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2Fe5O12
Available online at www.sciencedirect.com
Materials Letters 62 (2008) 911 – 913 www.elsevier.com/locate/matlet
Structural and dielectric properties of Y3/2Bi3/2Fe5O12 K. Jawahar, R.N.P. Choudhary ⁎ Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Received 17 April 2007; accepted 6 July 2007 Available online 13 July 2007
Abstract Polycrystalline Y3/2Bi3/2Fe5O12 compound was prepared by a standard high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of a single-phase compound in a tetragonal crystal system at room temperature. The elemental composition of the prepared compound has been confirmed by energy dispersive X-ray spectroscopy (EDS) microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the pellet. Detailed studies of dielectric properties suggest that the compound has frequency dependent dielectric anomaly (with a shift in dielectric peak of different frequencies on increasing temperature). © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; X-ray diffraction; Dielectric; Microwave
1. Introduction Yttrium iron garnet (YIG) is a well-known ferrimagnetic material of the garnet family because of its importance as a microwave material [1]. A lot of work has been carried out in the past on YIG, by partially/fully replacing yttrium by rare earths and other ions [2–7]. Recently, bismuth iron garnet (BIG) based materials are emerging as promising candidates for magnetoelectric, magneto-optical and multifunctional devices [8–11]. It is clear that whatever work carried out on BIG so far, gives much attention on magnetic and magneto-optical measurements [12,13]. It has been reported that the crystallization temperature of YIG decreases on inclusion of Bi3+ ions at the Y3+ site [14]. The main purpose of the present work is to study structural and dielectric properties of a new composition of YIG (i.e., Y3/2Bi3/2Fe5O12).
technique using high purity precursors; Y2O3 (99.9%, M/s Indian Rare Earth Ltd., India), Bi2O3 (99.9%, M/s Loba Chemi Pvt. Ltd., India) and Fe2O3 (99.9%, M/s Loba Chemi Pvt. Ltd., India). These oxides were thoroughly mixed in agate mortar for 2 h in air atmosphere followed by the wet mixing in methanol media for 1 h. The dried (at 150 °C) mixture was calcined at 940 °C (optimized) in an alumina crucible for 8 h. The process of grinding and calcination was repeated until the formation of a single-phase compound was confirmed by XRD. The fine calcined powder was mixed with polyvinyl alcohol (PVA as binder) and pressed into cylindrical pellets of diameter 10 mm and thickness 3 mm, under an isostatic pressure of about 5 × 107 kg/m2 using a hydraulic press. These green pellets were then sintered at 980 °C (optimized) for 6 h in an air atmosphere. The sintered pellets were polished with zero grain emery paper and coated with high purity silver paste. The pellets were dried for 2 h at 150 °C prior to any electrical measurements.
2. Experimental 2.2. Material characterization 2.1. Material preparation The polycrystalline sample of Y3/2Bi3/2Fe5O12 (YBIG) was prepared by a standard high-temperature solid-state reaction ⁎ Corresponding author. Tel.: +91 03222 283814; fax: +91 03222 282282. E-mail address: [email protected] (R.N.P. Choudhary). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.009
X-ray diffraction (XRD) data were collected by using Rigaku X-ray powder diffractometer (model: Miniflex) in a wide range of the Bragg angles (20° ≤ 2θ ≤ 80°) with CuKα (λ = 1.5405 Å). The surface morphology and energy dispersive X-ray spectroscopy (quantitative elemental analysis) were recorded using scanning electron microscope JEOL (model: JSM-5800F). The
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K. Jawahar, R.N.P. Choudhary / Materials Letters 62 (2008) 911–913
3.2. Dielectric properties
Fig. 1. Room temperature XRD pattern of Y3/2Bi3/2Fe5O12.
