- Email: [email protected]
Synthesis and thermodynamic stability of multiferroic BiFeO3
Materials Letters 62 (2008) 3984–3986
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and thermodynamic stability of multiferroic BiFeO3 T.T. Carvalho, P.B. Tavares ⁎ Centro de Química, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, 5001 - 801 Vila Real, Portugal
A R T I C L E
I N F O
Article history: Received 2 April 2008 Accepted 16 May 2008 Available online 29 May 2008 Keywords: Multiferroics Perovskite Ceramics Crystal structure
A B S T R A C T Ceramic BiFeO3 samples were prepared by the sol gel combustion method using urea as fuel. The obtain powders were thermal treated at different temperatures (300–840 °C) and times (1–64 h) to investigate the best synthesis conditions of the material. The resulting materials were analysed by TGA, FTIR, SEM/EDS and XRD. Rietveld analysis was applied to the diffraction data. The temperature and time of the heat treatment are critical for a high BiFeO3 phase content. Thermal treatment of 1 h at 600 °C yielded 99% molar of the BiFeO3 phase with a mean particle size of 120 nm. Upper or lower calcinations temperatures yielded higher content of the secondary phases Bi2Fe4O9 and Bi25FeO39. Further heat treatment in air or in argon, up to 64 h, induces a decomposition of the BiFeO3 phase according to the reaction 49 BiFeO3 BiFeO3 → 12 Bi2Fe4O9 + Bi25FeO39 pointing out that BiFeO3 is not thermodynamically stable at 600 °C. The BiFeO3 decomposition follows Avrami– Erofeev law with a slope of 1 indicating a one-dimensional kinetics. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials, such as BiFeO3, present simultaneously ferroelectricity, ferromagnetism and/or ferroelasticity and have promising technological application in multiferroic memories, the so-called multiferroic memories [1]. The perovskite BiFeO3 is an attractive material with no lead in its composition, exhibits high Curie temperature (TC = 1103 K) and high Néel temperature (TN = 643 K) with a weak magnetism at room temperature due to a G-type canted spin AFM structure [2,3]. Usually the BiFeO3 (BFO) bulk ceramics presents low resistivity, high leakage current and high coercive electric field, which makes it very difficult to achieve saturated hysteresis loops P(E). This problem is manly caused by the Fe2+ and oxygen vacancies [4,5]. The ceramic BiFeO3 phase pure compound is very difficult to achieve. Secondary phases like Bi2O3, Bi2Fe4O9 and Bi25FeO39 are reported to systematically appear due to the kinetics of phase formation [6,7]. Different synthesis conditions have been reported like the solid-state route [8], the precipitation/coprecipitation [9], the soft chemical route [10], the modified Pechini method [11] and the liquid phase sintering with a high heating rate (100 °C/s) [12,13]. Often the impurity phases are removed with the addition of diluted HNO3 [8]. Quenching processes are reported to suppress the Fe2+ and oxygen vacancies [14]. Phase diagrams of the Bi2O3–Fe2O3 system report the presence of BiFeO3 phase from room temperature up to 830 °C. However, this phase seems to be metastable [15].
⁎ Corresponding author. E-mail address: [email protected] (P.B. Tavares). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.05.051
In this paper we report the synthesis of BiFeO3 using the sol–gel combustion method followed by different heat treatments to investigate the thermodynamic stability of the BiFeO3 phase. 2. Experimental Ceramic samples of BiFeO3 were prepared by the sol–gel combustion method. Experimental details can be seen elsewhere [16]. Analytical grade reagents Bi2O3 (Cerac 99.9%) and Fe2O3 (Cerac
Fig. 1. Thermogravimetric analysis of the uncalcinated powder. Inset the infrared spectra of powders treated at different temperatures.
