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Infrared spectral studies of Zn-substituted Li–Mg ferrites
June 2000
Materials Letters 44 Ž2000. 192–196 www.elsevier.comrlocatermatlet
Infrared spectral studies of Zn-substituted Li–Mg ferrites A.M. Shaikh ) , S.A. Jadhav, S.C. Watawe, B.K. Chougule Ferrite Materials Research Laboratory, Department of Physics, ShiÕaji UniÕersity, Kolhapur 416 004, India Received 7 December 1999; accepted 14 December 1999
Abstract The infrared spectra of Zn-substituted Li–Mg ferrites, having the general formula Li x Mg 0.4 Zn 0.6y2 x Fe 2qx O4 Žwhere x s 0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3., have been analyzed in the frequency range 200–800 cmy1. The ferrites were prepared by standard double sintering ceramic method and the single-phase formation was confirmed by X-ray diffraction studies. The IR spectra show two fundamental absorption bands n 1 and n 2 in the range 600–400 cmy1, corresponding tetrahedral and octahedral complexes, respectively. The bands n 1 and n 2 are found to shift gradually towards the lower frequency side with addition of Zn, which have been attributed to the increase in the lattice parameter. The bands do not show any sign of splitting indicating no excess formation of Fe 2q ions. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Li–Mg–Zn ferrites; IR spectral studies
1. Introduction Lithium ferrites and lithium-based ferrites have become important materials for microwave applications as a replacement for garnets due to their low costs, squareness of hysteresis loop, high Curie temperature and low dielectric losses w1x. The IR spectroscopy is used to determine the local symmetry in crystalline and non-crystalline solids and also to study the ordering phenomenon in ferrites. The IR absorption bands mainly appear due to vibrations of oxygen ions with cations producing various frequencies of the unit cell. The frequencies depend upon
)
Corresponding author. Fax: q91-231-656133. E-mail address: [email protected] ŽA.M. Shaikh..
cation masses, lattice parameter and cation–oxygen bonding, etc. Many researchers have reported studies on Li–Cd w2,3x, Li–Zn w4x, Li–Mg w5x and Li–Ni w1x ferrites. Bellad and Chougule w6x have studied IR spectra of some mixed Li–Cd ferrites. However, no reports have been found in literature regarding IR studies of Zn-substituted Li–Mg ferrites. Therefore, in the present communication, we report on the infrared studies of these ferrites. The applications of IR spectroscopy to ferrites are to detect the completion of the solid state reaction, to study the cation distribution, to study the deformation of the spinel structure, cation disordering and to study the force constants for tetrahedral and octahedral sites. The ferrites crystallize in their crystallographic form with space group Fd 3 m -O h7 . According to
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 2 5 - 2
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
group theoretical considerations, four infrared active fundamentals were expected in the vibrational spectra of normal as well as inverse spinel ferrites. In the frequency range 200–800 cmy1 , spinel ferrites usually show two absorption bands corresponding to tetrahedral and octahedral sites.
193
2. Experimental Zn-substituted Li–Mg ferrites, having the general formula Li x Mg 0.4 Zn 0.6-2 x Fe 2qxO4 Žwhere x s 0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3., have been prepared by the standard double sintering ceramic
Fig. 1. Infrared spectra of Li–Mg–Zn ferrites.
