular application. The IR spectra of the ferrite materials are an important tool to describe various ordering problems in the investigation of structural properties of the mixed ferrite. They give information not only about the positions of the ions in the spinel lattice but also about their vibration modes. The vibration spectra can be used to indicate the valance state of the ions and their occupation in the spinel lattice crystal. Furthermore, an increase in the concentration of divalent metal ions in some mixed ferrites may give rise to the structural change within the unit cell without affecting the spinel
structure as a whole. This structural change brought about by metal ions strongly influences the lattice vibrations. The application of IR spectroscopy to the ferrite materials can be used to detect the completion of solid-state reaction, cations distribution and the deformation of spinel structure [5]. The IR spectra absorption bands mainly appear due to the vibrations of the oxygen ions with the cations producing various frequencies in the unit cell. In a certain mixed ferrite materials, as the concentration of the divalent metal ions increases, it gives rise to the structural change or the cations distribution in spinel lattice crystal without affecting the spinel ferrite structure [1]. The structural changes brought by the metal ions that are either lighter or heavier than divalent ions in the ferrites strongly influence the lattice vibration. Also the vibration frequency depends on the cations’ mass, oxygen distance and the bonding force [6]. Many researchers have studied IR spectra for several ferrites. However, this work is carried out to study the effect of Zn2 + substitution on IR absorption bands by analyzing the IR spectra for such ferrite. The aim of this work is to study the compositional structure using IR spectra for ferrites with spinel structure from the series of chemical composition Cu1 xZnxFe2O4.
2. Experimental technique E-mail addresses: [email protected], [email protected] (H.M. Zaki). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.018
Polycrystalline mixed ferrite having the formula Cu1 xZnxFe2O4 (where x stepped by 0.2 according to 0.0rxr1.0) was investigated.
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
The above mixed ferrites were prepared by the standard double ceramic method. Weighted high-purity metal oxides were mixed, and then ground to a very fine powder for 5 h, then pre-sintered at 750 1C for 3 h soaking time. After that, the pre-fired powder was ground well. The mixture was pressed under a constant pressure (3 tons) in the form of a disc shape with diameter 11 mm. All samples were sintered for 5 h at 1100 1C and then cooled gradually to room temperature with a rate of 1 1C/min. For recording IR spectra, the powder was mixed with KBr in the ratio 1:200 and then pressed into a disc of 1 mm thickness. IR measurements were carried out at room temperature in the range from 200 up to 1000 cm 1 by using an infraspectrophotometer Perkin Elmer model 883.
3. Results and discussion The absorption bands of the system Cu1 xZnxFe2O4 are shown in Fig. 1. The band positions are listed in Table 1. The highfrequency band n1 lies in the range between 568 and 536 cm 1 and the lower-frequency band n2 lies around 395 cm 1. Absorption bands observed within this limit revealed the formation of single-phase spinel structure having two sublattices: tetrahedral (A) site and octahedral (B) site [7]. The absorption band n1 that was observed at around 550 cm 1 is attributed to the tetrahedral
th
x = 1.0
1sh. 4
x = 0.8
1 2sh.
3
x = 0.6
Transmittance
2
x = 0.4
x = 0.2
x = 0.0
Wave Number cm-1
1000 900
800
700
600
500
400
300
200
Fig. 1. IR absorption spectra of the mixed Cu–Zn ferrites.