T.T. Carvalho, P.B. Tavares / Materials Letters 62 (2008) 3984–3986
3985
Table 1 Calculated molar phase amount, at different temperatures of thermal treatment, by Rietveld refinement of the XRD patterns T(°C)
BiFeO3 (%n/n)
Bi2Fe4O9 (%n/n)
Bi25FeO39 (%n/n)
Rwp (%)
550 600 700 760 800 820
99.1 99.7 95.4 94.9 96.7 96.4
0.0 0.0 4.1 4.7 2.6 2.8
0.9 0.3 0.4 0.4 0.7 0.7
19.2 3.4 5.2 6.7 9.6 10.2
Rwp is the statistic parameter that indicates the quality of the refinement.
Fig. 2. X-ray diffraction patterns of samples treated at 500, 600, 700 and 800 °C. The peaks of the BiFeO3 phase are indexed.
99.95%) were dissolved in diluted nitric acid (Fluka 65%). Urea (PANREAC 99.0%) was used as fuel in a molar ratio [urea]/{[Bi] + [Fe]} of 3. After ignition a brown powder was obtained. The powder was pressed into discs and then treated at different temperatures, (300– 800 °C), times (1–64 h) and atmospheres (air and argon). After each treatment the samples were quenched to room temperature placing them in contact with a cold metal. The resulting materials were analysed by Infrared Spectroscopic (UNICAM Research Series), Thermogravimetric Analysis (TA Instruments Q50), X-ray diffraction (PANalytical X’Pert Pro with X’Celerator detector), Scanning Electron Microscopy (FEI Quanta 400). Rietveld analyses were performed to the X-ray diffraction patterns with PowderCell software [17] in order to quantify the phases present, using the structural data and the atomic positions collected from literature [18].
PowderCell software in order to quantify the presented phases. In Table 1 we see that higher molar fractions of the BiFeO3 phase were obtained at lower temperatures (550 and 600 °C). The qualities of the refinements are indicated by the Rwp values that are in the 4–19% range. Higher temperatures increase the secondary phases, Bi2Fe4O9 and Bi25FeO39. The particle size obtained (from the Scherrer equation) is ~ 120 nm for samples treated at 600–760 °C, increasing in samples treated at 780–840 °C due to sinterization. These results are also clearly visible in SEM analysis (not shown). To study the thermodynamic stability of the BiFeO3 phase at 600 °C we performed several thermal treatments in air at this temperature during different times. Fig. 3 shows that for thermal treatments longer than 2 h the BiFeO3 phases decomposes into Bi2Fe4O9 and Bi25FeO39 phases according to the reaction 49BiFeO3 →12Bi2 Fe4 O9 þ Bi25 FeO39 : which indicates that BiFeO3 is not thermodynamically stable at this temperature. The kinetics of the decomposition reaction was studied according to the Avrami– Erofeev model (inset of Fig. 3) [20]. This model yields a slope of −1 for the BiFeO3 decomposition and a slope of 1 for the formation of Bi2Fe4O9 and Bi25FeO39 phases, suggesting that the decomposition reaction is a one-dimensional process. This result is in accordance with SEM/EDS analysis because it was not possible to find individual particles of the secondary phases even in samples treated for 64 h. The same analyses were performed in samples treated with argon atmospheres but the results evidence no difference in XRD pattern in air or argon atmosphere.