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
194
n 2 in the range of 430–420 cmy1 as the concentration of Zn increases from 0–60%. The band position obtained in the present case are found to be in the range reported w1x as well as for Li–Cd ferrite w6x. Slight variation in n 1 and n 2 is due to the fact that the method of preparation, grain size and porosity influence the band positions. In addition to this, it is observed that, with the addition of Zn intensity of lower band decreases, the difference between the intensities of the two bands also decreases, and the bands n 1 and n 2 become less pronounced. Absence of double band near 600 cmy1 indicates that excessive Fe 2q ions are not present in the system. Splitting of n 1 and n 2 into shoulders has not been observed in the present system, which also confirms the absence of excessive Fe 2q ions. Similar results have been also observed by Shaikh et al. w7x in the case of Li–Zn–Zr system. The decrease in intensity and increase in broadness are explained on the basis of cation distribution. The cation distribution in the present case is given as 3q 2q 3q . w 1q x 2y ŽZn2q 0.6y2 x Fe 0.9 xq0.7 A Li x Mg 0.4 Fe 0.1 xq1.3 B O4 . It can be seen that there is a 1:3 order of cations on the octahedral site. As the content of Zn increases, Zn2q ions consistently replace Fe 3q ions from A to B site. At the same time, the content of Li 1q ions on the octahedral site decreases. This disturbs the 1:3 order on the octahedral site with increase in Zn content. The disordered system gives rise to broad bands in their spectrum w9x. It is clear from Table 1, as well as from Fig 1, that the center frequency of the principle bands n 1 and n 2 shifts gradually towards the low-frequency side. From the X-ray diffraction study, it is revealed that the lattice parameter Ž a. and bond lengths R A and R B increase linearly with the increase in Zn
method in air medium using AR grade Li 2 CO 3 , MgO, ZnO and Fe 2 O 3 as starting materials. The oxides were mixed in stoichiometric proportions and the mixtures were pre-sintered at 873 K for 12 h. The resultant powders were granulated into fine particles using a agate mortar. The samples were finally sintered at 1273 K for 24 h and were furnace cooled. The completion of solid state reaction was confirmed by X-ray diffraction studies carried out on powder samples with Philips PW-1710 diffractometer using ˚ . radiation. All the samples CuK a Ž l s 1.5418 A show single-phase formation. The IR spectra were recorded on a Perkin-Elemer IR spectrometer Žmodel-783. in the range 200–800 cmy1 in the KBr medium.
3. Results and discussion Fig 1 shows the IR spectra of the samples. All the samples show two prominent absorption bands n 1 and n 2 in the range 600 and 400 cm -1 , respectively. Absorption at n 1 is caused by stretching of tetrahedral metal ion and oxygen bonding, while n 2 is caused by vibrations of oxygen in the direction perpendicular to the axis joining the tetrahedral ion and oxygen. Patil et al. w8x has studied the IR spectra of ferrites and has attributed the occurrence of band n 1 around 600 cmy1 to the intrinsic vibrations of the tetrahedral complexes corresponding to the highest restoring force, and band n 2 around 400 cmy1 to intrinsic vibrations of octahedral complexes, which are bond-bending vibrations. Therefore, it is expected that n 1 ) n 2 . In the present system, the band n 1 is found to lie in the range of 610–565 cmy1 and the lower band
Table 1 Data on lattice parameter Ž a., bond lengths Ž RA , R B ., the positions of IR absorption bands Ž n 1 , n 2 . and the force constants Ž K t , K o . Zn
˚. ‘a’ ŽA
˚. RA ŽA
˚. R B ŽA
n 1 Žcmy1 .
n 2 Žcmy1 .
K t = 10 5 dynesrcm2
K o = 10 5 dynesrcm2
0 0.1 0.2 0.3 0.4 0.5 0.6
8.328 8.337 8.348 8.360 8.368 8.384 8.392
1.875 1.877 1.879 1.882 1.884 1.887 1.889
2.039 2.042 2.044 2.048 2.049 2.053 2.055
610 605 600 585 580 570 565
432 430 428 425 422 420 418
1.8286 1.8534 1.8563 1.8582 1.8598 1.8752 1.8943
1.1002 1.1210 1.1308 1.1415 1.1512 1.16110 1.1682
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
195
Fig. 2. Variation of lattice parameter with Zn content. Fig. 4. Variation of force constant, K t with bond length, RA .
content as shown in Figs. 2 and 3, respectively. It is well known that increase in bond length reduces the fundamental vibrational frequency. Therefore, the shift in the center frequency of the bands n 1 and n 2 towards the low-frequency side, with the addition of Zn, could be due to the increase in the lattice parameter and bond lengths with the addition of Zn. The force constant is a second-order derivative of potential energy with respect to the bond length, the other independent parameters being kept constant.