temperature dependence of dielectric response was measured at some selected frequencies (1, 10, 100 and 1000 kHz) using a computer-controlled Hioki LCR Hitester (model: 3532). 3. Results and discussion 3.1. Structural analysis The sharp and single peaks of the XRD pattern (Fig. 1), which were different in position and intensity from those of ingredients, confirmed the formation of single-phase of a new composition. All the prominent peaks were indexed, and the lattice parameters were refined using the least-squares refinement subroutine of a computer program ‘PowdMult’ [15]. The best agreement between observed (obs) and calculated (cal) interplanar spacing (d) was found in the tetragonal crystal system. However a few small peaks of XRD pattern were identified as pyrochlore phase, which were difficult to remove as in BiFeO3 perovskite [16,17]. In fact, this pyrochlore phase sometimes helps to improve the spontaneous magnetization [18,19]. The refined lattice parameters of YBIG are: a = 11.2755 (24) Å and c = 13.6752 (24) Å (with estimated standard deviation in parenthesis). The Scherrer equation [20] was used to calculate the crystallite/particle size. The average particle size was found in the range of 25–40 nm. The energy dispersive spectroscopy (EDS) was carried out for the verification of the presence of individual elements of YBIG. SEM micrograph of the compound (Fig. 2) confirmed the polycrystalline nature of the material. Highly distinctive, nearly uniform and compactness of grain distributions are the special features of the micrograph.
Fig. 2. SEM micrograph of Y3/2Bi3/2Fe5O12 pellet.
The variation of dielectric constant (ɛ) and loss tangent (tanδ) with temperature at selected frequencies is shown in Fig. 3. A dielectric anomaly or phase transition was observed at 325 °C (Fig. 3a). A shift in the transition temperature towards high temperature side with increase of frequency was observed. The broad dielectric anomaly indicates that the phase transition is of diffuse-type. The broadening of dielectric peaks may be attributed to the disorder and defects present in the system. It may be consider as antiferrodistortive structural phase transition [21]. This phase transition may also be correlated to the structural distortion of the material. The preparation of BIG in its bulk form is thermodynamically unstable [8]. But the inclusion of Y3+ ions at the Bi site improves the structural stability with some distortion. As per ICDD data (Card No. 77-1998), YIG has cubic structure with cell parameter a = 12.3760 Å. This cubic structure has been distorted to tetragonal structure (with cell parameters given above), by the partial replacement of Y3+ at the Bi3+ site in YBIG. It is now inferred that the structural distortion plays a crucial role of the dielectric anomaly in the material. As the maxima of the dielectric constant (ɛmax) drops down from 12,000 (at 1 kHz) to 1500 (at 1 MHz), the influence of interfacial polarization also plays an important role. The charge accumulation at the grain boundaries is responsible for higher values of dielectric constant at low frequencies [22,23]. The loss tangent (Fig. 3b) increases slowly with rise of temperature and attains a maximum value at the
Fig. 3. Variation of dielectric constant ɛ (a) and tanδ (b) with temperature.
K. Jawahar, R.N.P. Choudhary / Materials Letters 62 (2008) 911–913
temperature close to transition temperature and decreases. The low values of tanδ at higher frequencies suggest application of this material for microwave filters/IR detectors.
4. Conclusion A single-phase Y3/2Bi3/2Fe5O12 ceramic (with 2–3% pyrochlore) was prepared by a high-temperature solid-state reaction technique. The microstructure of the material shows the uniform distribution of grains throughout the surface. The prepared compound has a tetragonal structure at room temperature with larger unit cell parameters, which differs from the YIG material. The dielectric anomaly was observed well above the room temperature. The correlation between the dielectric phase transition and the structural phase transition was established on the basis of crystal structure analysis. The accumulation of charges on the grain boundaries will be responsible for the high temperature dielectric relaxation behavior. References [1] A.J. Moulson, J.M. Herbert, Electroceramics, Chapman and Hall, London, 1990, p. 390. [2] D.E. Lacklison, G.B. Scott, J.L. Page, Solid State Commun. 14 (1974) 861–863. [3] J.L. Rehspringer, J. Bursik, D. Niznansky, A. Klarikova, J. Magn. Magn. Mater. 211 (2000) 291–295. [4] K. Matsumoto, S. Sasaki, Y. Asahara, K. Yamaguchi, T. Fujii, J. Magn. Magn. Mater. 104-107 (1992) 451–452.
