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The phase transition and phase stability of magnetoelectric BiFeO3
Materials Science and Engineering A 438–440 (2006) 346–349
The phase transition and phase stability of magnetoelectric BiFeO3 M.C. Li a,b,∗ , Judith Driscoll b , L.H. Liu c , L.C. Zhao a a
School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, China b Department of Materials Science, University of Cambridge, Cambridge, UK c Fasten Group, Tongjiang North Road, Jiangyin, Jiangsu, China Received 22 April 2005; received in revised form 29 November 2005; accepted 16 January 2006
Abstract A study of phase stability of BiFeO3 has been carried out employing a solid-state ionic titration technique, which involved yttria-stabilized zirconia (YSZ) solid electrolyte. As oxygen was intermittently titrated out of the sample, the oxygen activity monotonically decreased in the system until the oxygen nonstoichiometry in the BiFeO3 phase reached its limiting value. We have observed the appearance of a plateau at this point where the BiFeO3 phase started to undergo a decomposition reaction. X-ray diffraction data show that BiFeO3 phase is transforming to a new Bi25 FeO40 phase. Especially, an interesting result is that Bi2 Fe4 O9 was determined in the decomposed components. Therefore, further research is needed about the decomposition reaction of BiFeO3 . This work is useful for the production of novel devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Phase stability; BiFeO3 ; Multiferroic; Magnetoelectric materials
1. Introduction In recent years there is an increasing interest in a new class of materials, in which both electrical and magnetic ordering can coexist simultaneously. Such so-called magnetoelectric materials are rare in nature, but they have many potential applications including new memory and sensor devices. BiFeO3 is one of only a few materials in which (anti)ferromagnetism and ferroelectricity coexist. In order to understand the factors which couple the charge and magnetic order, it is necessary to undertake the kinetics and stability of phase transformation. BiFeO3 possesses a rhombohedrally distorted perovskite structure with space group R3m and a = 0.396 nm, α = 89.5◦ [1,2]. Bulk BiFeO3 materials was reported to be antiferromagnetic below the Neel temperature of 647 K and ferroelectric with a high Curie temperature of 1043 K [3,4]. At first, the isopleth Bi2 O3 –Fe2 O3 , which includes several corrosion products of the T91 was characterized. In more recent works concerning Bi2 O3 –Fe2 O3 , physico-chemical characteristics have been determined without questioning the phase equilibria. But the phase equilibria affect the preparation of the novel materials. We carried out the systematical researches of phase stability of BiFeO3 using a solid-state ionic titration tech∗
Corresponding author. Tel.: +86 451 86418745; fax: +86 451 86415776. E-mail address: [email protected] (M.C. Li).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.117
nique, which involved the yttria-stabilized zirconia (YSZ) solid electrolyte. 2. Experimental The purity of raw materials used was 99.9 wt.%. Mixtures of oxides were prepared under air by weighting and mixing the components in an agate mortar before introducing them into quartz tubes, which were then flowed by Ar gas to limit weight losses, i.e., composition changes during heat treatments. The first treatment of heating at 4 K/min is up to 580 ◦ C, leaving the sample for 8 h at that temperature. The second treatment of heating at 4 K/min is up to 800 ◦ C, followed by 4 h at that temperature, and then cooling at 4 K/min ensuring the reaction between the components. The temperature cycles are composed of heating and cooling at a rate of 4 K/min with an annealing of 12 h all together, at temperatures of 580 and 800 ◦ C. To carry out the systematical studies of phase stability of BiFeO3 , a coulometric titration system is used. The apparatus consists of a long quartz tube with an yttria-stabilized zirconia YSZ electrolyte tube sealed into the end. A sample is placed in a crucible at the other end of the tube. This end is sealed with a thermocouple tube and a rubber O-ring. Two clam furnaces and four thermocouples are used to control the temperatures of the system. One clam shell furnace is used to hold the YSZ
M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
347
Fig. 3. XRD pattern of BiFeO3 after titration at 800 ◦ C.