The force constants are calculated by using the standard formulae w10x given below
Fig. 3. Variation of bond lengths, Ž RA , R B . containing zinc.
Fig. 5. Variation of force constant, K 0 with bond length, R B .
K 0 s 0.942128 M1 n 22r Ž M1q32 . K t s 0.04416n 12 M2
Ž VrVq3.
Ž 1. Ž 2.
where V s 64.2 M1 urM2 and u s 2 K orŽ n 12 M1 y 2 K o ., K o s force constant on octahedral site, K t s
196
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
force constant on tetrahedral site, M1 s molecular weight of tetrahedral site, M2 s molecular weight of octahedral site, n 1 s the corresponding center frequency on tetrahedral site, and n 2 s the corresponding center frequency on octahedral site. The molecular weights M1 and M2 for each sample can be calculated from the cation distribution. The variation of force constants K t and K o with the corresponding bond lengths R A and R B , respectively, is shown in Figs. 4 and 5. The values of bond lengths R A , R B and the force constants K t , K o are listed in Table 1. From Figs. 4 and 5, it is observed that the force constants K t and K o increase with the increase in bond lengths. It is well known that the increase in bond lengths normally leads to decrease in force constants. However, Srivastava and Srinivasan w11x have observed some unexpected results in Cu–Zn ferrites. They observed an increase in force constant with the bond length. They attributed the results to the fact that oxygen can from under favourable conditions, stronger bonds with the metal ions even at larger internuclear separations. Similar behaviour has been reported for transition metal oxides with atomic numbers in the range of 26–29 w12x.
4. Conclusions IR absorption spectrum show two fundamental bands n 1 and n 2 in the frequency range 600–400
cmy1 , respectively, corresponding to the tetrahedral and octahedral complexes, respectively. These bands shift towards the low-frequency side with the increase in content on Zn, which can be attributed to the increase in lattice parameter and bond lengths R A and R B with the Zn content. The force constants K t and K o also increase with the increase in bond lengths.
References w1x P.V. Reddy, V.D. Reddy, J. Magn. Magn. Mater. 136 Ž1994. 279. w2x S.S. Bellad, R.B. Pujar, B.K. Chougule, Mater. Chem. Phys. 52 Ž1998. 166. w3x K. Radha, D. Ravinder, Indian J. Pure Appl. Phys. 33 Ž1995. 74. w4x D. Ravindra, J. Mater. Sci. Lett. 11 Ž1992. 1948. w5x Y. Purushothan, M.B. Reddy, P.P. Kishan, D. Sagar, V. Reddy, Mater. Lett. 17 Ž1993. 341, ŽNorth-Holland.. w6x S.S. Bellad, B.K. Chougule, Mater. Res. Bull. 33 Ž8. Ž1998. 1165. w7x A.M. Shaikh, S.S. Bellad, B.K. Chougule, J. Magn. Magn. Mater. 195 Ž1999. 384–390. w8x R.S. Patil, S.V. Kakatkar, A.M. Sankpal, S.R. Sawant, Indian J. Pure Appl. Phys. 32 Ž1994. 193. w9x S.A. Patil, S.M. Otari, V.C. Mahajan et al., Solid State Commun. 78 Ž1. Ž1991. 39. w10x S.R. Sawant, S.S. Suryavanshi, Curr. Sci. 57 Ž1998. 12. w11x C.M. Srivastava, G. Srinivasan, J. Appl. Phys. 53 Ž1982. 8148. w12x G. Hertzburg, in: Molecular Spectra Molecular Structure, Van Nostrand, 1950, p. 458.