4. Conclusions
3. Results and discussion Fig. 1 shows the TG curve of uncalcinated powder that exhibits approximately 40% weight loss from 170 to 470 °C due to the loss of water (25–150 °C), urea decomposition (150–400 °C) and nitrate decomposition (400–470 °C). The nitrate decomposition is confirmed in IR spectrum (inset on Fig. 1), by the disappearance of the peak at 1384 cm− 1 (that corresponds to nitrates) between 400 and 500 °C. The peak at 550 cm− 1 can be assigned to Fe-O stretching and bending vibration from FeO6 octahedral, starting at 300 °C, and completely formed at 500 °C indicative of formation of the perovskite [19]. To promote the formation of BiFeO3 phase, the samples were thermally treated at temperatures ranging from 500 up to 840 °C. Fig. 2 presents the XRD patterns for BiFeO3 powders with different treatment temperatures (500 °C, 600 °C, 700 °C and 800 °C). All the diffraction patterns were analysed by Rietveld refinement with
The sol–gel combustion using urea as a fuel is a suitable and fast method to prepare BiFeO3 powders and ceramics. Higher BiFeO3 phase content can be achieved with a low temperature (600 °C) and fast (1 hr) treatment that minimise the appearance of secondary phases, Bi2Fe4O9 and Bi25FeO39. However BiFeO3 is a metastable phase at 600 °C, which decomposes according to the reaction 49 BiFeO3 BiFeO3 → 12 Bi2Fe4O9 + Bi25FeO39 in a one-dimensional process, so one must take care to avoid long heat treatments. Acknowledgments The authors thank to Fundação para a Ciência e Tecnologia (FCT) for the (i) XRD and TGA equipment (REEQ/1183/CTM/2005), (ii) the project PTDC/CTM/67575/2006 and iii) SFRH/BD/41331/2007. References
Fig. 3. Representation of the kinetic data of the BiFeO3 decomposition. Inset shows the application of the Avrami–Erofeev model to the kinetic data.
[1] Scott JF. Nat Maters 2007;6:256. [2] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Science 2003;299:1719. [3] Neaton JB, Ederer C, Waghmare UV, Spaldin NA, Rabe KM. Phys Rev 2005;B 71:14113. [4] Das SR, Choudhary RN, Bhattacharya P, Katiyar RS, Dutta P, Manivannan A, et al. J Appl Phys 2007;101:34104. [5] Qi X, Dho J, Tomov R, Blamire MG, MacManus-Driscoll JL. Appl Phys Lett 2005;86:62903. [6] Chen C, Cheng J, Yu S, Che L, Meng Z. J Cryst Growth 2006;291:135. [7] Nalwa KS, Garg A, Upadhyaya A. Mater Lett 2007;62:878. [8] Kumar MM, Palkar VR, Srinivas K, Suryanarayana SV. Appl Phys Lett 2000;76:2764. [9] Ghosh S, Dasgupta S, Sen A, Maiti HS. J Am Ceram Soc 2005;88:1349. [10] Ghosh S, Dasgupta S, Sen A, Maiti HS. Mater Res Bull 2005;40:2073. [11] Jiang Qing-hui, Nan Ce-wen, Wang Yao, Liu Yu-heng, Shen Zhi-jian. J Electroceramics in press, doi:10.1007/s10832-007-9265-5. [12] Wang YP, Zhou L, Zhang MF, Chen XY, Liu JM, Liu ZG. Appl Phys Lett 2004;84:1731. [13] Pradhan AK, Zhang K, Hunter D, Dadson JB, Loutts GB, Bhattacharya P, et al. J Appl Phys 2005;97:93903.
3986 [14] [15] [16] [17]
T.T. Carvalho, P.B. Tavares / Materials Letters 62 (2008) 3984–3986
Zhang ST, Lu MH, Wu D, Chen YF, Ming NB. Appl Phys Lett 2005;87:262907. Morozov MI, Lomanova NA, Gusarov VV. Russ J Gen Chem 2003;73:1676. Mardare CC, Tavares PB, Mardare1 AI, Savu R. Mat Sci Forum 2006;514-516:328. Kraus W, Nolze G, Powder Cell for Windows, version 2.4, 2000, available at http:// www.ccp14.ac.uk/ccp/web-mirrors/powdcell/a_v/v_1/powder/e_cell.html.