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[5] T. Tepper, C.A. Ross, J. Cryst. Growth 255 (2003) 324–331. [6] C.Y. Tsay, C.Y. Liu, K.S. Liu, I.N. Lin, L.J. Hu, T.S. Yeh, J. Magn. Magn. Mater. 239 (2002) 490–494. [7] V.D. Murumkar, K.B. Modi, K.M. Jadhav, G.K. Bichile, R.G. Kulkarni, Mater. Lett. 32 (1997) 281–285. [8] T. Fujii, M. Takano, R. Katano, Y. Bando, J. Magn. Magn. Mater. 92 (1990) 261–264. [9] N. Adachi, T. Okuda, V.P. Denysenkov, A.J. Roudsar, A.M. Grishin, J. Magn. Magn. Mater. 242-245 (2002) 775–777. [10] X. Ma, Mater. Lett. 43 (2000) 170–173. [11] J.K. Kim, S.S. Kim, W.J. Kim, Mater. Lett. 59 (2005) 4006–4009. [12] P. Hansen, J.P. Krumme, Thin Solid Films 114 (1984) 69–107. [13] P. Hansen, C.P. Klages, K. Witter, J. Magn. Magn. Mater. 54-57 (1986) 1405–1406. [14] B. Dong, Y. Cui, H. Yang, L. Yu, W. Jin, S. Feng, Mater. Lett. 60 (2006) 2094–2097. [15] E. Wu: POWD, An Interactive Powder Diffraction Data Interpretation and Indexing Program, version 2.1, School of Physical Sciences, Flinders Univ. of South Australia. [16] C.M. Michel, J.M. Moreau, G.D. Achenbach, R. Gerson, W.J. James, Solid. State. Commun. 7 (1969) 701–704. [17] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Appl. Phys. Lett. 87 (2005) 262907. [18] S. Geller, M.A. Gilleo, J. Phys. Chem. Solids 3 (1957) 30–36. [19] S. Geller, R.M. Bozorth, M.A. Gilleo, C.E. Miller, J. Phys. Chem. Solids. 12 (1959) 111–118. [20] B.D. Cullity, Elements of X-ray Diffraction, Addision-Wesley Publishing Co. Inc., 1978. [21] J.F. Scott, JETP Lett. 49 (1989) 233–235. [22] K. Jawahar, R.N.P. Choudhary, Solid State Commun. 142 (2007) 449–452. [23] Y.J. Hsiao, Y.H. Chang, T.H. Fang, Y.S. Chang, Appl. Phys. Lett., 87 (2005) 142906.
Materials Letters 62 (2008) 911 – 913 www.elsevier.com/locate/matlet
Structural and dielectric properties of Y3/2Bi3/2Fe5O12 K. Jawahar, R.N.P. Choudhary ⁎ Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Received 17 April 2007; accepted 6 July 2007 Available online 13 July 2007
Abstract Polycrystalline Y3/2Bi3/2Fe5O12 compound was prepared by a standard high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of a single-phase compound in a tetragonal crystal system at room temperature. The elemental composition of the prepared compound has been confirmed by energy dispersive X-ray spectroscopy (EDS) microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the pellet. Detailed studies of dielectric properties suggest that the compound has frequency dependent dielectric anomaly (with a shift in dielectric peak of different frequencies on increasing temperature). © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramics; X-ray diffraction; Dielectric; Microwave
1. Introduction Yttrium iron garnet (YIG) is a well-known ferrimagnetic material of the garnet family because of its importance as a microwave material [1]. A lot of work has been carried out in the past on YIG, by partially/fully replacing yttrium by rare earths and other ions [2–7]. Recently, bismuth iron garnet (BIG) based materials are emerging as promising candidates for magnetoelectric, magneto-optical and multifunctional devices [8–11]. It is clear that whatever work carried out on BIG so far, gives much attention on magnetic and magneto-optical measurements [12,13]. It has been reported that the crystallization temperature of YIG decreases on inclusion of Bi3+ ions at the Y3+ site [14]. The main purpose of the present work is to study structural and dielectric properties of a new composition of YIG (i.e., Y3/2Bi3/2Fe5O12).
technique using high purity precursors; Y2O3 (99.9%, M/s Indian Rare Earth Ltd., India), Bi2O3 (99.9%, M/s Loba Chemi Pvt. Ltd., India) and Fe2O3 (99.9%, M/s Loba Chemi Pvt. Ltd., India). These oxides were thoroughly mixed in agate mortar for 2 h in air atmosphere followed by the wet mixing in methanol media for 1 h. The dried (at 150 °C) mixture was calcined at 940 °C (optimized) in an alumina crucible for 8 h. The process of grinding and calcination was repeated until the formation of a single-phase compound was confirmed by XRD. The fine calcined powder was mixed with polyvinyl alcohol (PVA as binder) and pressed into cylindrical pellets of diameter 10 mm and thickness 3 mm, under an isostatic pressure of about 5 × 107 kg/m2 using a hydraulic press. These green pellets were then sintered at 980 °C (optimized) for 6 h in an air atmosphere. The sintered pellets were polished with zero grain emery paper and coated with high purity silver paste. The pellets were dried for 2 h at 150 °C prior to any electrical measurements.