Materials Letters 44 Ž2000. 192–196 www.elsevier.comrlocatermatlet
Infrared spectral studies of Zn-substituted Li–Mg ferrites A.M. Shaikh ) , S.A. Jadhav, S.C. Watawe, B.K. Chougule Ferrite Materials Research Laboratory, Department of Physics, ShiÕaji UniÕersity, Kolhapur 416 004, India Received 7 December 1999; accepted 14 December 1999
Abstract The infrared spectra of Zn-substituted Li–Mg ferrites, having the general formula Li x Mg 0.4 Zn 0.6y2 x Fe 2qx O4 Žwhere x s 0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3., have been analyzed in the frequency range 200–800 cmy1. The ferrites were prepared by standard double sintering ceramic method and the single-phase formation was confirmed by X-ray diffraction studies. The IR spectra show two fundamental absorption bands n 1 and n 2 in the range 600–400 cmy1, corresponding tetrahedral and octahedral complexes, respectively. The bands n 1 and n 2 are found to shift gradually towards the lower frequency side with addition of Zn, which have been attributed to the increase in the lattice parameter. The bands do not show any sign of splitting indicating no excess formation of Fe 2q ions. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Li–Mg–Zn ferrites; IR spectral studies
1. Introduction Lithium ferrites and lithium-based ferrites have become important materials for microwave applications as a replacement for garnets due to their low costs, squareness of hysteresis loop, high Curie temperature and low dielectric losses w1x. The IR spectroscopy is used to determine the local symmetry in crystalline and non-crystalline solids and also to study the ordering phenomenon in ferrites. The IR absorption bands mainly appear due to vibrations of oxygen ions with cations producing various frequencies of the unit cell. The frequencies depend upon
)
Corresponding author. Fax: q91-231-656133. E-mail address: [email protected] ŽA.M. Shaikh..
cation masses, lattice parameter and cation–oxygen bonding, etc. Many researchers have reported studies on Li–Cd w2,3x, Li–Zn w4x, Li–Mg w5x and Li–Ni w1x ferrites. Bellad and Chougule w6x have studied IR spectra of some mixed Li–Cd ferrites. However, no reports have been found in literature regarding IR studies of Zn-substituted Li–Mg ferrites. Therefore, in the present communication, we report on the infrared studies of these ferrites. The applications of IR spectroscopy to ferrites are to detect the completion of the solid state reaction, to study the cation distribution, to study the deformation of the spinel structure, cation disordering and to study the force constants for tetrahedral and octahedral sites. The ferrites crystallize in their crystallographic form with space group Fd 3 m -O h7 . According to
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 2 5 - 2
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
group theoretical considerations, four infrared active fundamentals were expected in the vibrational spectra of normal as well as inverse spinel ferrites. In the frequency range 200–800 cmy1 , spinel ferrites usually show two absorption bands corresponding to tetrahedral and octahedral sites.
193
2. Experimental Zn-substituted Li–Mg ferrites, having the general formula Li x Mg 0.4 Zn 0.6-2 x Fe 2qxO4 Žwhere x s 0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3., have been prepared by the standard double sintering ceramic
Fig. 1. Infrared spectra of Li–Mg–Zn ferrites.