[18] Institute of Experimental Mineralogy, Russian Academy of Sciences, http://www. iem.ac.ru/, and American Mineralogist Crystal Structure Database, 2008, http:// rruff.geo.arizona.edu/AMS/amcsd.php. [19] Chen C, Cheng J, Yu S, Che L, Meng Z. J Cryst Growth 2006;291:135. [20] West AR. Solid State Chemistry and its Applications. 2nd ed. John Wiley; 1989.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and thermodynamic stability of multiferroic BiFeO3 T.T. Carvalho, P.B. Tavares ⁎ Centro de Química, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, 5001 - 801 Vila Real, Portugal
A R T I C L E
I N F O
Article history: Received 2 April 2008 Accepted 16 May 2008 Available online 29 May 2008 Keywords: Multiferroics Perovskite Ceramics Crystal structure
A B S T R A C T Ceramic BiFeO3 samples were prepared by the sol gel combustion method using urea as fuel. The obtain powders were thermal treated at different temperatures (300–840 °C) and times (1–64 h) to investigate the best synthesis conditions of the material. The resulting materials were analysed by TGA, FTIR, SEM/EDS and XRD. Rietveld analysis was applied to the diffraction data. The temperature and time of the heat treatment are critical for a high BiFeO3 phase content. Thermal treatment of 1 h at 600 °C yielded 99% molar of the BiFeO3 phase with a mean particle size of 120 nm. Upper or lower calcinations temperatures yielded higher content of the secondary phases Bi2Fe4O9 and Bi25FeO39. Further heat treatment in air or in argon, up to 64 h, induces a decomposition of the BiFeO3 phase according to the reaction 49 BiFeO3 BiFeO3 → 12 Bi2Fe4O9 + Bi25FeO39 pointing out that BiFeO3 is not thermodynamically stable at 600 °C. The BiFeO3 decomposition follows Avrami– Erofeev law with a slope of 1 indicating a one-dimensional kinetics. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials, such as BiFeO3, present simultaneously ferroelectricity, ferromagnetism and/or ferroelasticity and have promising technological application in multiferroic memories, the so-called multiferroic memories [1]. The perovskite BiFeO3 is an attractive material with no lead in its composition, exhibits high Curie temperature (TC = 1103 K) and high Néel temperature (TN = 643 K) with a weak magnetism at room temperature due to a G-type canted spin AFM structure [2,3]. Usually the BiFeO3 (BFO) bulk ceramics presents low resistivity, high leakage current and high coercive electric field, which makes it very difficult to achieve saturated hysteresis loops P(E). This problem is manly caused by the Fe2+ and oxygen vacancies [4,5]. The ceramic BiFeO3 phase pure compound is very difficult to achieve. Secondary phases like Bi2O3, Bi2Fe4O9 and Bi25FeO39 are reported to systematically appear due to the kinetics of phase formation [6,7]. Different synthesis conditions have been reported like the solid-state route [8], the precipitation/coprecipitation [9], the soft chemical route [10], the modified Pechini method [11] and the liquid phase sintering with a high heating rate (100 °C/s) [12,13]. Often the impurity phases are removed with the addition of diluted HNO3 [8]. Quenching processes are reported to suppress the Fe2+ and oxygen vacancies [14]. Phase diagrams of the Bi2O3–Fe2O3 system report the presence of BiFeO3 phase from room temperature up to 830 °C. However, this phase seems to be metastable [15].
⁎ Corresponding author. E-mail address: [email protected] (P.B. Tavares). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.05.051
In this paper we report the synthesis of BiFeO3 using the sol–gel combustion method followed by different heat treatments to investigate the thermodynamic stability of the BiFeO3 phase. 2. Experimental Ceramic samples of BiFeO3 were prepared by the sol–gel combustion method. Experimental details can be seen elsewhere [16]. Analytical grade reagents Bi2O3 (Cerac 99.9%) and Fe2O3 (Cerac
Fig. 1. Thermogravimetric analysis of the uncalcinated powder. Inset the infrared spectra of powders treated at different temperatures.