4477
Table 1 Absorption band frequency for the Td and the Oh sites of the mixed Cu–Zn ferrites. x
0.0 0.2 0.4 0.6 0.8 1.0
Td sites
Oh sites
v1sh (cm 1)
v2sh (cm 1)
v1 (cm 1)
v2sh (cm 1)
v3sh (cm 1)
v4sh (cm 1)
– – 700 700 700 700
600 600 600 600 600 600
568 564 556 546 540 536
395 395 395 395 395 395
300 305 310 315 320 325
265 265 267 270 266 266
site whereas n2 that was observed at around 395 cm 1 is assigned to octahedral group complexes. A difference in the band position of n1 and n2 is expected because of the difference in the Fe3 + –O2 distances for the tetrahedral and octahedral complexes. It was ˚ is smaller than found that the Fe O distance of A-site (1.89 A) ˚ [8]. This has been interpreted by more that of the B-site (1.99 A) covalent bonding of Fe3 + ions at the A-sites. From Table 1, it is found that the positions of n1 (tetrahedral bond) are composition dependent and show a close agreement with the previous reports [9,10]. The wave number of band n1 shows a decrease with an increase in zinc concentration (x). This variation in the band positions may be due to variation in the cation oxygen bond (A O) lengths [11], and this is discussed briefly later. A shoulder appearing closer to n2 denoted by n3 at around 310 cm 1, which increases by an increase in Zn content, is observed as shown in Fig. 1. Many researchers [10,12,13] have reported this shoulder in case of zinc rich compositions, assigning it to the divalent octahedral metal ion–oxygen group complexes. This shoulder appears relatively narrow and weak in its intensity starting from x ¼0.2, which increases with an increase in Zn2 + ions. This leads to the narrowing of octahedral band and n3 becomes more intense. Therefore, n3 could be seen very clearly at high concentration of the Zn ions. This appearance of shoulder for x40.0 confirms that a part of Zn ions could be present in B-site. However Amer [14] and many other authors reported that many Zn ions are resident at A-site and this reflects the presence of two shoulders in the tetra-site at around 600 and 700 cm 1. These shoulders begin to appear for x 40.0 and increase while Zn2 + ions increase. A change in the place of two shoulders may attribute to the presence of two covalent states for Zn ions. It has been shown by Potakova et al. [15] that the presence of Fe2 + ions in ferrites can produce splitting or shoulders on absorption bands. This is attributed to the Jahn–Teller distortion produced by the Fe2 + ions, which cause local deformations in the lattice owing to the noncubic components of the crystal field potential. Thus the shoulder n4 can be assigned to Fe2 + –O2 octahedral complexes. It can be seen that the shoulder is transferred to a band as zinc concentration increases, which means the increase in Fe2 + . According to Tarte and Preudhommed. [16] and Reddy and Solagram [17] this band may be attributed to some types of lattice vibrations. Its frequency depends on the mass of the foreign atom occupying this site in the ferrite samples and the mass of the divalent cations. Table 1 shows the position of absorption bands as wave numbers. From this table, it is found that the positions of tetrahedral bands are composition dependent and show a close agreement with the previous work reports by Ladgaonkar et al. [9]. The wave number of band n1 shows a decrease with an increase in zinc concentration (x). This variation in the band positions may be due to the variation in the cation oxygen bond length [11]. When zinc ion substitutes its residues on tetrahedral (A) site, it displaces proportional amount of Fe3 + ion from A to B
4478
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
600
820
ν1
800 vth (cm-1)
(Wavenumber) cm-1
550
500
450
780
760 ν2
400
740 0.2
0
0.4 0.6 Composition (x)
0.8
1
350 0
0.2
0.8
0.4 0.6 Composition (x)
Fig. 3. Change in threshold frequency uth with composition x.
1
2.3
Fig. 2. Behavior of the tetrahedral absorption bands u1 and octahedral absorption bands u2 with composition x.
2.2 2.1
Table 2 Calculated values of the force constants FCT and FCO. FCT 105 (dyne cm 1)
FCO 105 (dyne cm 1)
0.0 0.2 0.4 0.6 0.8 1.0
2.361 2.312 2.247 2.182 2.125 2.103
1.142 1.136 1.136 1.136 1.136 1.136
2
(A /B)
x
1.9 1.8 1.7 1.6
site. This leads to an increase in the cation oxygen bond length of tetrahedral site of the spinel [4], which results in a decrease in the wave number of n1 band and this is shown in Fig. 2. It can also be seen from Fig. 2 that the octahedral band has a constant value of around 395 cm 1 and this may be due to the neutralization effect for the migration of both Zn2 + and Fe3 + ions to this site. The force constants for tetrahedral and octahedral sites were calculated employing the following equation [18]: 2 2 2
F e ¼ 4p c n M
ð1Þ
where c is the velocity of light, n the vibration frequency for tetrahedral and octahedral sites and M the reduced mass for Fe3 + and O2 ions. The calculated values for the force constant are tabulated in Table 2. It is observed that the force constants of tetrahedral site (A-site) decrease on increasing zinc concentration. This behavior can be attributed to the variation in cation oxygen bond length [9]. Since the bond length (A O) increased with an increase in zinc concentration, the energy required to break longer bonds is less, and this supports a decrease in the force constant of tetrahedral sites. According to Waldron [7] the threshold frequency for the electronic transition can be determined from the maximum point of the absorption spectra at which it reaches a limiting value. Table 1 shows these threshold values, which are represented in Fig. 3. It is found that the threshold frequency for the electronic transition seems to increase with an increase in Zn2 + content and with a decrease in Cu2 + concentration. From IR spectra, the values of half bandwidth for each site are calculated and the ratio GA/GB is represented in Fig. 4. It can be noticed that the increase in GA/GB is linear with the increase in Zn
1.5 0
0.2
0.4
0.6
0.8
1
Composition (x) Fig. 4. Change in half bandwidth ratio (GA/GB) against composition x.