Fig. 1. Oxygen partial pressure vs. the amount of oxygen (charge) removed for sample at 800 ◦ C. The titration curve demonstrates that there is an oxygen pressure plateaus near BiFeO3 .
at a stable temperature, with one thermocouple in the furnace and another pushed into the end of the YSZ tube. The second clam shell furnace is used to heat the sample independently of the temperature of the YSZ. Again, one thermocouple is in the furnace and the other is held in the thermocouple tube as close to the sample as possible. The temperature of the YSZ and sample can be controlled to ±1 ◦ C. Pt paint was applied to the ends of the YSZ tube. Pt wire was then wrapped around the outside end of the tube to form the outer electrode, and a coil of Pt wire on the end of a fine quartz tube was held onto the inside of the YSZ tube by a spring to form the inner electrode. Wires from these electrodes led to a potentiostat. The YSZ has two uses. Firstly,
to measure the oxygen partial pressure within the tube, which is carried out by measuring the potential difference between the outer and inner electrode. Secondly, to titrate oxygen into or out of the system by applying a voltage across the electrodes as represented by the magnified view of the tip of the YSZ tube. The potential E, across the YSZ electrodes is related to the difference in oxygen partial pressures at the two sides of the YSZ electrodes by the Nernst equation: pO12 −zEF = RT ln (1) pO22 where z is the number of moles of electrons transferred per mole of oxygen. In this case, for the reaction O2 + 4e− → 2O2−
(2)
z = 4, F is the Faraday constant, R the molar gas constant, T the absolute temperature of the YSZ electrolyte, pO12 the oxygen partial pressure within the system, and pO22 is the reference oxygen partial pressure (=0.21 atm). The oxygen partial pressure within the tube can be determined to an accuracy of ±0.5% for oxygen partial pressures in the region of 10−5 atm when the YSZ is held at 800 ◦ C. The whole system is controlled using a potentiostat in parallel with a digital voltmeter (DVM). The potentiostat is used to fix a potential across the YSZ electrodes, and the DVM is used to monitor the pO2 in the system during processing. Table 1 The at.% content of BiFeO3 before and after titration Element
Bi–M
O–K
Fe–K
Before titration (at.%)
19.80
61.03
19.18
Ratio After titration (at.%)
Fig. 2. XRD pattern of BiFeO3 before titration.
1 23.17 21.38 37.15 44.15 42.75
3 54.35 57.83 60.17 54.16 55.29
1 21.89 20.78 2.68 1.70 1.96
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M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
Fig. 4. SEM micrograph of BiFeO3 before titration (a), and after titration (b).
The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS).
Before the coulometric titration, the pure BiFeO3 phase was determined by XRD, as shown by Fig. 2. The cell parameters were calculated as following:
3. Results and discussions
a = b = 0.5574 nm,
The Bi2 O3 –Fe2 O3 phase diagram was determined by Koizumi et al. [5] and Speranskaya et al. [6], respectively. But there are two contradictory versions; especially in the intermediate compound near Bi2 O3 , the invariants, and the allotropic transformation in BiFeO3 . Based on the study of phase equilibria, phase physico-chemical characteristics have been analyzed. Coulometric titration is a method for precisely changing the composition of a sample by passing a readily controlled and measured amount of charge through an appropriately designed electrochemical cell. The compositional variation of thermodynamic parameters is obtained by doing the titration step wise and equilibrating between each step. This technique was first determined on Ag2 S in 1953 [7]. The equilibration time often expanded to several days. However, when new phases formed during invariant reactions, oxygen was titrated both in and out of the samples on the oxygen pressure plateaus to ensure that the plateaus were real and reproducible. Fig. 1 shows the oxygen partial pressure versus the amount of oxygen (charge) removed for sample at 800 ◦ C. This titration curve demonstrates that there is an oxygen pressure plateau near BiFeO3 . And the plateau corresponds to an invariant reaction. The reactions occurring at the oxygen pressure plateau were determined by quenching samples from just above and just below plateau pressure and identifying the phases present. X-ray diffraction and visual observation were used for phase identification in these cases. When the sample was not wrapped by silver during the titration at 800 ◦ C, the new phase of Bi2 Fe4 O9 was found, which was mentioned in Reference [6], and in Reference [5] there are no this phase. In this process of titration without silver wrap, Bi was evaporated, therefore the Bi2 Fe4 O9 became forming.