T.T. Carvalho, P.B. Tavares / Materials Letters 62 (2008) 3984–3986
3985
Table 1 Calculated molar phase amount, at different temperatures of thermal treatment, by Rietveld refinement of the XRD patterns T(°C)
BiFeO3 (%n/n)
Bi2Fe4O9 (%n/n)
Bi25FeO39 (%n/n)
Rwp (%)
550 600 700 760 800 820
99.1 99.7 95.4 94.9 96.7 96.4
0.0 0.0 4.1 4.7 2.6 2.8
0.9 0.3 0.4 0.4 0.7 0.7
19.2 3.4 5.2 6.7 9.6 10.2
Rwp is the statistic parameter that indicates the quality of the refinement.
Fig. 2. X-ray diffraction patterns of samples treated at 500, 600, 700 and 800 °C. The peaks of the BiFeO3 phase are indexed.
99.95%) were dissolved in diluted nitric acid (Fluka 65%). Urea (PANREAC 99.0%) was used as fuel in a molar ratio [urea]/{[Bi] + [Fe]} of 3. After ignition a brown powder was obtained. The powder was pressed into discs and then treated at different temperatures, (300– 800 °C), times (1–64 h) and atmospheres (air and argon). After each treatment the samples were quenched to room temperature placing them in contact with a cold metal. The resulting materials were analysed by Infrared Spectroscopic (UNICAM Research Series), Thermogravimetric Analysis (TA Instruments Q50), X-ray diffraction (PANalytical X’Pert Pro with X’Celerator detector), Scanning Electron Microscopy (FEI Quanta 400). Rietveld analyses were performed to the X-ray diffraction patterns with PowderCell software [17] in order to quantify the phases present, using the structural data and the atomic positions collected from literature [18].
PowderCell software in order to quantify the presented phases. In Table 1 we see that higher molar fractions of the BiFeO3 phase were obtained at lower temperatures (550 and 600 °C). The qualities of the refinements are indicated by the Rwp values that are in the 4–19% range. Higher temperatures increase the secondary phases, Bi2Fe4O9 and Bi25FeO39. The particle size obtained (from the Scherrer equation) is ~ 120 nm for samples treated at 600–760 °C, increasing in samples treated at 780–840 °C due to sinterization. These results are also clearly visible in SEM analysis (not shown). To study the thermodynamic stability of the BiFeO3 phase at 600 °C we performed several thermal treatments in air at this temperature during different times. Fig. 3 shows that for thermal treatments longer than 2 h the BiFeO3 phases decomposes into Bi2Fe4O9 and Bi25FeO39 phases according to the reaction 49BiFeO3 →12Bi2 Fe4 O9 þ Bi25 FeO39 : which indicates that BiFeO3 is not thermodynamically stable at this temperature. The kinetics of the decomposition reaction was studied according to the Avrami– Erofeev model (inset of Fig. 3) [20]. This model yields a slope of −1 for the BiFeO3 decomposition and a slope of 1 for the formation of Bi2Fe4O9 and Bi25FeO39 phases, suggesting that the decomposition reaction is a one-dimensional process. This result is in accordance with SEM/EDS analysis because it was not possible to find individual particles of the secondary phases even in samples treated for 64 h. The same analyses were performed in samples treated with argon atmospheres but the results evidence no difference in XRD pattern in air or argon atmosphere.