2. Experimental 2.2. Material characterization 2.1. Material preparation The polycrystalline sample of Y3/2Bi3/2Fe5O12 (YBIG) was prepared by a standard high-temperature solid-state reaction ⁎ Corresponding author. Tel.: +91 03222 283814; fax: +91 03222 282282. E-mail address: [email protected] (R.N.P. Choudhary). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.009
X-ray diffraction (XRD) data were collected by using Rigaku X-ray powder diffractometer (model: Miniflex) in a wide range of the Bragg angles (20° ≤ 2θ ≤ 80°) with CuKα (λ = 1.5405 Å). The surface morphology and energy dispersive X-ray spectroscopy (quantitative elemental analysis) were recorded using scanning electron microscope JEOL (model: JSM-5800F). The
912
K. Jawahar, R.N.P. Choudhary / Materials Letters 62 (2008) 911–913
3.2. Dielectric properties
Fig. 1. Room temperature XRD pattern of Y3/2Bi3/2Fe5O12.
temperature dependence of dielectric response was measured at some selected frequencies (1, 10, 100 and 1000 kHz) using a computer-controlled Hioki LCR Hitester (model: 3532). 3. Results and discussion 3.1. Structural analysis The sharp and single peaks of the XRD pattern (Fig. 1), which were different in position and intensity from those of ingredients, confirmed the formation of single-phase of a new composition. All the prominent peaks were indexed, and the lattice parameters were refined using the least-squares refinement subroutine of a computer program ‘PowdMult’ [15]. The best agreement between observed (obs) and calculated (cal) interplanar spacing (d) was found in the tetragonal crystal system. However a few small peaks of XRD pattern were identified as pyrochlore phase, which were difficult to remove as in BiFeO3 perovskite [16,17]. In fact, this pyrochlore phase sometimes helps to improve the spontaneous magnetization [18,19]. The refined lattice parameters of YBIG are: a = 11.2755 (24) Å and c = 13.6752 (24) Å (with estimated standard deviation in parenthesis). The Scherrer equation [20] was used to calculate the crystallite/particle size. The average particle size was found in the range of 25–40 nm. The energy dispersive spectroscopy (EDS) was carried out for the verification of the presence of individual elements of YBIG. SEM micrograph of the compound (Fig. 2) confirmed the polycrystalline nature of the material. Highly distinctive, nearly uniform and compactness of grain distributions are the special features of the micrograph.
Fig. 2. SEM micrograph of Y3/2Bi3/2Fe5O12 pellet.
The variation of dielectric constant (ɛ) and loss tangent (tanδ) with temperature at selected frequencies is shown in Fig. 3. A dielectric anomaly or phase transition was observed at 325 °C (Fig. 3a). A shift in the transition temperature towards high temperature side with increase of frequency was observed. The broad dielectric anomaly indicates that the phase transition is of diffuse-type. The broadening of dielectric peaks may be attributed to the disorder and defects present in the system. It may be consider as antiferrodistortive structural phase transition [21]. This phase transition may also be correlated to the structural distortion of the material. The preparation of BIG in its bulk form is thermodynamically unstable [8]. But the inclusion of Y3+ ions at the Bi site improves the structural stability with some distortion. As per ICDD data (Card No. 77-1998), YIG has cubic structure with cell parameter a = 12.3760 Å. This cubic structure has been distorted to tetragonal structure (with cell parameters given above), by the partial replacement of Y3+ at the Bi3+ site in YBIG. It is now inferred that the structural distortion plays a crucial role of the dielectric anomaly in the material. As the maxima of the dielectric constant (ɛmax) drops down from 12,000 (at 1 kHz) to 1500 (at 1 MHz), the influence of interfacial polarization also plays an important role. The charge accumulation at the grain boundaries is responsible for higher values of dielectric constant at low frequencies [22,23]. The loss tangent (Fig. 3b) increases slowly with rise of temperature and attains a maximum value at the
Fig. 3. Variation of dielectric constant ɛ (a) and tanδ (b) with temperature.