and
c = 1.3864 nm
It was called as -BiFeO3 , which is rhombohedral (R3c). After three titrations, there is a plateau. The sample was quenching from just above or just below plateau pressure. The phase was identified by X-ray diffraction as shown by Fig. 3. From the patterns, it can be seen that there is still some part of BiFeO3 existing. Its cell parameters were shown as below: a = b = 0.5588 nm,
and
c = 1.3897 nm
It was still -BiFeO3 [8], which is rhombohedral (R3c). In Fig. 3, there are many patterns for other phases. It is hard to determine the exact type of phase. Therefore, the energy disperse spectroscopy (EDS) was used to investigate the possible compositions and phases. From Table 1 we know that after titration, some pars of BiFeO3 phase still leave, while there are some new phases, including Bi25 FeO40 , which was determined from Fig. 3 by XRD. Fig. 4 is the SEM graphs of BiFeO3 before and after titration. It can be seen that the scale of BiFeO3 crystals increases after the titration. And there are some little crystals of other new phases in the interfaces. 4. Conclusions Phase equilibria near BiFeO3 in reduced oxygen pressures were investigated by coulometric titration. The results show that the phase relations near BiFeO3 depend strongly on oxygen pressure. The titration curve shows an invariant reaction. And Bi25 FeO40 is one of the producing phases from BiFeO3 . In the process, the Bi2 Fe4 O9 was determined also. There are still some points that need clarification, including the detailed reaction process and determination of other phases.
M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
Acknowledgements This study is supported in part by project 50272019 funded by the National Natural Science Foundation of China. And the U.K. Royal Society is also appreciated. References [1] A.G. Tutov, Fiz Tverd. Tela (FTVTA) 11 (1969) 2681–2684. [2] G.D. Achenbach, R. Gerson, W.J. James, J. Am. Ceram. Soc. 50 (1967) 437–439.
349
[3] V.A. Murashav, D.N. Rakov, V.M. Ionov, L.S. Dubenko, Y.U. Titov, Ferroelectrics 162 (1994) 11–14. [4] Yu.F. Popov, A.M. Kadomtseva, G.P. Vorobev, A.K. Zvezdin, Ferroelectrics 162 (1994) 135–137. [5] H. Koizumi, N. Nirizaki, T. Ikeda, Jpn. Appl. Phys. 3 (1964) 495– 496. [6] E.J. Speranskaya, V.M. Skorikov, E.Ya. Kode, V.A. Terektova, Bull. Acad. Sci. U.S.S.R. 5 (1965) 873–874. [7] C. Wagner, J. Chem. Phys. 21 (1953) 1819–1821. [8] A. Maitre, M. Francois, J.C. Gachon, J. Phase Equilib. Diffusion 25 (2004) 59–67.
The phase transition and phase stability of magnetoelectric BiFeO3 M.C. Li a,b,∗ , Judith Driscoll b , L.H. Liu c , L.C. Zhao a a
School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, China b Department of Materials Science, University of Cambridge, Cambridge, UK c Fasten Group, Tongjiang North Road, Jiangyin, Jiangsu, China Received 22 April 2005; received in revised form 29 November 2005; accepted 16 January 2006
Abstract A study of phase stability of BiFeO3 has been carried out employing a solid-state ionic titration technique, which involved yttria-stabilized zirconia (YSZ) solid electrolyte. As oxygen was intermittently titrated out of the sample, the oxygen activity monotonically decreased in the system until the oxygen nonstoichiometry in the BiFeO3 phase reached its limiting value. We have observed the appearance of a plateau at this point where the BiFeO3 phase started to undergo a decomposition reaction. X-ray diffraction data show that BiFeO3 phase is transforming to a new Bi25 FeO40 phase. Especially, an interesting result is that Bi2 Fe4 O9 was determined in the decomposed components. Therefore, further research is needed about the decomposition reaction of BiFeO3 . This work is useful for the production of novel devices. © 2006 Elsevier B.V. All rights reserved. Keywords: Phase stability; BiFeO3 ; Multiferroic; Magnetoelectric materials
1. Introduction In recent years there is an increasing interest in a new class of materials, in which both electrical and magnetic ordering can coexist simultaneously. Such so-called magnetoelectric materials are rare in nature, but they have many potential applications including new memory and sensor devices. BiFeO3 is one of only a few materials in which (anti)ferromagnetism and ferroelectricity coexist. In order to understand the factors which couple the charge and magnetic order, it is necessary to undertake the kinetics and stability of phase transformation. BiFeO3 possesses a rhombohedrally distorted perovskite structure with space group R3m and a = 0.396 nm, α = 89.5◦ [1,2]. Bulk BiFeO3 materials was reported to be antiferromagnetic below the Neel temperature of 647 K and ferroelectric with a high Curie temperature of 1043 K [3,4]. At first, the isopleth Bi2 O3 –Fe2 O3 , which includes several corrosion products of the T91 was characterized. In more recent works concerning Bi2 O3 –Fe2 O3 , physico-chemical characteristics have been determined without questioning the phase equilibria. But the phase equilibria affect the preparation of the novel materials. We carried out the systematical researches of phase stability of BiFeO3 using a solid-state ionic titration tech∗