4. Conclusions
3. Results and discussion Fig. 1 shows the TG curve of uncalcinated powder that exhibits approximately 40% weight loss from 170 to 470 °C due to the loss of water (25–150 °C), urea decomposition (150–400 °C) and nitrate decomposition (400–470 °C). The nitrate decomposition is confirmed in IR spectrum (inset on Fig. 1), by the disappearance of the peak at 1384 cm− 1 (that corresponds to nitrates) between 400 and 500 °C. The peak at 550 cm− 1 can be assigned to Fe-O stretching and bending vibration from FeO6 octahedral, starting at 300 °C, and completely formed at 500 °C indicative of formation of the perovskite [19]. To promote the formation of BiFeO3 phase, the samples were thermally treated at temperatures ranging from 500 up to 840 °C. Fig. 2 presents the XRD patterns for BiFeO3 powders with different treatment temperatures (500 °C, 600 °C, 700 °C and 800 °C). All the diffraction patterns were analysed by Rietveld refinement with
The sol–gel combustion using urea as a fuel is a suitable and fast method to prepare BiFeO3 powders and ceramics. Higher BiFeO3 phase content can be achieved with a low temperature (600 °C) and fast (1 hr) treatment that minimise the appearance of secondary phases, Bi2Fe4O9 and Bi25FeO39. However BiFeO3 is a metastable phase at 600 °C, which decomposes according to the reaction 49 BiFeO3 BiFeO3 → 12 Bi2Fe4O9 + Bi25FeO39 in a one-dimensional process, so one must take care to avoid long heat treatments. Acknowledgments The authors thank to Fundação para a Ciência e Tecnologia (FCT) for the (i) XRD and TGA equipment (REEQ/1183/CTM/2005), (ii) the project PTDC/CTM/67575/2006 and iii) SFRH/BD/41331/2007. References
Fig. 3. Representation of the kinetic data of the BiFeO3 decomposition. Inset shows the application of the Avrami–Erofeev model to the kinetic data.
[1] Scott JF. Nat Maters 2007;6:256. [2] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Science 2003;299:1719. [3] Neaton JB, Ederer C, Waghmare UV, Spaldin NA, Rabe KM. Phys Rev 2005;B 71:14113. [4] Das SR, Choudhary RN, Bhattacharya P, Katiyar RS, Dutta P, Manivannan A, et al. J Appl Phys 2007;101:34104. [5] Qi X, Dho J, Tomov R, Blamire MG, MacManus-Driscoll JL. Appl Phys Lett 2005;86:62903. [6] Chen C, Cheng J, Yu S, Che L, Meng Z. J Cryst Growth 2006;291:135. [7] Nalwa KS, Garg A, Upadhyaya A. Mater Lett 2007;62:878. [8] Kumar MM, Palkar VR, Srinivas K, Suryanarayana SV. Appl Phys Lett 2000;76:2764. [9] Ghosh S, Dasgupta S, Sen A, Maiti HS. J Am Ceram Soc 2005;88:1349. [10] Ghosh S, Dasgupta S, Sen A, Maiti HS. Mater Res Bull 2005;40:2073. [11] Jiang Qing-hui, Nan Ce-wen, Wang Yao, Liu Yu-heng, Shen Zhi-jian. J Electroceramics in press, doi:10.1007/s10832-007-9265-5. [12] Wang YP, Zhou L, Zhang MF, Chen XY, Liu JM, Liu ZG. Appl Phys Lett 2004;84:1731. [13] Pradhan AK, Zhang K, Hunter D, Dadson JB, Loutts GB, Bhattacharya P, et al. J Appl Phys 2005;97:93903.
3986 [14] [15] [16] [17]
T.T. Carvalho, P.B. Tavares / Materials Letters 62 (2008) 3984–3986
Zhang ST, Lu MH, Wu D, Chen YF, Ming NB. Appl Phys Lett 2005;87:262907. Morozov MI, Lomanova NA, Gusarov VV. Russ J Gen Chem 2003;73:1676. Mardare CC, Tavares PB, Mardare1 AI, Savu R. Mat Sci Forum 2006;514-516:328. Kraus W, Nolze G, Powder Cell for Windows, version 2.4, 2000, available at http:// www.ccp14.ac.uk/ccp/web-mirrors/powdcell/a_v/v_1/powder/e_cell.html.
[18] Institute of Experimental Mineralogy, Russian Academy of Sciences, http://www. iem.ac.ru/, and American Mineralogist Crystal Structure Database, 2008, http:// rruff.geo.arizona.edu/AMS/amcsd.php. [19] Chen C, Cheng J, Yu S, Che L, Meng Z. J Cryst Growth 2006;291:135. [20] West AR. Solid State Chemistry and its Applications. 2nd ed. John Wiley; 1989.