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
194
n 2 in the range of 430–420 cmy1 as the concentration of Zn increases from 0–60%. The band position obtained in the present case are found to be in the range reported w1x as well as for Li–Cd ferrite w6x. Slight variation in n 1 and n 2 is due to the fact that the method of preparation, grain size and porosity influence the band positions. In addition to this, it is observed that, with the addition of Zn intensity of lower band decreases, the difference between the intensities of the two bands also decreases, and the bands n 1 and n 2 become less pronounced. Absence of double band near 600 cmy1 indicates that excessive Fe 2q ions are not present in the system. Splitting of n 1 and n 2 into shoulders has not been observed in the present system, which also confirms the absence of excessive Fe 2q ions. Similar results have been also observed by Shaikh et al. w7x in the case of Li–Zn–Zr system. The decrease in intensity and increase in broadness are explained on the basis of cation distribution. The cation distribution in the present case is given as 3q 2q 3q . w 1q x 2y ŽZn2q 0.6y2 x Fe 0.9 xq0.7 A Li x Mg 0.4 Fe 0.1 xq1.3 B O4 . It can be seen that there is a 1:3 order of cations on the octahedral site. As the content of Zn increases, Zn2q ions consistently replace Fe 3q ions from A to B site. At the same time, the content of Li 1q ions on the octahedral site decreases. This disturbs the 1:3 order on the octahedral site with increase in Zn content. The disordered system gives rise to broad bands in their spectrum w9x. It is clear from Table 1, as well as from Fig 1, that the center frequency of the principle bands n 1 and n 2 shifts gradually towards the low-frequency side. From the X-ray diffraction study, it is revealed that the lattice parameter Ž a. and bond lengths R A and R B increase linearly with the increase in Zn
method in air medium using AR grade Li 2 CO 3 , MgO, ZnO and Fe 2 O 3 as starting materials. The oxides were mixed in stoichiometric proportions and the mixtures were pre-sintered at 873 K for 12 h. The resultant powders were granulated into fine particles using a agate mortar. The samples were finally sintered at 1273 K for 24 h and were furnace cooled. The completion of solid state reaction was confirmed by X-ray diffraction studies carried out on powder samples with Philips PW-1710 diffractometer using ˚ . radiation. All the samples CuK a Ž l s 1.5418 A show single-phase formation. The IR spectra were recorded on a Perkin-Elemer IR spectrometer Žmodel-783. in the range 200–800 cmy1 in the KBr medium.
3. Results and discussion Fig 1 shows the IR spectra of the samples. All the samples show two prominent absorption bands n 1 and n 2 in the range 600 and 400 cm -1 , respectively. Absorption at n 1 is caused by stretching of tetrahedral metal ion and oxygen bonding, while n 2 is caused by vibrations of oxygen in the direction perpendicular to the axis joining the tetrahedral ion and oxygen. Patil et al. w8x has studied the IR spectra of ferrites and has attributed the occurrence of band n 1 around 600 cmy1 to the intrinsic vibrations of the tetrahedral complexes corresponding to the highest restoring force, and band n 2 around 400 cmy1 to intrinsic vibrations of octahedral complexes, which are bond-bending vibrations. Therefore, it is expected that n 1 ) n 2 . In the present system, the band n 1 is found to lie in the range of 610–565 cmy1 and the lower band
Table 1 Data on lattice parameter Ž a., bond lengths Ž RA , R B ., the positions of IR absorption bands Ž n 1 , n 2 . and the force constants Ž K t , K o . Zn
˚. ‘a’ ŽA
˚. RA ŽA
˚. R B ŽA
n 1 Žcmy1 .
n 2 Žcmy1 .
K t = 10 5 dynesrcm2
K o = 10 5 dynesrcm2
0 0.1 0.2 0.3 0.4 0.5 0.6
8.328 8.337 8.348 8.360 8.368 8.384 8.392
1.875 1.877 1.879 1.882 1.884 1.887 1.889
2.039 2.042 2.044 2.048 2.049 2.053 2.055
610 605 600 585 580 570 565
432 430 428 425 422 420 418
1.8286 1.8534 1.8563 1.8582 1.8598 1.8752 1.8943
1.1002 1.1210 1.1308 1.1415 1.1512 1.16110 1.1682
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
195
Fig. 2. Variation of lattice parameter with Zn content. Fig. 4. Variation of force constant, K t with bond length, RA .
content as shown in Figs. 2 and 3, respectively. It is well known that increase in bond length reduces the fundamental vibrational frequency. Therefore, the shift in the center frequency of the bands n 1 and n 2 towards the low-frequency side, with the addition of Zn, could be due to the increase in the lattice parameter and bond lengths with the addition of Zn. The force constant is a second-order derivative of potential energy with respect to the bond length, the other independent parameters being kept constant.