Corresponding author. Tel.: +86 451 86418745; fax: +86 451 86415776. E-mail address: [email protected] (M.C. Li).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.117
nique, which involved the yttria-stabilized zirconia (YSZ) solid electrolyte. 2. Experimental The purity of raw materials used was 99.9 wt.%. Mixtures of oxides were prepared under air by weighting and mixing the components in an agate mortar before introducing them into quartz tubes, which were then flowed by Ar gas to limit weight losses, i.e., composition changes during heat treatments. The first treatment of heating at 4 K/min is up to 580 ◦ C, leaving the sample for 8 h at that temperature. The second treatment of heating at 4 K/min is up to 800 ◦ C, followed by 4 h at that temperature, and then cooling at 4 K/min ensuring the reaction between the components. The temperature cycles are composed of heating and cooling at a rate of 4 K/min with an annealing of 12 h all together, at temperatures of 580 and 800 ◦ C. To carry out the systematical studies of phase stability of BiFeO3 , a coulometric titration system is used. The apparatus consists of a long quartz tube with an yttria-stabilized zirconia YSZ electrolyte tube sealed into the end. A sample is placed in a crucible at the other end of the tube. This end is sealed with a thermocouple tube and a rubber O-ring. Two clam furnaces and four thermocouples are used to control the temperatures of the system. One clam shell furnace is used to hold the YSZ
M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
347
Fig. 3. XRD pattern of BiFeO3 after titration at 800 ◦ C.
Fig. 1. Oxygen partial pressure vs. the amount of oxygen (charge) removed for sample at 800 ◦ C. The titration curve demonstrates that there is an oxygen pressure plateaus near BiFeO3 .
at a stable temperature, with one thermocouple in the furnace and another pushed into the end of the YSZ tube. The second clam shell furnace is used to heat the sample independently of the temperature of the YSZ. Again, one thermocouple is in the furnace and the other is held in the thermocouple tube as close to the sample as possible. The temperature of the YSZ and sample can be controlled to ±1 ◦ C. Pt paint was applied to the ends of the YSZ tube. Pt wire was then wrapped around the outside end of the tube to form the outer electrode, and a coil of Pt wire on the end of a fine quartz tube was held onto the inside of the YSZ tube by a spring to form the inner electrode. Wires from these electrodes led to a potentiostat. The YSZ has two uses. Firstly,
to measure the oxygen partial pressure within the tube, which is carried out by measuring the potential difference between the outer and inner electrode. Secondly, to titrate oxygen into or out of the system by applying a voltage across the electrodes as represented by the magnified view of the tip of the YSZ tube. The potential E, across the YSZ electrodes is related to the difference in oxygen partial pressures at the two sides of the YSZ electrodes by the Nernst equation: pO12 −zEF = RT ln (1) pO22 where z is the number of moles of electrons transferred per mole of oxygen. In this case, for the reaction O2 + 4e− → 2O2−
(2)
z = 4, F is the Faraday constant, R the molar gas constant, T the absolute temperature of the YSZ electrolyte, pO12 the oxygen partial pressure within the system, and pO22 is the reference oxygen partial pressure (=0.21 atm). The oxygen partial pressure within the tube can be determined to an accuracy of ±0.5% for oxygen partial pressures in the region of 10−5 atm when the YSZ is held at 800 ◦ C. The whole system is controlled using a potentiostat in parallel with a digital voltmeter (DVM). The potentiostat is used to fix a potential across the YSZ electrodes, and the DVM is used to monitor the pO2 in the system during processing. Table 1 The at.% content of BiFeO3 before and after titration Element
Bi–M
O–K
Fe–K
Before titration (at.%)
19.80
61.03
19.18
Ratio After titration (at.%)
Fig. 2. XRD pattern of BiFeO3 before titration.
1 23.17 21.38 37.15 44.15 42.75
3 54.35 57.83 60.17 54.16 55.29
1 21.89 20.78 2.68 1.70 1.96
348
M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
Fig. 4. SEM micrograph of BiFeO3 before titration (a), and after titration (b).
The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS).