Table 3 Values of vth, GA, GB and GA/GB for the mixed Cu–Zn ferrites. x
vth (cm 1)
GA (cm 1)
GB (cm 1)
GA/GB
0.0 0.2 0.4 0.6 0.8 1.0
760 770 780 790 800 810
140 145 150 155 160 165
90 85 81 78 76 75
1.56 1.71 1.85 1.98 2.1 2.2
content (Table 3). The half bandwidth depends on the statistical distribution of various cations over the two sites [19]. This distribution depends on the replacement process between smaller ionic radii Cu2 + (0.072 nm) and larger ionic radii Zn2 + (0.082 nm). In the light of the above consideration and according to IR spectra, the cation distribution for the mixed Cu–Zn ferrites can be written as follows: (Znx tFe1 x) [Cu1 xZntFe1 + x]O4
(2)
where the parenthesis represents the so-called tetrahedral site (A-site), while the square bracket represents the octahedral site
H.M. Zaki, H.A. Dawoud / Physica B 405 (2010) 4476–4479
0.848 0.070
0.846
rB
0.068
0.844 0.842 ra
0.840
0.064
0.838 0.062
Theorotical lattice constant, nm
Tetrahedral and octaredral radius
0.850
0.072
0.066
4. Conclusion
0.852
0.074
0.836
ath
0.060
0.834 0.0
0.2
0.4 0.6 Composition (x)
0.8
1.0
Fig. 5. Variation in tetrahedral (rA) and octahedral (rB) radii for each site and theoretical lattice parameter (ath) against composition x.
(B-site), t has the values between 0 rt r0.1 [20–22] and x is the cation distribution parameter (0.0rx r1.0). According to the above cation distribution and reported results in Refs. [21,23], the ionic radius and the lattice constant (a) are correlated as follows: pffiffiffi pffiffiffi a ¼ 8½ðr A þ r O Þ þ 3ðr B þ r O Þ=3 3
4479
ð3Þ
where rA, rB and rO are the ionic radii of the A-site, B-site and the oxygen, respectively and rO is a constant with the value 0.132 nm. The values of rA and rB depend on the cation distribution and are calculated as follows: r A ¼ ðxtÞr Zn þð1xÞr Fe
ð4Þ
r B ¼ ½ð1x þ tr Zn þð1 þ xÞr Fe =2
ð5Þ
where rZn, rCu and rFe are the ionic radii of zinc, copper and ferric ions, respectively. These radii were substituted by 0.074, 0.072 and 0.064 nm, respectively. The theoretical values of the lattice parameters (ath) rA and rB (which depend on the cation distribution) have been calculated and are plotted versus the composition in Fig. 5. It can be seen from this figure that there is an increase in the lattice constant by the increasing value of x; this is in coherence with the bigger ionic radii of Zn ions, which has been replaced by a smaller one for the copper ions.
1. IR spectra indicated two main bands, one for the tetrahedral site at around 550 cm 1 and the other for octahedral site at around 395 cm 1. 2. New shoulders were observed by introducing Zn2 + ions, the first at around 600 cm 1 and the second at around 700 cm 1. 3. Another subsidiary absorption band was detected at around 310 cm 1 indicating the migration of some Zn2 + ions to the octahedral site. 4. Small and weak absorption band was observed at around 265 cm 1; its intensity increased with the increase in Zn content and this is related to Fe2 + ions. 5. The force constants for the two sites were calculated and found to decrease with the increase in Zn concentration. 6. Both threshold frequency and half bandwidth were determined and calculated. 7. On the basis of IR absorption band analysis, the cation distributions of the Cu–Zn ferrites were estimated.
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