K. Jawahar, R.N.P. Choudhary / Materials Letters 62 (2008) 911–913
temperature close to transition temperature and decreases. The low values of tanδ at higher frequencies suggest application of this material for microwave filters/IR detectors.
4. Conclusion A single-phase Y3/2Bi3/2Fe5O12 ceramic (with 2–3% pyrochlore) was prepared by a high-temperature solid-state reaction technique. The microstructure of the material shows the uniform distribution of grains throughout the surface. The prepared compound has a tetragonal structure at room temperature with larger unit cell parameters, which differs from the YIG material. The dielectric anomaly was observed well above the room temperature. The correlation between the dielectric phase transition and the structural phase transition was established on the basis of crystal structure analysis. The accumulation of charges on the grain boundaries will be responsible for the high temperature dielectric relaxation behavior. References [1] A.J. Moulson, J.M. Herbert, Electroceramics, Chapman and Hall, London, 1990, p. 390. [2] D.E. Lacklison, G.B. Scott, J.L. Page, Solid State Commun. 14 (1974) 861–863. [3] J.L. Rehspringer, J. Bursik, D. Niznansky, A. Klarikova, J. Magn. Magn. Mater. 211 (2000) 291–295. [4] K. Matsumoto, S. Sasaki, Y. Asahara, K. Yamaguchi, T. Fujii, J. Magn. Magn. Mater. 104-107 (1992) 451–452.
913
[5] T. Tepper, C.A. Ross, J. Cryst. Growth 255 (2003) 324–331. [6] C.Y. Tsay, C.Y. Liu, K.S. Liu, I.N. Lin, L.J. Hu, T.S. Yeh, J. Magn. Magn. Mater. 239 (2002) 490–494. [7] V.D. Murumkar, K.B. Modi, K.M. Jadhav, G.K. Bichile, R.G. Kulkarni, Mater. Lett. 32 (1997) 281–285. [8] T. Fujii, M. Takano, R. Katano, Y. Bando, J. Magn. Magn. Mater. 92 (1990) 261–264. [9] N. Adachi, T. Okuda, V.P. Denysenkov, A.J. Roudsar, A.M. Grishin, J. Magn. Magn. Mater. 242-245 (2002) 775–777. [10] X. Ma, Mater. Lett. 43 (2000) 170–173. [11] J.K. Kim, S.S. Kim, W.J. Kim, Mater. Lett. 59 (2005) 4006–4009. [12] P. Hansen, J.P. Krumme, Thin Solid Films 114 (1984) 69–107. [13] P. Hansen, C.P. Klages, K. Witter, J. Magn. Magn. Mater. 54-57 (1986) 1405–1406. [14] B. Dong, Y. Cui, H. Yang, L. Yu, W. Jin, S. Feng, Mater. Lett. 60 (2006) 2094–2097. [15] E. Wu: POWD, An Interactive Powder Diffraction Data Interpretation and Indexing Program, version 2.1, School of Physical Sciences, Flinders Univ. of South Australia. [16] C.M. Michel, J.M. Moreau, G.D. Achenbach, R. Gerson, W.J. James, Solid. State. Commun. 7 (1969) 701–704. [17] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Appl. Phys. Lett. 87 (2005) 262907. [18] S. Geller, M.A. Gilleo, J. Phys. Chem. Solids 3 (1957) 30–36. [19] S. Geller, R.M. Bozorth, M.A. Gilleo, C.E. Miller, J. Phys. Chem. Solids. 12 (1959) 111–118. [20] B.D. Cullity, Elements of X-ray Diffraction, Addision-Wesley Publishing Co. Inc., 1978. [21] J.F. Scott, JETP Lett. 49 (1989) 233–235. [22] K. Jawahar, R.N.P. Choudhary, Solid State Commun. 142 (2007) 449–452. [23] Y.J. Hsiao, Y.H. Chang, T.H. Fang, Y.S. Chang, Appl. Phys. Lett., 87 (2005) 142906.