The force constants are calculated by using the standard formulae w10x given below
Fig. 3. Variation of bond lengths, Ž RA , R B . containing zinc.
Fig. 5. Variation of force constant, K 0 with bond length, R B .
K 0 s 0.942128 M1 n 22r Ž M1q32 . K t s 0.04416n 12 M2
Ž VrVq3.
Ž 1. Ž 2.
where V s 64.2 M1 urM2 and u s 2 K orŽ n 12 M1 y 2 K o ., K o s force constant on octahedral site, K t s
196
A.M. Shaikh et al.r Materials Letters 44 (2000) 192–196
force constant on tetrahedral site, M1 s molecular weight of tetrahedral site, M2 s molecular weight of octahedral site, n 1 s the corresponding center frequency on tetrahedral site, and n 2 s the corresponding center frequency on octahedral site. The molecular weights M1 and M2 for each sample can be calculated from the cation distribution. The variation of force constants K t and K o with the corresponding bond lengths R A and R B , respectively, is shown in Figs. 4 and 5. The values of bond lengths R A , R B and the force constants K t , K o are listed in Table 1. From Figs. 4 and 5, it is observed that the force constants K t and K o increase with the increase in bond lengths. It is well known that the increase in bond lengths normally leads to decrease in force constants. However, Srivastava and Srinivasan w11x have observed some unexpected results in Cu–Zn ferrites. They observed an increase in force constant with the bond length. They attributed the results to the fact that oxygen can from under favourable conditions, stronger bonds with the metal ions even at larger internuclear separations. Similar behaviour has been reported for transition metal oxides with atomic numbers in the range of 26–29 w12x.
4. Conclusions IR absorption spectrum show two fundamental bands n 1 and n 2 in the frequency range 600–400
cmy1 , respectively, corresponding to the tetrahedral and octahedral complexes, respectively. These bands shift towards the low-frequency side with the increase in content on Zn, which can be attributed to the increase in lattice parameter and bond lengths R A and R B with the Zn content. The force constants K t and K o also increase with the increase in bond lengths.
References w1x P.V. Reddy, V.D. Reddy, J. Magn. Magn. Mater. 136 Ž1994. 279. w2x S.S. Bellad, R.B. Pujar, B.K. Chougule, Mater. Chem. Phys. 52 Ž1998. 166. w3x K. Radha, D. Ravinder, Indian J. Pure Appl. Phys. 33 Ž1995. 74. w4x D. Ravindra, J. Mater. Sci. Lett. 11 Ž1992. 1948. w5x Y. Purushothan, M.B. Reddy, P.P. Kishan, D. Sagar, V. Reddy, Mater. Lett. 17 Ž1993. 341, ŽNorth-Holland.. w6x S.S. Bellad, B.K. Chougule, Mater. Res. Bull. 33 Ž8. Ž1998. 1165. w7x A.M. Shaikh, S.S. Bellad, B.K. Chougule, J. Magn. Magn. Mater. 195 Ž1999. 384–390. w8x R.S. Patil, S.V. Kakatkar, A.M. Sankpal, S.R. Sawant, Indian J. Pure Appl. Phys. 32 Ž1994. 193. w9x S.A. Patil, S.M. Otari, V.C. Mahajan et al., Solid State Commun. 78 Ž1. Ž1991. 39. w10x S.R. Sawant, S.S. Suryavanshi, Curr. Sci. 57 Ž1998. 12. w11x C.M. Srivastava, G. Srinivasan, J. Appl. Phys. 53 Ž1982. 8148. w12x G. Hertzburg, in: Molecular Spectra Molecular Structure, Van Nostrand, 1950, p. 458.