Before the coulometric titration, the pure BiFeO3 phase was determined by XRD, as shown by Fig. 2. The cell parameters were calculated as following:
3. Results and discussions
a = b = 0.5574 nm,
The Bi2 O3 –Fe2 O3 phase diagram was determined by Koizumi et al. [5] and Speranskaya et al. [6], respectively. But there are two contradictory versions; especially in the intermediate compound near Bi2 O3 , the invariants, and the allotropic transformation in BiFeO3 . Based on the study of phase equilibria, phase physico-chemical characteristics have been analyzed. Coulometric titration is a method for precisely changing the composition of a sample by passing a readily controlled and measured amount of charge through an appropriately designed electrochemical cell. The compositional variation of thermodynamic parameters is obtained by doing the titration step wise and equilibrating between each step. This technique was first determined on Ag2 S in 1953 [7]. The equilibration time often expanded to several days. However, when new phases formed during invariant reactions, oxygen was titrated both in and out of the samples on the oxygen pressure plateaus to ensure that the plateaus were real and reproducible. Fig. 1 shows the oxygen partial pressure versus the amount of oxygen (charge) removed for sample at 800 ◦ C. This titration curve demonstrates that there is an oxygen pressure plateau near BiFeO3 . And the plateau corresponds to an invariant reaction. The reactions occurring at the oxygen pressure plateau were determined by quenching samples from just above and just below plateau pressure and identifying the phases present. X-ray diffraction and visual observation were used for phase identification in these cases. When the sample was not wrapped by silver during the titration at 800 ◦ C, the new phase of Bi2 Fe4 O9 was found, which was mentioned in Reference [6], and in Reference [5] there are no this phase. In this process of titration without silver wrap, Bi was evaporated, therefore the Bi2 Fe4 O9 became forming.
and
c = 1.3864 nm
It was called as -BiFeO3 , which is rhombohedral (R3c). After three titrations, there is a plateau. The sample was quenching from just above or just below plateau pressure. The phase was identified by X-ray diffraction as shown by Fig. 3. From the patterns, it can be seen that there is still some part of BiFeO3 existing. Its cell parameters were shown as below: a = b = 0.5588 nm,
and
c = 1.3897 nm
It was still -BiFeO3 [8], which is rhombohedral (R3c). In Fig. 3, there are many patterns for other phases. It is hard to determine the exact type of phase. Therefore, the energy disperse spectroscopy (EDS) was used to investigate the possible compositions and phases. From Table 1 we know that after titration, some pars of BiFeO3 phase still leave, while there are some new phases, including Bi25 FeO40 , which was determined from Fig. 3 by XRD. Fig. 4 is the SEM graphs of BiFeO3 before and after titration. It can be seen that the scale of BiFeO3 crystals increases after the titration. And there are some little crystals of other new phases in the interfaces. 4. Conclusions Phase equilibria near BiFeO3 in reduced oxygen pressures were investigated by coulometric titration. The results show that the phase relations near BiFeO3 depend strongly on oxygen pressure. The titration curve shows an invariant reaction. And Bi25 FeO40 is one of the producing phases from BiFeO3 . In the process, the Bi2 Fe4 O9 was determined also. There are still some points that need clarification, including the detailed reaction process and determination of other phases.
M.C. Li et al. / Materials Science and Engineering A 438–440 (2006) 346–349
Acknowledgements This study is supported in part by project 50272019 funded by the National Natural Science Foundation of China. And the U.K. Royal Society is also appreciated. References [1] A.G. Tutov, Fiz Tverd. Tela (FTVTA) 11 (1969) 2681–2684. [2] G.D. Achenbach, R. Gerson, W.J. James, J. Am. Ceram. Soc. 50 (1967) 437–439.
349
[3] V.A. Murashav, D.N. Rakov, V.M. Ionov, L.S. Dubenko, Y.U. Titov, Ferroelectrics 162 (1994) 11–14. [4] Yu.F. Popov, A.M. Kadomtseva, G.P. Vorobev, A.K. Zvezdin, Ferroelectrics 162 (1994) 135–137. [5] H. Koizumi, N. Nirizaki, T. Ikeda, Jpn. Appl. Phys. 3 (1964) 495– 496. [6] E.J. Speranskaya, V.M. Skorikov, E.Ya. Kode, V.A. Terektova, Bull. Acad. Sci. U.S.S.R. 5 (1965) 873–874. [7] C. Wagner, J. Chem. Phys. 21 (1953) 1819–1821. [8] A. Maitre, M. Francois, J.C. Gachon, J. Phase Equilib. Diffusion 25 (2004) 59–67.